Tom's Hardware and Que Publishing are partnering up to give you four chapters from Scott Mueller's Upgrading And Repairing PCs, 20th Edition. This fifth installment is the beginning of the forth chapter we're making available from Scott's book, which covers Power Supply Fundamentals. Don't forget to check out the previous chapters published on Tom's Hardware, Computer History 101: The Development Of The PC, Hard Drives 101: Magnetic Storage, LAN 101: Networking Basics, and LAN 102: Network Hardware And Assembly. In the days to come, we'll also present a comprehensive look at Power Supply usage factors.
The Power Supply
The power supply is not only one of the most important parts in a PC, it is unfortunately one of the most overlooked. Although most enthusiasts who build their own systems understand its importance, the mainstream PC buyer generally does not. Some that do pay any mind seem concerned only with how many watts of power it is rated to put out (even though no practical way exists to verify those ratings), without regard to whether the power being produced is clean and stable or whether it is full of noise, spikes, and surges.
I have always placed great emphasis on selecting a power supply for my systems. I consider the power supply the foundation of the system and am willing to spend a little extra to get a more robust and reliable unit. The power supply is critical because it supplies electrical power to every other component in the system. In my experience, the power supply is also one of the most failure-prone components in any computer system. Over the years I have replaced more power supplies in PCs than any other part. A malfunctioning power supply not only can cause other components in the system to malfunction, but it also can damage the other components in your computer by delivering improper or erratic voltages. Because of its importance to proper and reliable system operation, you should understand both the function and limitations of a power supply, as well as its potential problems and their solutions.
Primary Function and Operation
The basic function of the power supply is to convert the electrical power available at the wall socket to that which the computer circuitry can use. The power supply in a conventional desktop system is designed to convert either 120 V (nominal) 60 Hz AC (alternating current) or 240 V (nominal) 50 Hz AC power into +3.3 V, +5 V, and +12 V DC (direct current) power. Some power supplies require you to switch between the two input ranges, whereas others auto-switch.
Technically, the power supply in most PCs is described as a constant voltage switching power supply unit (PSU), which is defined as follows:
- Constant voltage means the power supply puts out the same voltage to the computer’s internal components, no matter the voltage of AC current running it or the capacity (wattage) of the power supply.
- Switching refers to the design and power regulation technique that most suppliers use. Compared to other types of power supplies, this design provides an efficient and inexpensive power source and generates a minimum amount of heat. It also maintains a small size and
low price.
The PSU normally supplies +3.3 V, +5 V, and +12 V to the system. These voltages are often called rails, referring to the fact that although there are multiple wires carrying a specific voltage, they are normally tied to a single rail (or tap) in the PSU. Multiple wires are used because, if all of the current were carried over a single wire, the wire and the terminals, connectors, and even the traces on the circuit boards would all have to be extremely large and thick to handle the load. Instead, it is cheaper and more efficient to spread the load out among multiple smaller and thinner wires.
The digital electronic components and circuits in the system (motherboard, adapter cards, and disk drive logic boards) typically use the +3.3 V or +5 V power, and the motors (disk drive motors and any fans) use the +12 V power. In addition, voltage regulators on the motherboard or in other components convert these standard voltages to others as necessary. The following table lists the devices typically powered by the various voltage rails.
| Voltage Rail Usage in a PC | |
|---|---|
| Rail | Devices Powered |
| +3.3 V | Chipsets, some DIMMs, PCI/AGP/PCIe cards, miscellaneous chips |
| +5 V | Disk drive logic, low-voltage motors, SIMMs, PCI/AGP/ISA cards, voltage |
| +12 V | Motors, high-output voltage regulators, AGP/PCIe cards |
| SIMM = Single Inline Memory Module DIMM = Dual Inline Memory Module PCI = Peripheral Component Interconnect PCIe = PCI Express AGP = Accellerated Graphics Port ISA = Industry Standard Architecture | |
You can think of each rail as a separate power circuit, kind of like a power supply within the power supply. Normally each rail is rated for a specified maximum amount of current in amperes. Because the extreme amount of 12 V current required by newer CPU voltage regulators and high-end video cards can exceed the output of common 12 V rails, some power supply designs use multiple +12 V rails. This means that essentially they have two or more separate 12 V circuits internally, with some wires tapping off of one circuit and others tapping off of another. Unfortunately, this can lead to power problems, especially if you fail to balance the loads on both rails or to ensure you don’t exceed the load capacity on one or the other. In other words, it is far better to have a single 12 V rail that can supply 40 amps than two 12 V rails supplying 20 amps each because with the single rail you don’t have to worry which connectors derive power from which rail and then try to ensure that you don’t overload one or the other.
Whereas the +3.3 V, +5 V, and +12 V rails are technically independent inside the power supply, many cheaper designs have them sharing some circuitry, making them less independent than they should be. This manifests itself in voltage regulation problems in which a significant load on one rail causes a voltage drop on the others. Components such as processors and video cards can vary their power consumption greatly by their activity. Transitioning from sitting at the Windows desktop to loading a 3D game can cause both the processor and video card to more than double the draw on the +12 V rail. On some cheaper power supplies, this can cause the voltages on the other rails to fall out of spec (drop greater than 5%), making the system crash. Better-designed power supplies feature truly independent rails with tighter regulation in the 1% to 3% range.
Voltage Regulators
The power supply must deliver a good, steady supply of DC power so the system can operate properly. Devices that run on voltages other than these directly must then be indirectly powered through on-board voltage regulators, which take the 5 V or 12 V from the power supply and convert that to the lower voltages required by various components. For example, older DDR (double data rate) dual inline memory modules (DIMMs) and Rambus inline memory modules (RIMMs) require 2.5 V, whereas DDR2 and DDR3 DIMMs require 1.8 V and 1.5 V, legacy AGP 4x/8x cards require 1.5 V, and current PCI Express cards use only 0.8 V differential signaling—all of which are supplied by simple on-board regulators. Processors also require a variety of voltages (as low as 1.3 V or less) that are supplied by a sophisticated voltage regulator module (VRM) that is either built into or plugged into the motherboard. You’ll commonly find three or more different voltage regulator circuits on a modern motherboard.
Note: When Intel began releasing processors that required a +3.3 V power source, power supplies that supplied the additional output voltage were not yet available. As a result, motherboard manufacturers began adding voltage regulators to their boards, which converted +5 V to +3.3 V for the processor. When other chips began using 3.3 V as well, Intel created the ATX power supply specification, which supplied 3.3 V to the motherboard. You would think that having 3.3 V direct from the power supply would have eliminated the need for on-board voltage regulators, but by that time, processors, memory, and other components began running on a voltages lower than 3.3 V. Motherboard manufacturers then included adaptable regulator circuits called voltage regulator modules to accommodate the widely varying processor voltage requirements. Additional regulators are also used to power other devices on the motherboard that don’t use +3.3 V, +5 V, or +12 V.
For more information see Scott Mueller's Upgrading And Repairing PCs, 20th Edition, “CPU Operating Voltages,” p. 83 (Chapter 3, “Processor Types and Specifications”).
Negative DC Voltages
If you look at a specification sheet for a typical PC power supply, you can see that the supply generates not only +3.3 V, +5 V, and +12 V, but also –12 V and possibly –5 V. Although –12 V and (possibly) –5 V are supplied to the motherboard via the power supply connectors, the motherboard normally uses only the +3.3 V, +5 V, and +12 V. If present, the –5 V is simply routed to the ISA bus on pin B5 so any ISA cards can use it, even though very few ever have. However, as an example, the analog data separator circuits found in older floppy controllers did use –5 V. The motherboard logic typically doesn’t use –12 V either; however, it might be used in some board designs for serial port or local area network (LAN) circuits.
The positive voltages seemingly power everything in the system (logic and motors), so what are the negative voltages used for? The answer is, not much! In fact, –5 V was removed from the ATX12V 1.3 and later specifications. The only reason it remained in most power supply designs for many years is that –5 V was required on the ISA bus for full backward compatibility. Because modern PCs no longer include ISA slots, the –5 V signal was deemed as no longer necessary. However, if you are installing a new power supply in a system with an older motherboard that incorporates ISA bus slots, you want a supply that does include the –5 V signal.
Note: The load placed on the –12 V output by an integrated LAN adapter is small. For example, the integrated 10/100 Ethernet adapter in the Intel D815EEAL motherboard uses only 10 mA of +12 V and 10 mA of –12 V (0.01 amps each) to operate.
Although older serial port circuits used +/–12 V outputs, today most run only on +3.3 V or +5 V.
The main function of the +12 V power is to run disk drive motors as well as the higher-output processor voltage regulators in some of the newer boards. Usually, a large amount of +12 V current is available from the power supply, especially in those designed for systems with a large number of drive bays (such as in a tower configuration). Besides disk drive motors and newer CPU voltage regulators, the +12 V supply is used by any cooling fans in the system—which, of course, should always be running. A single cooling fan can draw between 100 mA and 250 mA (0.1–0.25 amps); however, most newer fans use the lower 100 mA figure. Note that although most fans in desktop systems run on +12 V, portable systems can use fans that run on +5 V or even +3.3 V.
Systems with modern form factors based on the ATX or BTX standards include another special signal. This feature, called PS_ON, can turn the power supply (and thus the system) on or off via software. It is sometimes known as the soft-power feature. PS_ON is most evident when you use it with an operating system (OS) such as Windows that supports the Advanced Power Management (APM) or Advanced Configuration and Power Interface (ACPI) specification. When you shut down a PC from the Start menu, Windows automatically turns off the computer after it completes the OS shutdown sequence. A system without this feature only displays a message that it’s safe or ready for you to shut down the computer manually.
The Power Good Signal
In addition to supplying electrical power to run the system, the power supply ensures that the system does not run unless the voltages supplied are sufficient to operate the system properly. In other words, the power supply actually prevents the computer from starting up or operating until all the power supply voltages are within the proper ranges.
The power supply completes internal checks and tests before allowing the system to start. If the tests are successful, the power supply sends a special signal to the motherboard called Power_Good. This signal must be continuously present for the system to run. Therefore, when the AC voltage dips and the power supply can’t maintain outputs within regulation tolerance, the Power_Good signal is withdrawn (goes low) and forces the system to reset. The system does not restart until the Power_Good signal returns.
The Power_Good signal (sometimes called Power_OK or PWR_OK) is a +5 V (nominal) active high signal (with a variation from +2.4 V through +6.0 V generally being considered acceptable) that is supplied to the motherboard when the power supply has passed its internal self-tests and the output voltages have stabilized. This typically takes place anywhere from 100 ms to 500 ms (0.1–0.5 seconds) after you turn on the power supply switch. The power supply then sends the Power_Good signal to the motherboard, where the processor timer chip that controls the reset line to the processor receives it.
In the absence of Power_Good, the timer chip holds the reset line on the processor, which prevents the system from running under bad or unstable power conditions. When the timer chip receives the Power_Good signal, it releases the reset and the processor begins executing whatever code is at address FFFF0h (occupied by the motherboard ROM).
If the power supply can’t maintain proper outputs (such as when a brownout occurs), the Power_Good signal is withdrawn and the processor is automatically reset. When the power output returns to its proper levels, the power supply regenerates the Power_Good signal and the system again begins operation (as if you had just powered on). By withdrawing Power_Good before the output voltages fall out of regulation, the system never sees the bad power because it is stopped quickly (reset) rather than being allowed to operate using unstable or improper power levels, which can cause memory parity errors and other problems.
On pre-ATX systems, the Power_Good connection is made via connector P8-1 (P8 pin 1) from the power supply to the motherboard. ATX, BTX, and later systems use pin 8 of the 20/24-pin main power connector, which is usually a gray wire.
A properly designed power supply delays the arrival of the Power_Good signal until all the voltages stabilize upon turning on the system. Poorly designed power supplies, which are found in many low-cost systems, often do not delay the Power_Good signal properly and enable the processor to start too soon. (The normal Power_Good delay is 0.1–0.5 seconds.) Improper Power_Good timing also causes CMOS memory corruption in some systems.
Note: If you find that a system consistently fails to boot up properly the first time you turn on the switch, but that it subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with the Power_Good timing. You should install a new, higher-quality power supply and see whether that solves the problem.
Some cheaper power supplies do not have proper Power_Good circuitry and might just tie any +5 V line to that signal. Some motherboards are more sensitive to an improperly designed or improperly functioning Power_Good signal than others. Intermittent startup problems are often the result of improper Power_Good signal timing. A common example is when you replace a motherboard in a system and then find that the system intermittently fails to start properly when you turn on the power. This can be difficult to diagnose, especially for the inexperienced technician, because the problem appears to be caused by the new motherboard. Although it seems as though the new motherboard is defective, it usually turns out that the power supply is poorly designed. It either can’t produce stable enough power to properly operate the new board, or it has an improperly wired or timed Power_Good signal (which is more likely). In these situations, replacing the supply with a higher-quality unit, in addition to the new motherboard, is the proper solution.
The shape and general physical layout of a component is called the form factor. Items that share a form factor are generally interchangeable, at least as far as their sizes and fits are concerned. When designing a PC, the engineers can choose to use one of the popular standard PSU form factors, or they can elect to build their own custom design. Choosing the former means that a virtually inexhaustible supply of inexpensive replacement parts will be available in a variety of quality and power output levels. Going the custom route means additional time and expense for development. In addition, the power supply is unique to the system and generally available for replacement only from the original manufacturer. This precludes any upgrades as well, such as installing higher-output replacement models.
If you can’t tell already, I am a fan of the industry-standard form factors! Having standards and then following them allows us to upgrade and repair our systems by easily replacing physically (and electrically) interchangeable components. Having interchangeable parts means that we have a better range of choices for replacement items, and the competition makes for better pricing, too.
In the PC market, IBM originally defined the form factor standards, and everybody else copied them; this included power supplies. All the popular PC power supply form factors up through 1995 were based on one of three IBM models, including the PC/XT, AT, and PS/2 Model 30. The interesting thing is that these three original IBM power supply form factors had the same motherboard connectors and pinouts; where they differed was mainly in shape, maximum power output, the number of peripheral power connectors, and switch mounting. PC systems using knock-offs of one of those three designs were popular up through 1996 and beyond; in fact, even the current industry standard ATX12V models are based on the PS/2 Model 30 physical form factor, but with different connectors.
Intel defined a new power supply form factor in 1995 with the introduction of the ATX form factor. ATX became popular in 1996 and started a shift away from the previous IBM-based standards. ATX and the standards that have followed since use different connectors with additional voltages and signals that allow systems with greater power consumption and additional features that would otherwise not be possible with the AT-style supplies.
Note: Although two power supplies can share the same basic design and form factor, they can differ greatly in quality and efficiency. Later in this chapter, you’ll learn about some of the features and specifications to look for when evaluating PC power supplies.
More than 10 different power supply form factors have existed that can be called industry standards. Many of these are or were based on designs IBM created in the 1980s, whereas the rest are based on Intel designs from the 1990s to the present. The industry-standard form factors can be broken down into two main categories: those that are currently in use in modern systems and those that are largely obsolete.
Note that although the names of some of the power supply form factors seem to be the same as those of motherboard form factors, the power supply form factor relates more to the system chassis (case) than to the motherboard. That is because all the form factors use one of only two main types of connector designs: either AT or ATX, with subtle variations on each. So, although a particular power supply form factor might be typically associated with a particular motherboard form factor, many other power supplies would plug in as well.
For example, all modern ATX form factor motherboards with PCI Express slots have two main power connectors, including a 24-pin ATX main connector along with a four-pin +12 V connector. All the modern power supply form factors include these same connectors and therefore are capable of plugging into the same motherboards. In other words, no matter what the form factor of the motherboard (ATX, BTX, or any of the smaller variants of either), virtually any of the modern industry-standard power supplies will plug in.
Plugging the power supply connectors into the motherboard is one thing, but for the power supply to work in the system, it must physically fit inside the chassis or case—and that is what the different modern power supply form factors are all about. The bottom line is that it is up to you to ensure that the power supply you purchase not only plugs in to your motherboard, but also fits into the chassis or case you plan to use.
The following two tables show the industry-standard power supply form factors, their connector types, and the motherboard form factors with which they are usually associated.
| Modern Industry-Standard Power Supply Form Factors | |||
|---|---|---|---|
| Modern Power Supply Form Factors | Year Introduced | Normally Associated Motherboard Connector Types | Form Factors |
| ATX/ATX12V | 1995 | 20/24-pin Main, 4-pin +12V | ATX, microATX, BTX, microBTX |
| SFX/SFX12V*/PS3 | 1997 | 20/24-pin Main, 4-pin +12V | microATX, FlexATX, microBTX, picoBTX, Mini-ITX, DTX |
| EPS/EPS12V | 1998 | 24-pin Main, 8-pin +12V | ATX, extended ATX |
| TFX12V | 2002 | 20/24-pin Main, 4-pin +12V | microATX, FlexATX, microBTX, picoBTX, Mini-ITX, DTX |
| CFX12V | 2003 | 20/24-pin Main, 4-pin +12V | microBTX, picoBTX, DTX |
| LFX12V | 2004 | 24-pin Main, 4-pin +12V | picoBTX, nanoBTX, DTX |
| Flex ATX | 2007 | 24-pin Main, 4-pin +12V | ‑microATX, FlexATX, microBTX, picoBTX, nanoBTX, Mini-ITX, DTX |
| *SFX12V also includes the PS3 form factor, which is a shortened version of ATX12V. | |||
12 V versions include a four-pin or eight-pin +12 V connector.
You may encounter power supplies using obsolete form factors if you work on PCs built in the 1980s through the mid-1990s. I cover them in more detail in the 18th and earlier editions of this book.
| Obsolete Industry Standard Power Supply Form Factors | |||
|---|---|---|---|
| Obsolete Power Supply Form Factors | Year Introduced | Normally Associated Motherboard Connector Types | Form Factors |
| *PC/XT | 1981 | PC/XT | PC/XT, Baby-AT |
| AT/Desk | 1984 | AT | Full-size AT, Baby-AT |
| AT/Tower | 1984 | AT | Full-size AT, Baby-AT |
| Baby-AT | 1984 | AT | Full-size AT, Baby-AT |
| **LPX (PS/2) | 1987 | AT | Baby-AT, Mini-AT, LPX |
| *PC/XT connectors were the same as AT connectors, except one +5 V pin (P8 pin 2) was not used. **LPX is also sometimes called PS/2 or Slimline. | |||
Each of these power supply form factors is, or has been, available in numerous configurations and power output levels. The obsolete LPX form factor supply originated in the IBM PS/2 Model 30 in April 1987 and was the standard used on most systems from the late 1980s to mid-1996, when the ATX form factor started to gain in popularity. Since then, ATX and the many variants based on ATX have become by far the dominant form factors for power supplies. It is interesting to note that IBM’s legacy lives on even now because ATX, PS3, and EPS are all based on the LPX (PS/2) physical form factor. Any power supply that does not conform to one of these standards is considered proprietary. In general, avoid systems that use proprietary power supply designs because replacements are difficult to obtain and upgrades are generally not available. When you consider that the power supply is one of the most failure-prone components, purchasing systems that use proprietary designs can be a significant liability in the future. If you need a replacement for a proprietary form factor supply, one of the best sources is ATXPowerSupplies.com. They maintain replacement models that cover a huge number of both proprietary and industry standard designs.
The power supply form factors detailed in the following sections are the standards used in current systems. ATX is far and away the most common of these, but if you work on a variety of PC types, you are likely to encounter the other types listed here.
ATX/ATX12V
In 1995, Intel saw that the existing power supply designs were literally running out of power. The problem was that the existing standards used two connectors with a total of only 12 pins providing power to the motherboard. In addition, the connectors used were difficult to properly key, and plugging them in improperly resulted in short-circuiting and damage to both the motherboard and the power supply. To solve these problems, in 1995 Intel took the existing popular LPX (PS/2) design and simply changed the internal circuitry and connectors (while leaving the mechanical shape the same), giving birth to the ATX power supply form factor.
Intel released the ATX specification in 1995, and in 1996, it started to become increasingly popular in Pentium and Pentium Pro–based PCs, capturing 18% of the market within the first year. Since 1996, ATX variants have become both the dominant motherboard and power supply form factors, replacing the previously popular Baby-AT/LPX designs. ATX12V power supplies are also used with newer motherboard form factors such as BTX, ensuring that ATX and its derivatives will remain the most popular power supply form factors for several years to come. The ATX12V specification defines the physical or mechanical form as well as the electrical connectors for the power supply.
From 1995 through early 2000, the ATX power supply form factor was defined as part of the ATX motherboard specification. However, in February 2000, Intel took the power supply specification out of the then-current ATX 2.03 motherboard/chassis specification and created the ATX/ATX12V power supply specification 1.0, adding an optional four-pin +12 V connector at the same time (those with the +12 V connector were called ATX12V supplies). The +12 V connector was made a requirement in version 1.3 (April 2002), whereupon the specification became only ATX12V. The ATX12V 2.0 specification (February 2003) dropped the six-pin auxiliary connector, changed the main power connector to 24 pins, and made Serial ATA power connectors a requirement. The current version is ATX12V 2.2, which was released in March 2005 and contains only minor changes from the previous releases, such as the use of Molex High Current System (HCS) terminals in the connectors.
As the ATX power supply specification has evolved, there have been some changes in the cooling fan orientation and design. The ATX specification originally called for an 80 mm fan to be mounted along the inner side of the supply, where it could draw air in from the rear of the chassis and blow it inside across the motherboard. This kind of airflow runs in the opposite direction than most standard supplies, which exhaust air out the back of the supply through a hole in the case where the fan protrudes. The idea was that the reverse-flow design could cool the system more efficiently with only a single fan, eliminating the need for a fan (active) heatsink on the CPU.
Another benefit of the reverse-flow cooling is that the system would run cleaner, freer from dust and dirt. The case would be pressurized, so air would be continuously forced out of the cracks in the case—the opposite of what happens with a negative pressure design. For this reason, the reverse-flow cooling design is often referred to as a positive-pressure-ventilation design. On an ATX system with reverse-flow cooling, the air is blown out away from the drive because the only air intake is the single fan vent on the power supply at the rear. For systems that operate in extremely harsh environments, you can add a filter to the fan intake vent to further ensure that all the air entering the system is clean and free of dust.
Although this sounds like a good way to ventilate a system, the positive-pressure design needs to use a more powerful fan to pull the required amount of air through a filter and pressurize the case. Also, if a filter is used, it must be serviced periodically; depending on operating conditions, it could need changing or cleaning as often as every week. In addition, the heat load from the power supply on a fully loaded system heats the air being ingested, blowing warm air over the CPU and reducing the overall cooling capability.
As CPUs evolved to generate more and more heat, the cooling capability of the system became more critical and the positive-pressure design was simply not up to the task. Therefore, subsequent versions of the ATX specification were rewritten to allow both positive- and negative-pressure designs, but they emphasized the standard negative-pressure system with an exhaust fan on the power supply and an additional high-quality cooling fan blowing cool air right on the CPU as the best solution.
Because a standard negative-pressure system offers the greatest cooling capacity for a given fan’s airspeed and flow, virtually all recent ATX-style power supplies use a negative-pressure design, in which air flows out the back of the power supply. Most use an 80 mm fan mounted on the rear of the unit blowing outward, but some use an 80 mm, a 92 mm, or a 120 mm fan mounted on the inside upper or lower surface, with open vents on the rear of the system. In either example, the flow of air is such that air is always exhausted out of the system through the rear of the supply.
The ATX power supply form factor addressed several problems with the previous PC/XT, AT, and LPX-type supplies. One is that the power supplies used with PC/XT/AT boards had only two connectors that plugged into the motherboard. If you inserted these connectors backward or out of their normal sequence, you would usually fry both the motherboard and the power supply! Most responsible system manufacturers tried to “key” the motherboard and power supply connectors so you couldn’t install them backward or out of sequence. However, most vendors of cheaper systems did not feature this keying on the boards or supplies they used. The ATX form factor includes intelligently designed and keyed power plugs to prevent users from incorrectly plugging in their power supplies. The ATX connectors also supply +3.3 V, reducing the need for voltage regulators on the motherboard to power +3.3 V-based circuits.
Besides the new +3.3 V outputs, ATX power supplies furnish another set of outputs that is not typically seen on standard power supplies. The set consists of the Power_On (PS_ON) and 5V_Standby (5VSB) outputs mentioned earlier, known collectively as Soft Power. This enables features to be implemented, such as Wake on Ring or Wake on LAN, in which a signal from a modem or network adapter can actually cause a PC to wake up and power on. Many such systems also have the option of setting a wakeup time, at which the PC can automatically turn itself on to perform scheduled tasks. These signals also can enable the optional use of the keyboard to power the system on—an option you can set on some systems. These features are possible because the +5 V Standby power is always active, giving the motherboard a limited source of power even when off. Check your BIOS Setup for control over these types of features.
SFX/SFX12V
Intel released the smaller microATX motherboard form factor in December 1997. At the same time, it released the small form factor (SFX) power supply design to go with it. Even so, most microATX chassis continued to use the standard ATX power supply instead. Then in March 1999, Intel released the FlexATX addendum to the microATX specification, which was a small board designed for low-end PCs or PC-based appliances. Since then, the SFX supply has found use in many new compact system designs. Unlike most of the power supply form factor specifications in which a single mechanical or physical outline is defined, the SFX standard actually defines five different physical shapes, some of which are not directly interchangeable. In addition, there have been changes to the connectors required as the specification has evolved. Therefore, when replacing an SFX/SFX12V-type supply, you need to ensure you are purchasing the correct type—which is to say the type that will physically install in your chassis—as well as have the correct connectors for your motherboard.
The number and types of connectors have changed over the life of the specification. The original SFX power supply specification included a single 20-pin motherboard connector. The four-pin +12 V connector to provide independent CPU power was added as an option in the 2.0 revision (May 2001) and was made a requirement in revision 2.3 (April 2003), causing the spec to be renamed as SFX12V in the process. SFX12V version 3.0 changed the main motherboard power connector from 20 pins to 24 pins and made Serial ATA power connectors a requirement. The current SFX12V version 3.1 was released in March 2005 and contains a few additional minor revisions, including a change to High Current System (HCS) terminals in the connectors. SFX12V includes several physical designs, including one called the PS3 form factor.
On a standard SFX/SFX12V power supply, a 60 mm diameter cooling fan is located inside the power supply housing, facing the inside of the computer’s case. The fan draws the air into the power supply housing from the system cavity and expels it through a port at the rear of the system. Internalizing the fan in this way reduces system noise and results in a standard negative-pressure design. The system can also use additional processor and chassis cooling fans, which are separate from the power supply.
SFX/SFX12V standard power supply with internal 60 mm fan.
For systems that require more cooling capability, a version that allows for a larger, 80 mm top-mounted cooling fan also is available. The larger fan provides more cooling capability and airflow for systems that need it.
SFX/SFX12V standard power supply with a recessed, top-mounted 80 mm fan.
Another variation of SFX12V also uses a recessed top-mounted 80 mm cooling fan, but it has the body of the power supply rotated for greater width but reduced depth, as shown in the following image.
A special low-profile version of SFX12V designed for a slim chassis is only 50 mm tall with an internal 40 mm cooling fan, as shown in the image below that.
Finally, a more recent variation on SFX is called the PS3 form factor, defined in Appendix E of the SFX12V specification. Although defined as part of SFX12V, the PS3 form factor is actually a shortened version of ATX12V and is generally used in systems with microATX chassis and motherboards that require higher power output than the smaller SFX variants can supply.
SFX/SFX12V rotated power supply with a recessed top-mounted 80 mm fan.
SFX/SFX12V low-profile power supply with internal 40 mm fan.
PS3 (SFX/SFX12V) power supply with internal 80 mm fan.
SFX12V power supplies are specifically designed for use in small systems containing a limited hardware and limited upgradeability. Most SFX supplies are designed to provide 80–300 watts of continuous power in four voltages (+5 V, +12 V, –12 V, and +3.3 V). This amount of power has proven to be sufficient for a small system with a processor, an AGP or PCI Express x16 interface, up to four expansion slots, and three peripheral devices—such as hard drives and optical drives.
Although Intel designed the SFX12V power supply specification with the microATX and FlexATX motherboard form factors in mind, SFX is a wholly separate standard that is compliant with other motherboards as well. For example, the PS3 variant of SFX12V can replace standard ATX12V power supplies as long as the output capabilities and provided connectors match the system requirements. SFX power supplies use the same 20-pin or 24-pin connectors defined in the ATX/ATX12V standards and include both the Power_On and 5V_Standby outputs. SFX12V power supplies add the four-pin +12 V connector for CPU power, just as ATX12V supplies do. Whether you will use an ATX- or SFX-based power supply in a given system depends more on the case or chassis than the motherboard. Each has the same basic electrical connectors; the main difference is which type of power supply the case is physically designed to accept.
EPS/EPS12V
In 1998, a group of companies including Intel, Hewlett-Packard, NEC, Dell, Data General, Micron, and Compaq created the Server System Infrastructure (SSI), an industry initiative to promote industry-standard form factors covering common server hardware elements such as chassis, power supplies, motherboards, and other components. The idea was to be able to design network servers that could use industry-standard interchangeable parts. You can find out more about SSI at www.ssiforum.org. Although this book does not cover network servers, in many ways a low-end server is a high-end PC, and many high-end components that were once found only on servers have trickled down to standard PCs. This trickle-down theory is especially true when it comes to power supplies.
In 1998, the SSI created the entry-level power supply (EPS) specification, which defines an industry-standard power supply form factor for entry-level pedestal (standalone tower chassis) servers. The initial EPS standard was based on ATX, but with several enhancements. The first major enhancement was the use of a 24-pin main power connector, which eventually trickled down to the ATX12V as well as other power supply form factor specifications in 2003. EPS also originally called for the use of HCS terminals in the Molex Mini-Fit Jr.–based power supply connectors, which became standard in ATX12V in March 2005. In addition, the (now-obsolete) auxiliary six-pin power connector, the four-pin +12 V power connector, and a variation of the six-pin graphics power connector all appeared in the EPS specifications before ending up in ATX.
The EPS specification originally used a mechanical form factor identical to ATX, but the EPS form factor was later extended to support higher power outputs by allowing the body of the supply to be deeper if necessary. The ATX and the original EPS standards call for a supply that is 86 mm tall by 150 mm wide by 140 mm deep, the same dimensions as the LPX or PS/2 form factors. EPS later added optional extended depths of 180 mm and 230 mm total. Most power supplies with true ratings of 500 watts or more are made in the EPS12V form factor, because it isn’t really possible to fit more power than that into the standard ATX size. You may think these would require a custom EPS chassis, but in fact many (if not most) full-sized ATX tower chassis can handle these greater depths without interference, especially when using one of the newer breed of shorter-length optical drives (because one or more of the optical drives are often inline with the power supply).
With the improvements in EPS/EPS12V power supplies trickling down to ATX/ATX12V, I have studied the SSI EPS specifications to see what potential improvements might come to ATX. The main difference today between ATX and EPS with respect to connectors is the use of an eight-pin dual +12 V connector in EPS12V instead of a four-pin +12 V connector in ATX12V. The eight-pin dual +12 V connector is essentially the equivalent of two four-pin connectors mated together and is used by entry-level servers to power multiple processors. Because of the way the connectors are designed, an eight-pin +12 V connector can plug into a four-pin +12 V connector on a motherboard, with the unused pins simply hanging unused, offset to one side or the other.
The only other major difference between EPS12V and ATX12V is that EPS power supplies can be up to 180 mm or 230 mm deep, whereas ATX supplies are technically limited to 140 mm depth according to the specification. An example of an EPS12V type supply from PC Power and Cooling is shown below.
EPS12V form factor power supply (www.pcpower.com).
This power supply is basically a 230 mm-deep EPS12V supply that works in place of or as an upgrade to an ATX12V supply as long as the chassis can accommodate the additional depth. EPS12V supplies are sometimes called extended ATX power supplies because of their optional extended length. If you plan to use one of these EPS12V power supplies in a standard ATX chassis, it’s important that you measure the available space in your chassis to ensure you have the room behind the supply for the additional depth.
Connector compatibility isn’t generally a problem because, by virtue of the Molex Mini-Fit Jr. connector design, you can plug a 24-pin main power connector into a 20-pin socket, as well as an eight-pin dual +12 V connector into a four-pin +12 V socket.
If you have the room, an EPS12V power supply can be used with most ATX chassis and motherboards for the ultimate in high-output capabilities.
TFX12V
The TFX12V (thin form factor) power supply was originally introduced by Intel in April 2002 and is designed for small form factor (SFF) systems of about 9–15 liters in volume, primarily those using low-profile SFF chassis and microATX, FlexATX, or Mini-ITX motherboards. The basic shape of TFX12V is longer and narrower than the ATX- or SFX-based form factors, allowing it to more easily fit into low-profile systems. The dimensions of the TFX12V form factor are shown in the figure below.
TFX12V power supply form factor dimensions.
TFX12V power supplies are designed to deliver nominal power output capabilities of 180–300 watts, which is more than adequate for the smaller systems for which they are designed. TFX12V supplies include a side-mounted internal 80 mm fan that is usually thermostatically controlled, so as to run both coolly and quietly. A symmetrically designed mounting system allows the fan to be oriented facing either side inside the system for optimum cooling and flexibility in accommodating different chassis layouts.
TFX12V power supplies are symmetrical and can be mounted with the fan facing either left or right.
Unlike SFX-based supplies, only one standard mechanical form factor exists for TFX12V supplies. TFX12V supplies have also always included the four-pin +12 V connector since the standard appeared in April 2002, well after the +12 V connector had been included in other power supply form factors. TFX12V 1.2 (April 2003) added the Serial ATA power connector as an option, whereas the TFX12V 2.0 release (February 2004) made them mandatory and changed the main power connector from 20 pins to 24 pins. Revision 2.1 (July 2005) includes only minor updates and changes from the previous version.
CFX12V
The CFX12V (compact form factor) power supply was originally introduced by Intel in November 2003 and is designed for mid-sized balanced technology extended (BTX) systems of about 10–15 liters in volume, primarily using microBTX or picoBTX motherboards.
CFX12V power supplies are designed to deliver nominal power output capabilities of 220–300 watts, which is more than adequate for the mid-sized systems for which they are designed. CFX12V supplies include a rear-mounted internal 80 mm fan that is typically thermostatically controlled, which enables it to run both coolly and quietly. The shape of the supply includes a ledge such that part of the supply can extend over the motherboard, reducing the overall size of the system. The dimensions of the CFX12V form factor are shown in the image below.
CFX12V power supply dimensions.
CFX12V supplies have included the four-pin +12 V connector since the standard first appeared in November 2003, well after the +12 V connector had been included in other power supply form factors. TFX12V also included the 24-pin main power connector and Serial ATA power connectors as mandatory since its inception. The current CFX12V 1.2 release dates from 2005 and has only minor revisions over previous versions, including a change to HCS terminals in the connectors.
LFX12V
Intel originally introduced the LFX12V (low profile form factor) power supply in April 2004. It’s designed for ultra-small BTX systems of about 6–9 liters in volume, primarily using picoBTX or nanoBTX motherboards.
Figure 18.12 LFX12V power supply.
LFX12V power supplies are designed to deliver nominal power output capabilities of 180–260 watts, which is ideal for the tiny systems for which they are designed. LFX12V supplies include an internal 60 mm fan, which is 20 mm smaller than that of the CFX12V design. Similar to the CFX12V fan, it is usually thermostatically controlled to ensure quiet operation while still providing adequate cooling. The shape of the supply includes a ledge such that part of the supply can extend over the motherboard, reducing the overall size of the system. The dimensions of the LFX12V form factor are shown below.
LFX12V power supply dimensions.
All LFX12V supplies include a 24-pin main motherboard power connector, a four-pin +12 V connector, and Serial ATA connectors. The current LFX12V 1.1 release dates from April 2005 and has only minor revisions over the previous version.
Flex ATX
A power supply company called FSP (Fortron Source Power) originally introduced variations of what was to become the Flex ATX power supply form factor in 2001 in the form of proprietary designs for SFF desktop and thin (1U) server systems. These power supplies became popular in systems from Shuttle, but they have also been used by HP/Compaq, IBM, SuperMicro, and others. In an effort to make this form factor an official standard, Intel introduced the Flex ATX power supply form factor in March 2007 as part of the 1.1 and later revisions of the “Power Supply Design Guide for Desktop Platform Form Factors” document, which is available on the www.formfactors.org site. These are also sometimes called 1U (one unit) power supplies because they are used in many 1U server chassis.
Flex ATX power supplies, like the one shown below, are designed to deliver nominal power output capabilities of between 180 and 270 watts, which is ideal for the small systems for which they are designed. Flex ATX supplies usually include one or two internal 40 mm fans; however, larger fans can be mounted horizontally, and even fanless models exist.
Flex ATX power supply dimensions.
Flex ATX power supplies include either a 20-pin or 24-pin main motherboard power connector and a four-pin +12 V connector for the motherboard. They also usually include standard peripheral and floppy power connectors, with newer units having Serial ATA power connectors as well.
Three main types of power switches are used on PCs. They can be described as follows:
- Front panel motherboard-controlled switch (ATX and newer)
- Front panel power supply AC switch (AT/LPX; obsolete)
- Integral power supply AC switch (PC/XT/AT; obsolete)
ATX and Newer
All ATX and newer power supplies that employ the 20- or 24-pin motherboard connector use the PS_ON signal to power up the system. In this design, the power supply runs in standby mode when plugged in with the system off. The PS_ON signal is routed from the power supply through the motherboard to a low-voltage momentary contact DC switch on the front panel. As a result, the remote power switch does not physically control the power supply’s access to the 120 V AC power, as in older-style power supplies. Instead, the power supply’s on or off status is toggled by the PS_ON signal received on the ATX Main power connector. This is sometimes called a soft-off switch because this is the name of the Advanced Configuration Power Interface (ACPI) state when the system is off but still receiving standby power.
The PS_ON signal can be manipulated physically by the computer’s power switch or electronically by the motherboard under software control. PS_ON is an active low signal, meaning the power supply voltage outputs are disabled (the system is off) when the PS_ON is high (greater than or equal to 2.0 V). This excludes the +5 VSB (Standby) on pin nine of the ATX main power connector, which is active whenever the power supply is connected to an AC power source. The power supply maintains the PS_ON signal at either 3.3 V or +5 V. This signal is then routed through the motherboard to the remote switch on the front of the case. When the switch is pressed, the PS_ON signal is grounded. When the power supply sees the PS_ON signal drop to 0.8 V or less, the power supply (and system) is turned on. Thus, the remote switch in a system using an ATX or newer power supply carries up to only +5 V of DC power, rather than the full 120 V–240 V AC current like that of the older AT/LPX form factors.
The actual power switch used in ATX systems is normally a tiny momentary contact push button switch, which is connected to the motherboard front panel header via a tiny two-pin connector. When the button is pushed, the motherboard then grounds the PS_ON signal in the main 20/24-pin power connector, causing the power supply to turn on.
Caution: The continuous presence of the +5 VSB power on pin nine of the ATX main connector means the motherboard is always receiving standby power from the power supply when connected to an AC source, even when the computer is turned off. As a result, it is even more important to unplug an ATX system from the power source before working inside the case than it is on earlier model systems.
The remote switch on ATX and newer designs can only put the system in a soft-off state, in which the system appears off but is still receiving standby power. Some ATX and newer power supplies include a hard override AC power switch on the back, which essentially disconnects AC power from the system when turned off. With the AC switch off, the system no longer receives standby power and is essentially the same as being completely unplugged from an AC outlet.
Tip: The design of the ATX power switch is such that the motherboard actually controls the status of the power supply. On systems with full support for ACPI, when you press the power switch, the motherboard informs the OS to perform an orderly shutdown before the power is actually turned off. However, if the system is locked up or corrupted, it can remain running when you press the switch. In that situation, you can manually override the ACPI control by pressing the switch continuously for more than four seconds, which overrides the software control and forcibly turns off the system.
PC/XT/AT and LPX Power Switches
The earliest systems had power switches integrated or built directly into the power supply, which turned the main AC power to the system on and off. This was a simple design, but because the power supply was mounted in the rear right of the system, it required reaching around to the right side near the back of the system to actuate the switch. Also, switching the AC power directly meant the system couldn’t be remotely started without special hardware.
Starting in the late 1980s, systems with LPX power supplies began using remote front panel switches. These were still AC switches; the only difference was that the AC switch was now mounted remotely (usually on the front panel of the chassis), rather than integrated in the power supply unit. The switch was connected to the power supply via a four-wire cable, and the ends of the cable were fitted with spade connector lugs, which plugged onto the spade connectors on the power switch. The cable from the power supply to the switch in the case contained four color-coded wires. In addition, a fifth wire supplying a ground connection to the case was sometimes included. The switch was usually included with the power supply and heavily shrink-wrapped or insulated where the connector lugs attached, to prevent electric shock.
This solved the ergonomic problem of reaching the switch, but it still didn’t enable remote or automated system power-up without special hardware. Plus, you now had a 120 V AC switch mounted in the chassis, with wires carrying dangerous voltage through the system. Some of these wires are hot anytime the system is plugged in (all are hot when the system’s turned on), creating a dangerous environment for the average person when messing around inside the system.
Caution: At least two of the remote power switch leads to a remote-mounted AC power switch in an AT/LPX supply are energized with 120 V AC at all times. You can be electrocuted if you touch the ends of these wires with the power supply plugged in, even if the unit is turned off! For this reason, always make sure the power supply is unplugged before connecting or disconnecting the remote power switch or touching any of the wires connected to it.
The four or five wires are usually color-coded as follows:
- Brown and blue—These wires are the live and neutral feed wires from the 120 V power cord to the power supply. These are always hot when the power supply is plugged in.
- Black and white—These wires carry the AC feed from the switch back to the power supply. These leads should be hot only when the power supply is plugged in and the switch is turned on.
- Green or green with a yellow stripe—This is the ground lead. It should be connected to the PC case and should help ground the power supply to the case.
On the switch, the tabs for the leads are usually color-coded; if not, you’ll find that most switches have two parallel tabs and two angled tabs. If no color-coding is on the switch, plug the blue and brown wires onto the tabs that are parallel to each other and the black and white wires to the tabs that are angled away from each other. If none of the tabs are angled, simply make sure the blue and brown wires are plugged into the most closely spaced tabs on one side of the switch and the black and white wires on the most closely spaced tabs on the other side (see the following image).
Power supply remote pushbutton switch connections.
Caution: Although these wire color-codings and parallel/angled tabs are used on most power supplies, they are not necessarily 100% universal. I have encountered power supplies that do not use the same coloring or tab placement scheme described here. One thing is sure: two of the wires will be hot with potentially fatal AC voltage anytime the power supply is plugged in. No matter what, always disconnect the power supply from the wall socket before handling any of these wires. Be sure to insulate the connections with electrical tape or heat-shrink tubing so you won’t be able to touch the wires when working inside the case in the future.
As long as the blue and brown wires are on one set of tabs and the black-and-white leads are on the other, the switch and supply will work properly. If you incorrectly mix the leads, you will likely blow the circuit breaker for the wall socket because mixing them can create a direct short circuit.
Every PC power supply has connectors that attach to the motherboard, providing power to the motherboard, processor, memory, chipset, integrated components (such as video, LAN, universal serial bus [USB], and FireWire), and any cards plugged into bus slots. These connectors are important; not only are these the main conduit through which power flows to your system, but attaching these connectors improperly can have a devastating effect on your PC, including burning up both your power supply and motherboard. Just as with the mechanical shape of the power supply, these connectors are usually designed to conform to one of several industry-standard specifications, which dictate the types of connectors used as well as the pinouts of the individual wires and terminals. Unfortunately, just as with the mechanical form factors, some PC manufacturers use power supplies with custom connectors or, worse yet, use standard connector types but with modified (incompatible) pinouts (meaning the signals and voltages are rearranged from standard specifications). Plugging a power supply with an incompatible pinout into a motherboard that uses a standard pinout (or vice versa) usually results in the destruction of either the board or the power supply—or both.
Just as I insist on industry-standard mechanical form factors in my systems, I also want to ensure that they use industry-standard connectors and pinouts. By only purchasing components that conform to industry standards, I can ensure the greatest flexibility and lowest cost for future upgrades and repairs.
Two main sets of motherboard power connectors have been used over the years: what I would call AT/LPX type and the ATX type. Each of these has minor variations; for example, the ATX type has evolved over the years, with new connectors coming (and some going) and modifications to existing connectors. This section details the motherboard power connectors used by various types of industry-standard (and some not-so-standard) power supplies.
AT/LPX Power Supply Connectors
Industry-standard PC, XT, AT, Baby-AT, and LPX motherboards use the same type of main power supply connectors. AT/LPX power supplies feature two main power connectors (P8 and P9), each with six pins that attach the power supply to the motherboard. The terminals used in these connectors are rated to handle up to five amps at up to 250 V (even though the maximum used in a PC is +12 V). These two connectors are shown in the following figure.
AT/LPX main P8/P9 (also called P1/P2) power connectors, side and terminal end view.
All AT/LPX power supplies that use the P8 and P9 connectors have them installed end to end so that the two black wires (ground connections) on both power connectors are next to each other when properly plugged in. Note the designations P8 and P9 are not fully standardized, although most use those designations because that is what IBM stamped on the originals. Some power supplies have them labeled as P1/P2 instead. Because these connectors usually have a clasp that prevents them from being inserted backward on the motherboard’s pins, the major concern is getting the two connectors in the correct orientation side by side and also not offsetting by one or more pins side to side. Following the black-to-black rule and ensuring they are on-center keeps you safe. You must take care to ensure that no remaining unconnected motherboard pins exist between or on either side of the two connectors after you install them. A properly installed connector connects to and covers every motherboard power pin. If any power pins are showing on either side of or between the connectors, the entire connector assembly is installed incorrectly, which can result in catastrophic failure for the motherboard and everything plugged into it at the time of power-up. This next image shows the P8 and P9 connectors (sometimes also called P1/P2) in their proper orientation when connected to a motherboard.
This table shows typical AT/LPX power supply connections.
| AT/LPX Power Supply Connectors (Wire End View) | |||||||
|---|---|---|---|---|---|---|---|
| Connector | Pin | Signal | Color2 | Connector | Pin | Signal | Color2 |
| P8 (or P1) | 1 | Power_Good (+5V) | Orange | P9 (or P2) | 1 | Ground | Black |
| 2 | +5V1 | Red | 2 | Ground | Black | ||
| 3 | +12V | Yellow | 3 | -5 V | White | ||
| 4 | -12V | Blue | 4 | +5 V | Red | ||
| 5 | Ground | Black | 5 | +5 V | Red | ||
| 6 | Ground | Black | 6 | +5 V | Red | ||
| 1. First-generation PC/XT motherboards and power supplies did not require this voltage, so the pin might be missing from the motherboard and terminal and the wire might be missing from the connector (P8 pin 2). 2. I have seen some supplies where the manufacturer did not follow industry-standard wire color-codes even though the signals were correct. | |||||||
Tip: Although older PC/XT power supplies do not have a connection at connector P8 pin 2, you still can use them on AT-type motherboards, or vice versa. The presence or absence of the +5 V on that pin has little or no effect on system operation because the remaining +5 V wires can usually carry the load.
Note that all the AT/LPX-type power supplies use the same connectors and pin configurations; to my knowledge there were never nonstandard variations.
ATX and ATX12V Motherboard Power Connectors
Power supplies conforming to the original ATX and ATX12V 1.x form factor standards or variations thereof use the following three motherboard power connectors:
- 20-pin main power connector
- Six-pin auxiliary power connector
- Four-pin +12 V power connector
The main power connector is always required, but the other two are optional depending on the application. Consequently, a given ATX or ATX12V power supply can have up to four combinations of connectors, as listed here:
- Main power connector only
- Main and auxiliary
- Main and +12 V
- Main, auxiliary, and +12 V
The most common varieties are those including the main only and those with the main and +12 V connectors. Most motherboards that use the +12 V connector do not use the auxiliary connector, and vice versa.
20-Pin Main Power Connector
The 20-pin main power connector is standard for all power supplies conforming to the ATX and ATX12V 1.x power supply form factors and consists of a Molex Mini-Fit Jr. connector housing with female terminals. For reference, the connector is Molex part number 39-01-2200 (or equivalent), and the standard terminals are part number 5556 (see the following figure). This is a 20-pin keyed connector with pins configured as shown in the next table. The colors for the wires listed are those the ATX standard recommends; however, to enable them to vary from manufacturer to manufacturer, they are not required for compliance to the specification. I like to show these connector pinouts in a wire end view, which shows how the pins are arranged looking at the back of the connector (from the wire and not the terminal end). This way, you can see how they would be oriented if you were back-probing the connector with the connector plugged in.
ATX 20-pin motherboard main power connector, perspective view.
ATX 20-pin main power connector, side and terminal end view.
| ATX 20-pin Main Power Supply Connector Pinout (Motherboard Connector) | |||||
|---|---|---|---|---|---|
| Color | Signal | Pin | Pin | Signal | Color |
| Orange | +3.3 V | 111 | 1 | +3.3 V | Orange |
| Blue | –12 V | 12 | 2 | +3.3 V | Orange |
| Black | GND | 13 | 3 | GND | Black |
| Green | PS_On | 14 | 4 | +5 V | Red |
| Black | GND | 15 | 5 | GND | Black |
| Black | GND | 16 | 6 | +5 V | Red |
| Black | GND | 17 | 7 | GND | Black |
| White | -5 V | 182 | 8 | Power_Good | Gray |
| Red | +5 V | 19 | 9 | +5 VSB (Standby) | Purple |
| Red | +5 V | 20 | 10 | +12 V | Yellow |
| 1. Might have a second orange or brown wire, used for +3.3 V sense feedback. The power supply uses this wire to monitor 3.3 V regulation. 2. Pin 18 will be N/C (no connection, absent) on some later model supplies or motherboards because –5 V was removed from the ATX12V 1.3 and later specifications. Supplies with no connection at pin 18 should not be used with older motherboards that incorporate ISA Bus slots. | |||||
Note: The ATX supply features several voltages and signals not seen on earlier AT/LPX designs, such as the +3.3 V, PS_On, and +5V_Standby. Therefore, adapting a standard LPX form factor supply to make it work properly in an ATX system is impossible, even though the shapes of the power supplies are virtually identical.
However, because ATX is a superset of the older LPX power supply standard, you can use a connector adapter to allow an ATX power supply to connect to an older motherboard using AT/LPX connectors.
One of the most important issues with respect to power supply connectors is the capability to deliver sufficient power to the motherboard without overheating. It doesn’t help to have a 500-watt power supply if the cables and connectors supplying power to the motherboard can handle only 250 watts before they start to melt. When talking about specific connectors, the current rating is stated in amperes per circuit, which is a measure of the amount of current that can be passed through a mated terminal that will allow no more than a 30°C (86°F) temperature rise over ambient 22°C (72°F). In other words, at a normal ambient temperature of 22°C (72°F), when operating under the maximum rated current load, the temperature of the mated terminals will not exceed 52°C (126°F). Because the ambient temperature inside a PC can run 40°C (104°F) or higher, running power connectors at maximum ratings can result in extremely high temperatures in the connectors.
The maximum current level is further de-rated or adjusted for the number of circuits in a given connector housing due to the heat of any adjacent terminals. For example, a power connector might be able to carry eight amps per circuit in a four-position connector, but the same connector and terminal design might be able to handle only six amps per circuit in a 20-position connector.
All the modern form factor power supplies since ATX have standardized on the use of Molex Mini-Fit Jr. connectors for the main and +12 V connectors. A number of connector housings are used with anywhere from four to 24 positions or terminals. Molex makes three types of terminals for these connectors: a standard version, an HCS version, and a Plus HCS version. The current ratings for these terminals are shown below.
| Current Ratings for Mini-Fit Jr. Connectors | ||||
|---|---|---|---|---|
| Mini-Fit Jr. Terminal Type/No. | 2–3 Pins (Amps/Pin) | 4–6 Pins (Amps/Pin) | 7–10 Pins (Amps/Pin) | 12–24 Pins (Amps/Pin) |
| Standard/5556 | 9 | 8 | 7 | 6 |
| HCS/44476 | 12 | 11 | 10 | 9 |
| Plus HCS/45750 | 12 | 12 | 12 | 11 |
| All ratings assume Mini-Fit Jr. connectors with 12–24 circuits using 18-gauge wire under standard temperature conditions. | ||||
The ATX main power connector is either a 20-pin or 24-pin connector, which, if standard terminals are used, is rated for up to six amps of current per terminal. If the connector were upgraded to HCS terminals, the rating would increase to nine amps per terminal, and if upgraded to Plus HCS terminals, the rating would increase further to 11 amps per terminal. Prior to March 2005, all the power supply form factor specifications called for using standard terminals, but all the ratings from March 2005 to the present have changed to require HCS terminals instead. If your power supply connector has been overheating, you can easily install HCS or Plus HCS terminals to increase the power-handling capability of your connector by 50% or more.
By counting the number of terminals for each voltage level, you can calculate the power-handling capability of the connector, as shown below.
| ATX 20-pin Main Power Connector Maximum Power Handling Capabilities | ||||
|---|---|---|---|---|
| Volts | No. Pins | Using Std. Terminals (W) | Using HCS Terminals (W) | Using Plus HCS Terminals (W) |
| +3.3 V | 3 | 59.4 | 89.1 | 108.9 |
| +5 V | 4 | 120 | 180 | 220 |
| +12 V | 1 | 72 | 108 | 132 |
| Total watts: | 251.4 | 377.1 | 460.9 | |
| Standard terminals are rated six amps. HCS terminals are rated nine amps. Plus HCS terminals are rated 11 amps. All ratings assume Mini-Fit Jr. connectors with 12–24 circuits using 18-gauge wire under standard temperature conditions. | ||||
This means the total power-handling capacity of this connector is only 251 watts if standard terminals are being used, which is lower than most systems need today. Unfortunately, drawing more power than this maximum rating through the connector causes it to overheat. I’m sure you can appreciate how inadequate this has become today; for example, it certainly doesn’t make sense to manufacture a 400- or 500-watt power supply if the main power connector can handle only 251 watts without melting! That would be like building a car that could go 200 MPH and then equipping it with tires rated for only 100 MPH. Everything would be fine until you exceeded the tires’ rated speed, after which the results would not be pretty.
This is why the official power supply form factor specifications were updated in March 2005 to include HCS terminals, which have 50% greater power-handling capability than the standard terminals. Using HCS terminals, the power-handling capability of the 20-pin main connector alone increases to 377 watts, which is more than most systems need to run the entire system through all the connectors combined.
Six-Pin Auxiliary Power Connector
As motherboards and processors have evolved, the need for power has become greater. The terminals in the main power connector are rated for six amps (A) using standard terminals, which allows for a maximum supply of approximately 250 watts to the motherboard. Because motherboards with high-speed processors and multiple cards installed could draw more power than that and power supply manufacturers were building supplies with 300-watt and higher ratings, melted connectors were becoming more and more common. The terminals in the main connector overheated under such a load.
To allow for additional power from the supply to the motherboard, Intel modified the ATX specification to add a second auxiliary power connector for high power-drawing ATX motherboards and 250-watt or higher rated supplies. The criteria is such that, if the motherboard could draw more than 18 A of +3.3 V power or more than 24 A of +5 V power, the auxiliary connector is required to carry the additional load. These higher levels of power are needed in systems using 250- or 300-watt or greater supplies.
The six-pin auxiliary power connector was added as a safety or stopgap measure in the ATX motherboard 2.02/2.03 and ATX12V 1.x power supply specifications for systems in which the +3.3 V and +5 V power draw could exceed the respective 18 A and 24 A maximums allowed using only the main connector with standard terminals. These conditions would normally be met in systems requiring 300 W or higher output power supplies. The auxiliary power connector is a six-pin Molex 90331-0010 connector, which is similar to the motherboard power connectors used on older AT/LPX power supplies for Baby-AT motherboards.
The pinouts of the auxiliary connector are shown below.
| ATX Auxiliary Power Connector Pinout | |||||
|---|---|---|---|---|---|
| Pin | Signal | Color | Pin | Signal | Color |
| 1 | Gnd | Black | 4 | +3.3 V | Orange |
| 2 | Gnd | Black | 5 | +3.3 V | Orange |
| 3 | Gnd | Black | 6 | +5 V | Red |
Each terminal in the auxiliary power connector is rated to handle up to five amps of current, slightly less than the main power connector. By counting the number of terminals for each voltage level, you can calculate the power-handling capability of the connector, as shown below.
| Six-Pin Auxiliary Power Connector Maximum Power-Handling Capabilities | ||
|---|---|---|
| Volts | No. pins | Watts |
| +3.3 V | 2 | 33 |
| +5 V | 1 | 25 |
| Total watts: | 58 | |
| Terminals are rated five amps. Ratings assume 18-gauge wire under standard temperature conditions. | ||
This means the total power-handling capacity of this connector is only 58 watts. Drawing more power than this maximum rating through the connector will cause it to overheat.
Combining the 20-pin main plus the auxiliary power connector would result in a maximum power-delivery capability to the motherboard of 309 watts.
Few motherboards actually used this connector, and few power supplies included it. Generally, if a motherboard includes this connector, you need a power supply that has it as well, but if the power supply includes the auxiliary connector but the motherboard does not, it can be left unconnected.
Starting in 2000, both motherboards and power supplies began including a different additional connector that was a better solution than the auxiliary connector. The most recent power supply form factor specifications have removed the auxiliary connector, rendering it an obsolete standard in modern systems.
ATX12V 2.x 24-Pin Main Power Connector
Starting in June 2004, the PCI Express bus first appeared on motherboards. PCI Express is a type of serial bus with standard slots having a single channel or lane of communications. These single-lane slots are called x1 slots and are designed for peripheral cards such as network cards, sound cards, and the like. PCI Express also includes a special higher-bandwidth slot with 16 lanes (called an x16 slot), which is especially designed for use by video cards. During development, it was realized that PCI Express x16 video cards could draw more power than what was allowed by the existing 20-pin main and six-pin auxiliary power supply connectors, especially when it came to +12 V power.
The problem was that the 20-pin main connector had only a single +12 V pin, but the new video cards required more +12 V power than a single pin could safely deliver. The +12 V connector that had already been added, as discussed in the next section, was specifically for the CPU and was unavailable to other devices. Rather than add another supplemental or auxiliary connector as it had done before, Intel eventually decided that it was finally time to upgrade the main power connector to supply more power.
The result was officially called ATX12V 2.0 and was released in February 2003. ATX12V 2.0 included two major changes from the previous ATX12V 1.x specifications: a new 24-pin main power connector and the elimination of the six-pin auxiliary power connector. The new 24-pin main power connector included four more pins supplying additional +3.3 V, +5 V, and +12 V power plus a ground. The inclusion of these extra pins delivered extra power to satisfy the power requirements for PCI Express video cards drawing up to 75 watts, but it also made the older six-pin auxiliary connector unnecessary. The pinout of the new 24-pin main power connector started to be implemented in motherboards in mid-2004. The motherboard connector pinout is shown below, as is the PSU connector.
Note: Even though one of the design goals for increasing the main power connector to 24 pins was to provide extra power for PCI Express video cards, many if not most high-end video cards need more than the 75 watts available directly through the PCIe x16 slot. Video cards requiring more will have one or more additional power connectors on the card, which are used to draw power directly from the PSU.
ATX12V 2.x 24-pin main power connector.
| ATX12V 2.x 24-Pin Main Power Supply Connector Pinout (Motherboard Connector) | |||||
|---|---|---|---|---|---|
| Color | Signal | Pin | Pin | Signal | Color |
| Orange | +3.3 V | 131 | 1 | +3.3 V | Orange |
| Blue | –12 V | 14 | 2 | +3.3 V | Orange |
| Black | GND | 15 | 3 | GND | Black |
| Green | PS_On | 16 | 4 | +5 V | Red |
| Black | GND | 17 | 5 | GND | Black |
| Black | GND | 18 | 6 | +5 V | Red |
| Black | GND | 19 | 7 | GND | Black |
| - | N/C | 202 | 8 | Power_Good | Gray |
| Red | +5 V | 21 | 9 | +5 VSB (Standby) | Purple |
| Red | +5 V | 22 | 10 | +12 V | Yellow |
| Red | +5 V | 23 | 11 | +12 V | Yellow |
| Black | GND | 24 | 12 | +3.3 V | Orange |
| 1. Pin 13 might have a second orange or brown wire, used for +3.3 V sense feedback. The power supply uses this wire to monitor 3.3 V regulation. 2. Pin 20 will be N/C (no connection) because –5 V was removed from the ATX12V 1.3 and later specifications. | |||||
It is interesting to note that the 24-pin connector is not really that new; it first appeared in the SSI EPS specification released in 1998. SSI (http://ssiforum.org/) is an initiative designed to create standard interfaces for server components, including power supplies. The 24-pin main power connector was created for servers because, at the time, only servers needed the additional power. Today’s PCs draw the same power levels as servers did years ago, so rather than reinvent an incompatible connector, the ATX12V 2.0 standard merely incorporated the 24-pin connector already specified in the SSI EPS standard.
Compared to the previous 20-pin design, the 24-pin main power connector includes additional +3.3 V, +5 V, and +12 V terminals, allowing a substantially greater amount of power to be delivered to the motherboard. Each terminal in the main power connector is rated to handle up to six amps of current. By counting the number of terminals for each voltage level, you can calculate the power-handling capability of the connector, as shown in the following table.
| Maximum Power-Handling Capabilities of the 24-Pin Main Power Connector | ||||
|---|---|---|---|---|
| Volts | No. Pins | Using Std. Terminals (W) | Using HCS Terminals (W) | Using Plus HCS Terminals (W) |
| +3.3 V | 4 | 79.2 | 118.8 | 145.2 |
| +5 V | 5 | 150 | 225 | 275 |
| +12 V | 2 | 144 | 216 | 264 |
| Total watts: | 373.2 | 559.8 | 684.2 | |
| Standard terminals are rated six amps. HCS terminals are rated nine amps. Plus HCS terminals are rated 11 amps. All ratings assume Mini-Fit Jr. connectors with 12–24 circuits using 18-gauge wire under standard temperature conditions. | ||||
This means the total power-handling capacity of this connector is 373 watts using standard terminals or 560 watts using HCS terminals, which is substantially higher than the 251 watts available in the previous 20-pin connector. Combining the 24-pin main and the four-pin +12 V power connector results in up to 565 watts (standard terminals) or 824 watts (using HCS terminals) total power available to the motherboard and processor! This is more than enough to support the most power-hungry motherboards and processors on the market today.
Power for the processor comes from a device called the voltage regulator module (VRM), which is built into most modern motherboards. This device senses the CPU voltage requirements (usually via sense pins on the processor) and calibrates itself to provide the proper voltage to run the CPU. The design of a VRM enables it to run on either +5 V or +12 V for input power. Many have used +5 V over the years, but starting in 2000 most converted to +12 V because of the lower current requirements at that voltage. In addition, other devices might have already loaded the +5 V, whereas only drive motors typically used the +12 V prior to 2000. Whether the VRM on your board uses +5 V or +12 V depends on the particular motherboard or regulator design. Many modern voltage regulator ICs are designed to run on anything from a +4 V to a +36 V input, so it is up to the motherboard designer as to how they will be configured.
For example, I studied a system using a First International Computer (FIC) SD-11 motherboard, which used a Semtech SC1144ABCSW voltage regulator. This board design uses the +5 V to convert to the lower voltage the CPU needs. Most motherboards use voltage regulator circuits controlled by chips from Semtech (www.semtech.com) or Linear Technology (www.linear.com). You can visit their sites for more data on these chips.
That motherboard accepted an Athlon 1 GHz Cartridge version (Model 2), which according to AMD has a maximum power draw of 65 W and a nominal voltage requirement of 1.8 V, and 65 W at 1.8 V would equate to 36.1 A of current at that voltage (volts × amps = watts). If the voltage regulator used +5 V as a feed, 65 W would equate to only 13 A at +5 V. That would assume 100% efficiency in the regulator, which is impossible. Therefore, assuming 80% efficiency (which is typical), there would be about 16.25 A actual draw on the +5 V due to the regulator and processor combined.
When you consider that other circuits on the motherboard also use +5 V power—remember that ISA or PCI cards are drawing that power as well—you can see how easy it is to overload the +5 V lines from the supply to the motherboard.
Although most motherboard VRM designs up through the Pentium III and Athlon/Duron use +5 V-based regulators, most systems since then use +12 V-powered regulators. This is because the higher voltage significantly reduces the current draw. Using the same 65 W AMD Athlon 1 GHz CPU as an example, you would end up with the current draw at the various voltages shown below.
| Current Draw at Various Voltages | |||
|---|---|---|---|
| Watts | Volts | Amps | Amps at 80% Regulator Efficiency |
| 65 | 1.8 | 36.1 | - |
| 65 | 3.3 | 19.7 | 24.6 |
| 65 | 5.0 | 13.0 | 16.3 |
| 65 | 12.0 | 5.4 | 6.8 |
As you can see, using +12 V to power the chip results in only 5.4 A of draw, or 6.8 A assuming 80% efficiency on the part of the regulator.
So, modifying the motherboard VRM circuit to use the +12 V power feed would seem simple. But as you’ll recall from the preceding text, the ATX 2.03 specification has only a single +12 V lead in the main power connector. Even the short-lived auxiliary connector had no +12 V leads, so that was no help. Pulling up to 8 A more through a single 18-gauge wire supplying +12 V power to the motherboard is a recipe for a melted connector because the contacts in the main ATX connector are rated for only 6 A using standard terminals. Therefore, another solution was necessary.
Platform Compatibility Guide
The processor directly controls the amount of current drawn through the +12 V connector. Modern motherboards are designed to support a wide range of different processors; however the voltage regulator circuitry on a given motherboard may not have been designed to supply sufficient power to support all processors that might otherwise fit in the socket. To help eliminate the potential power problems that could result (including intermittent lockups or even damage such as damaged components or burned circuits), Intel created a power standard called the Platform Compatibility Guide (PCG). The PCG was marked on most Intel boxed (retail) processors and motherboards introduced from 2004 through 2009. It was designed for system builders to use as an easy way to know the power requirements of a processor and to ensure that the motherboard can meet those requirements.
The PCG is a two- or three-digit alphanumeric value (for example, 05A), where the first two digits represent the year the particular specification was introduced, and the optional third character stands for the market segment. PCG designations in which the third character is A apply to processors and motherboards that fall in the low-end market (requiring less power), whereas designations whose third character is B apply to processors and motherboards that fall in the high-end market (requiring more power). Motherboards that support high-end processors by default also support low-end processors, but not the other way around. For example, you can install a processor with a PCG specification of 05A in a motherboard with a PCG specification of 05B, but if you install a 05B processor in a motherboard rated 05A, power problems will result. In other words, you can always install a processor with lower power requirements in a higher-power-capable motherboard, but not the other way around.
Although the PCG figures were specifically intended to apply to processors and motherboards, they also can be used to specify minimum power supply requirements. The following table shows the PCG numbers and the power recommendations they prescribe. Intel stopped using the PCG numbers on processors and motherboards introduced after 2009.
| Intel Platform Compatibility Guide (PCG) +12 V Connector Power Recommendations | |||||
|---|---|---|---|---|---|
| PCG Number | Year Introduced | Market Segment | CPU Power Specification | Continuous +12 V Rating | Peak +12 V Rating |
| 04A | 2004 | Low-end | 84 W | 13 A | 16.5 A |
| 04B | 2004 | High-end | 115 W | 13 A | 16.5 A |
| 05A | 2005 | Low-end | 95 W | 13 A | 16.5 A |
| 05B | 2005 | High-end | 130 W | 16 A | 19 A |
| 06 | 2006 | All | 65 W | 8 A | 13 A |
| 08 | 2008 | High-end | 130 W | 16 A | 19 A |
| 09A | 2009 | Low-end | 65 W | 8 A | 13 A |
| 09B | 2009 | High-end | 95 W | 13 A | 16.5 A |
| The power supply should be able to supply peak current for at least 10 ms. Choosing a power supply with the required minimum output on the +12 V connector helps to ensure proper operation of the system. | |||||
Four-Pin +12 V CPU Power Connector
To augment the supply of +12 V power to the motherboard, Intel created a new ATX12V power supply specification. This added a third power connector, called the +12 V connector, specifically to supply additional +12 V power to the board. The four-pin +12 V power connector is specified for all power supplies conforming to the ATX12V form factor and consists of a Molex Mini-Fit Jr. connector housing with female terminals. For reference, the connector is Molex part number 39-01-2040, and the terminals are part number 5556. This is the same style of connector as the ATX Main power connector, except with fewer pins.
This connector has two +12 V power pins, each rated for 8 A total using standard terminals (or up to 11 A each using HCS terminals). This allows for up to 16 A or more of additional +12 V current to the motherboard, for a total of 22 A of +12 V when combined with the 20-pin main connector. The four-pin +12 V connector is shown in the image below.
+12 V four-pin CPU power connector, side and terminal end view.
The pinout of the +12 V power connector is shown below.
| +12 V Four-Pin CPU Power Connector Pinout (Wire End View) | |||||
|---|---|---|---|---|---|
| Pin | Signal | Color | Pin | Signal | Color |
| 3 | +12 V | Yellow | 1 | Gnd | Black |
| 4 | +12 V | Yellow | 2 | Gnd | Black |
Using standard terminals, each pin in the +12 V connector is rated to handle up to eight amps of current, 11 amps with HCS terminals, or up to 12 amps with Plus HCS terminals. Even though it uses the same design and the same terminals as the main power connector, the current rating per terminal is higher on this four-pin connector than on the 20-pin main because there are fewer terminals overall. By counting the number of terminals for each voltage level, you can calculate the power-handling capability of the connector.
| Maximum Power-Handling Capabilities of the Four-Pin +12 V Power Connector | ||||
|---|---|---|---|---|
| Volts | No. Pins | Using Std. Terminals (W) | Using HCS Terminals (W) | Using Plus HCS Terminals (W) |
| +12 V | 2 | 192 | 264 | 288 |
| Standard terminals are rated eight amps. HCS terminals are rated 11 amps. Plus HCS terminals are rated 12 amps. All ratings assume Mini-Fit Jr. connectors with 4–6 circuits using 18-gauge wire under standard temperature conditions. | ||||
This means the total power-handling capacity of this connector is 192 watts using standard terminals, which is available to and used only by the processor. Drawing more power than this maximum rating through the connector causes it to overheat, unless the HCS or Plus HCS terminals are used.
Combining the 20-pin main plus the four-pin +12 V power connector results in a maximum power-delivery capability to the motherboard of 443 watts (using standard terminals). The important thing to note is that adding the +12 V connector provides the capability to support power supplies of up to 500 watts or more without overloading and melting the connectors.
Peripheral to Four-Pin +12 V CPU Power Adapters
If you are installing a motherboard in a system that currently doesn’t have the +12 V connection for the CPU voltage regulator, an easy solution may be available; however, there are some caveats.
Power adapters are available that convert one of the extra peripheral power connectors found in most systems to a +12 V four-pin type. The drawback to this is that there are two +12 V terminals in a +12 V four-pin connector, and only one +12 V terminal in a peripheral connector. If the adapter uses only a single peripheral connector to power both +12 V pins of the +12 V connector, a serious power mismatch can result. Because the terminals in the peripheral connector are only rated for 11 A, and the two terminals in the +12 V connector are also rated for up to 11 A each, drawing more than 11 A total can result in melted connectors at the peripheral connector end. This is below the peak power requirements as recommended by the Power Supply Design Guide for Desktop Platform Form Factors (www.formfactors.org), meaning these adapters do not conform to the latest standards.
I did some calculations: assuming a motherboard VRM (voltage regulator module) efficiency of 80%, a CPU power draw of 105 W would just about equal 11 A, the absolute limit of the peripheral connector terminal. Because most CPUs can intermittently draw more than their nominal rating, I would hesitate to use one of these adapters on any processor rated at more than 75 watts. An example of a peripheral to +12 V adapter is shown below.
Peripheral to +12 V power adapter.
Eight-Pin +12 V CPU Power Connector
High-end motherboards often use multiple voltage regulators to supply power to the processor. To distribute the load among the additional voltage regulators, these boards may use two four-pin +12 V connectors; however, they are physically combined into a single eight-pin connector shell (see the figure below). This type of CPU power connector was first defined in the EPS12V power supply specification version 1.6 released in the year 2000. Although this specification is intended for file servers, the increased power requirements of some high-power PC processors has caused this connector to appear on desktop PC motherboards supporting these processors.
Eight-pin +12 V CPU power connector, side and terminal end view.
The pinout of the eight-pin +12 V CPU power connector is shown below.
| Eight-Pin +12 V CPU Power Connector Pinout (Wire End View) | |||||
|---|---|---|---|---|---|
| Color | Signal | Pin | Pin | Signal | Color |
| Yellow | +12 V | 5 | 1 | GND | Black |
| Yellow | +12 V | 6 | 2 | GND | Black |
| Yellow | +12 V | 7 | 3 | GND | Black |
| Yellow | +12 V | 8 | 4 | GND | Black |
Some motherboards that utilize an eight-pin +12 V CPU power connector must have signals connected to all eight pins for the voltage regulators to function properly, while most will work properly even if a four-pin PSU connector is attached. In the latter case you will often see that the eight-pin connector has a cap installed over four of the pins, meaning that a four-pin connector can be installed in the exposed portion. Consult the specific motherboard documentation to see if you can attach a single four-pin +12 V power connector (offset to one side or the other) when using lower power processors. If you are using a processor that draws more power than a four-pin connector can supply, then you should ensure you are using a power supply with an eight-pin connector to match the motherboard.
Four-Pin to Eight-Pin +12 V CPU Power Adapters
If your motherboard requires all eight-pins be connected, and you are using a lower power draw processor and a power supply that does not have a matching eight-pin +12 V connector, you can use an adapter to convert an existing four-pin connector to an eight-pin connector. An example of this is shown below.
Four-pin +12 V to eight-pin +12 V power adapter.
Adapters are also available that go the other way—that is, they convert an eight-pin CPU power connector to a four-pin version. However, these are not always required because one can plug an eight-pin connector from a power supply into a four-pin connector on a motherboard by simply offsetting the connector to one side. The only time the adapter would be needed is if there is a component on the board that is physically interfering with the portion of the connector that is offset.
Backward and Forward Compatibility
If you have reached this point, I’m sure you have some questions. For example, what happens if you purchase a new power supply that has a 24-pin main power connector but your motherboard has only a 20-pin main power socket? Likewise, what if you purchase a new motherboard that has a 24-pin main power socket but your power supply has only a 20-pin main power connector? The answers to these questions are surprising to say the least.
There are adapters that can convert a 24-pin connector to a 20-pin type, and the other way around, but surprisingly these adapters are not usually necessary. The truth is that compatibility has been engineered into the connectors, power supplies, and motherboards from the start.
If you look at the 24-pin main power connector diagram and compare it to the previous 20-pin design, you’ll see that the extra four pins are all placed on one end of the connector and all the other pins are defined the same as they were previously. The design of these connectors is such that it allows an interesting bit of backward compatibility. The result is that you can plug a 24-pin main connector directly into a motherboard that has a 20-pin socket (and vice versa) without using an adapter! The trick is to position the connector such that the four extra pins are empty. Depending on the latch design, the latch on the side might not engage, but the connector will otherwise plug in and operate properly.
The next figure shows how you would connect a new power supply with a 24-pin connector to a motherboard that has only a 20-pin socket. The terminals on the 24-pin connector that are highlighted in gray would plug directly into the 20-pin socket, whereas the white highlighted terminals would remain free and unconnected.
Connecting a 24-pin main power connector to a 20-pin motherboard socket.
Logically, this works because the first 20 pins of the 24-pin connector that match the 20-pin motherboard socket contain the correct signals in the correct positions. The only problem that might arise is if there are some components on the motherboard directly adjacent to the end of the 20-pin power socket that physically interfere with the four extra unused terminals on the 24-pin connector.
What about the opposite condition, in which you have a new motherboard with a 24-pin socket but your power supply has only a 20-pin connector? In this case, four terminals at the end of the motherboard socket are not connected. This also works because the 20-pin portion of both the connector and socket are the same. But this example raises another question: Will the motherboard operate properly without the extra power pins? Because the extra signals are merely additional voltage pins that are already present in the remaining part of the connector, the answer should be yes, but if the motherboard draws a lot of power, it can overload the remaining pins. After all, preventing overloads is the reason the extra pins were added in the first place.
Some motherboards sold from 2004 through 2010 that use a 24-pin main power connector also have an additional peripheral (that is, disk drive) power connector onboard designed to provide the extra power that would be missing if you connected a 20-pin main power connector from your power supply. The documentation for these motherboards refers to this as an alternate or auxiliary power connector. Some boards included both standard and SATA style drive connectors to supply this extra power.
If you plug a 24-pin main power connector into the 24-pin socket on the motherboard, the alternate or auxiliary power connection is probably unnecessary. However, if you plug a 20-pin main power connector into the 24-pin main power socket on the motherboard, and that board has one of these alternate or auxiliary power connectors on board, then you should probably use it. In that case simply select a spare peripheral (disk drive) power connector from the power supply and plug it into the alternate or auxiliary power connector. Most power supplies have several extra peripheral or SATA power connectors for supporting additional drives. Using a 20-pin main and the alternate or auxiliary power connector satisfies the power requirements for the motherboard and any PCI Express x16 video cards drawing up to 75 watts.
As a side note, you should be careful when plugging in the mismatched connectors so that they are offset properly. The main, +12 V, and PCI Express graphics connectors are Molex Mini-Fit Jr.–type connectors that are keyed by virtue of a series of differently shaped plastic protrusions used around the terminals, which fit similarly shaped holes in the mating connectors. This keying is designed to prevent backward or improper off-center insertion, but I have found two problems with the keying that should be noted. One is that some alternate low-quality connector brands are built to looser tolerances than the original high-quality Molex versions, and the sloppier fit of the low-quality versions sometimes allows improper insertion. The other problem is that, with sufficient force, the keying on even the high-quality versions can be overcome. Because plugging a 20-pin connector into a 24-pin socket—or a 24-pin connector into a 20-pin socket—is designed to work even though they don’t fully match up, you need to make sure you have the offsets correct or you risk damaging the board when you power it up.
Dell Proprietary (Nonstandard) ATX Design
Most of these systems are no longer in use, but if you upgrade or repair any Dell desktop systems made between 1996 and 2000, you should be aware that they used a non-standard design, and upgrading either the motherboard or power supply can result in the destruction of the motherboard, power supply, or both!
When Dell converted to the ATX motherboard form factor in mid-1996, it unfortunately defected from the newly released standard and began using specially modified Intel-supplied ATX motherboards with custom-wired power connectors. Inevitably, it also had custom power supplies made that duplicated the nonstandard pinout of the motherboard power connectors.
An even bigger crime than simply using nonstandard power connectors is that only the pinout is nonstandard; the connectors look like and are keyed the same as is dictated by true ATX. Therefore, nothing prevents you from plugging one of these Dell nonstandard power supplies into a new industry-standard ATX motherboard you installed in the Dell case as an upgrade, or even plugging a new upgraded industry-standard ATX power supply into the existing Dell motherboard. But mixing either a new ATX board with the non-standard Dell supply or a new ATX supply with the non-standard Dell motherboard is a recipe for silicon toast.
The following tables show the nonstandard Dell main and auxiliary power supply connections. This nonstandard wiring is used on some of Dell’s early pseudo-ATX systems.
| Dell Proprietary (Non-standard) 20-Pin ATX Main Power Connector Pinout (Wire End View) | |||||
|---|---|---|---|---|---|
| Color | Signal | Pin | Pin | Signal | Color |
| Grey | PS_On | 11 | 1 | +5 V | Red |
| Black | GND | 12 | 2 | GND | Black |
| Black | GND | 13 | 3 | +5 V | Red |
| Black | GND | 14 | 4 | GND | Black |
| White | -5 V | 15 | 5 | Power_Good | Orange |
| Red | +5 V | 16 | 6 | +5 VSB (Standby) | Purple |
| Red | +5 V | 17 | 7 | +12 V | Yellow |
| Red | +5 V | 18 | 8 | –12 V | Blue |
| KEY (blank) | - | 19 | 9 | GND | Black |
| Red | +5 V | 20 | 10 | GND | Black |
| Dell Proprietary (Non-standard) ATX Auxiliary Power Connector Pinout | |||||
|---|---|---|---|---|---|
| Pin | Signal | Color | Pin | Signal | Color |
| 1 | Gnd | Black | 4 | +3.3 V | Blue/White |
| 2 | Gnd | Black | 5 | +3.3 V | Blue/White |
| 3 | Gnd | Black | 6 | +3.3 V | Blue/White |
If you study the Dell main and auxiliary connector pinouts I’ve listed here and compare them to the industry-standard ATX pinouts listed earlier, you’ll see that not only are the voltage and signal positions changed, but the number of terminals carrying specific voltages and grounds has changed as well. You could possibly modify a Dell supply to work with a standard ATX board or modify a standard ATX supply to work with a Dell board, but you’d have to do some cutting and splicing in addition to swapping some terminals around. Usually, it isn’t worth the time and effort.
Systems known to have this nonstandard connector wiring include the following Dell models:
- Dimension 2100, 4100, B1000R, L Series, V350, V400, XPS B Series, XPS Dxxx, XPS Mxxx, XPS P133c MT, XPS Pro 180n, XPS Rxxx, XPS Txxx
- OptiPlex G1, GX1, GX110, GX115, GX300, GXa, GXi
- Power Edge 2100, 2200
- Precision Workstation 210, 400
If you do decide to upgrade the motherboard in any of these nonstandard Dell systems, just be sure you replace both the motherboard and power supply with industry-standard ATX components at the same time. That way nothing gets fried, and you’ll be back to having a true industry-standard ATX system. If you want to replace just the Dell motherboard, you’re out of luck unless you get your replacement board from Dell. On the other hand, if you wantto replace just the power supply, you have several alternatives. Both PC Power and Cooling (www.pcpower.com) and ATXPowerSupplies.com sell replacement power supplies with the modified Dell wiring.
Fortunately, starting in 2000, Dell switched to using industry-standard ATX power connections in its systems.
Besides the motherboard power connectors, all power supplies include a variety of additional power connectors, mainly used for internally mounted drives but usable by other components, such as graphics cards. Most of these connectors are industry-standard types required by the various power supply form factor specifications. This section discusses the various types of additional device power connectors you’re likely to find in your PC.
Peripheral Power Connectors
Perhaps the most common additional power connector seen on virtually all power supplies is the peripheral power connector, also called the disk drive power connector. What we know as the peripheral power connector was originally created by AMP as part of the commercial MATE-N-LOK series, although since it is also manufactured and sold by Molex, it is often incorrectly called a Molex connector.
To determine the location of pin one, carefully look at the connector. It is usually embossed in the plastic connector body; however, it is often tiny and difficult to read. Fortunately, these connectors are keyed and therefore difficult to insert incorrectly. The following image shows the keying with respect to pin numbers on the larger drive power connector.
This is the one connector type that has been on all PC power supplies from the original IBM PC to the latest systems built today. It is most commonly known as a disk drive connector, but it is also used in some systems to provide additional power to the motherboard, video card, cooling fans, or just about anything that can use +5 V or +12 V power.
A peripheral power connector is a four-pin connector with round terminals spaced 0.200 inches apart, rated to carry up to 11 amps per pin. Because there is one +12 V pin and one +5 V pin (the other two are grounds), the maximum power-handling capability of the peripheral connector is 187 watts. The plug is 0.830 inches wide, making it suitable for larger drives and devices.
The next table shows the peripheral power connector pinout and wire colors.
| Peripheral Power Connector Pinout (Large Drive Power Connector) | |||||
|---|---|---|---|---|---|
| Pin | Signal | Color | Pin | Signal | Color |
| 1 | +12 V | Yellow | 3 | Gnd | Black |
| 2 | Gnd | Black | 4 | +5 V | Red |
Floppy Power Connectors
When 3.5-inch floppy drives were first being integrated into PCs in the mid-1980s, it was clear that a smaller power connector was necessary. The answer came in what is now known as the floppy power connector, which was created by AMP as part of the economy interconnection (EI) series. These connectors are now used on all types of smaller drives and devices and feature the same +12 V, +5 V, and ground pins as the larger peripheral power connector. The floppy power connector has four pins spaced 2.5 mm (0.098 inches) apart, which makes the entire connector about half the overall width as the larger peripheral power connector. The pins are rated for only two amps each, giving a maximum power-handling capability of 34 watts.
This table shows the pinouts for the smaller floppy drive power connector.
| Pinout for the 3.5-Inch Floppy Power Connector (Small Drive Power Connector) | |||||
|---|---|---|---|---|---|
| Pin | Signal | Color | Pin | Signal | Color |
| 1 | +5 V | Red | 3 | Gnd | Black |
| 2 | Gnd | Black | 4 | +12 V | Yellow |
The peripheral and floppy power connectors are universal with regard to pin configuration and even wire color. Here we see the peripheral and floppy power connectors.
Peripheral and floppy power connectors.
The pin numbering and voltage designations are reversed on the floppy power connector. Be careful if you are making or using an adapter cable from one type of connector to another. Reversing the red and yellow wires fries the drive or device you plug into.
Early power supplies featured only two peripheral power connectors, whereas later power supplies featured four or more of the larger peripheral (disk drive) connectors and one or two of the smaller floppy power connectors. Depending on their power ratings and intended uses, some supplies have as many as eight or more peripheral or floppy power connectors.
If you are adding drives and need additional power connectors, Y splitter cables as well as peripheral-to-floppy power connector adapters are available from many electronics supply houses (including RadioShack). These cables can adapt a single power connector to service two drives or enable you to convert the large peripheral power connector to a smaller floppy drive power connector. If you are using several Y-adapters, be sure that your total power supply output is capable of supplying the additional power and that you don’t draw more power than a single connector can handle.
Serial ATA Power Connectors
If you want to add Serial ATA drives to an existing system, you will need a newer power supply that includes a Serial ATA (SATA) power connector. The SATA power connector is a special 15-pin connector fed by only five wires, meaning three pins are connected directly to each wire. The overall width is about the same as the peripheral power connector, but the SATA connector is significantly thinner. All the most recent power supply form factor specifications include SATA power connectors as mandatory for systems supporting SATA drives.
In the SATA power connector, each wire is connected to three terminal pins, and the wire numbering is not in sync with the terminal numbering, which can be confusing.
If your power supply does not feature SATA power connectors, you can use an adapter to convert a standard peripheral power connector to a SATA power connector. However, such adapters do not include the +3.3 V power. Fortunately, though, this is not a problem for most applications because most drives do not require +3.3 V and use only +12 V and +5 V instead.
Although the ATX12V 2.x specification includes a new 24-pin main power connector with more power for devices such as video cards, the design was intended to power a video card drawing up to 75 watts maximum through the PCIe x16 slot. That is adequate for most video cards, but high-end gaming or workstation cards usually need quite a bit more power. To accommodate graphics cards needing more than 75 watts, the PCI-SIG (Special Interest Group) introduced two standards for supplying additional power to a video card via additional graphics power connectors:
- PCI Express x16 Graphics 150 W-ATX Specification—Published in October 2004, this standard defines a six-pin (2x3) auxiliary power connector capable of delivering an additional 75 W to a graphics card directly from the power supply, for a total of 150 W to the card.
- PCI Express 225 W/300 W High Power Card Electromechanical Specification—Published in March 2008, this standard defines an eight-pin (2x4) auxiliary power connector capable of supplying an additional 150 W of power, for a total of either 225 watts (75+150) or 300 watts (75+150+75) of available power.
Cards requiring even more power can use multiple connectors.
| Graphics Card Auxiliary Power Connector Configurations | |
|---|---|
| Maximum Power Draw | Auxiliary Power Connector Configuration |
| 75 Watts | None |
| 150 Watts | One six-pin connector |
| 225 Watts | Two six-pin connectors* |
| 300 Watts | One eight-pin connector + one six-pin connector |
| 375 Watts | Two eight-pin connectors |
| 450 Watts | Two eight-pin connectors + one six-pin connector |
| *May optionally use one eight-pin connector instead. | |
The PCI Express auxiliary power connectors are six-pin (2 × 3) or eight-pin (2 × 4) Molex Mini-Fit Jr. connector housings with female terminals that provide power directly to a video card. For reference, the connector is similar to Molex part number 39-01-2060 (six-pin) or 39-01-2080 (eight-pin), but with different keying to prevent interchanging them with the +12 V motherboard power connectors. A diagram of the six-pin connector is shown below, as is the pinout below that. Note the Sense signal at pin five, which allows a graphics card to detect whether a six-pin power connector has been attached. Without the proper power connections being detected, the card may shut down or operate in a reduced functionality mode. Also note that pin two is technically listed as “no connection” in the official specification, but most power supplies do seem to include +12 V there.
PCI Express six-pin (2x3) auxiliary 75 W power supply connector.
| PCI Express Six-Pin (2x3) Auxiliary 75 W Power Connector Pinout (Graphics Card Socket) | |||||
|---|---|---|---|---|---|
| Color | Signal | Pin | Pin | Signal | Color |
| Black | GND | 4 | 1 | +12 V | Yellow |
| Black | Sense | 5 | 2 | N/C | - |
| Black | GND | 6 | 3 | +12 V | Yellow |
| N/C = No connection; however, many PSUs include a redundant +12V (yellow) wire at pin 2. | |||||
A diagram of the eight-pin connector is shown below, as is its pinout. Note the additional +12 V power at pin two and the two Sense signals at pins four and six, which allow a card to detect whether an eight-pin connector, a six-pin connector, or no connector is attached.
PCI Express eight-pin (2x4) auxiliary 150 W power supply connector.
| PCI Express Eight-Pin (2x4) Auxiliary 150 W Power Connector Pinout (Graphics Card Socket) | |||||
|---|---|---|---|---|---|
| Color | Signal | Pin | Pin | Signal | Color |
| Black | GND | 5 | 1 | +12 V | Yellow |
| Black | Sense0 | 6 | 2 | 12 V | Yellow |
| Black | GND | 7 | 3 | 12 V | Yellow |
| Black | GND | 8 | 4 | Sense1 | Black |
Because of both the physical design as well as the use of the sense signals, the six-pin power supply connector plug is backward compatible with the eight-pin graphics card socket. This means that if your graphics card has an eight-pin socket but your power supply has only six-pin connectors available, you can plug the six-pin connector into the eight-pin socket using an offset arrangement, as shown below. The connectors are keyed such that they should only plug in the correct way, but be careful because they can be forced together in an incorrect fashion, which can potentially damage the card.
Plugging a six-pin power supply connector into an eight-pin graphics card power socket.
The sense signals are used so that the graphics card can detect what types of connector(s) are attached, and therefore how much total power is available. For example, if a graphics card needs a full 300 W and has both an eight-pin and six-pin connector on board, if you were to attach two six-pin power supply connectors, the card would “sense” that it had only 225 W available and, depending on the design, it could either shut down or operate in a reduced functionality mode.
Due to special keying on the eight-pin connector, it cannot be plugged into a six-pin socket. Because of this, many power supply manufacturers include eight-pin connectors made in a “6+2” arrangement, where the portion containing the two extra pins can be disconnected, leaving a six-pin connector that will, of course, work in a six-pin socket.
Caution: The eight-pin PCI Express Auxiliary Power Connector and the eight-pin EPS12V CPU Power Connector use similar Molex Mini-Fit Jr. connector housings. Although they are keyed differently, the keying can be overcome by sufficient force such that you can plug an EPS12V power connector into a graphics card, or a PCI Express power connector into a motherboard. Either of these scenarios results in +12 V being directly shorted to ground, potentially destroying the motherboard, graphics card, or power supply.
The six-pin connector uses two +12 V wires to carry up to 75 W, whereas the eight-pin connector uses three +12 V wires to carry up to 150 W. Although these figures are what the specifications allow, the wires and terminals of each connector are technically capable of handling much more power. Each pin in the PCI Express auxiliary power connectors is rated to handle up to 8 amps of current using standard terminals—more if using HCS or Plus HCS terminals. By counting the number of terminals, you can calculate the power-handling capability of the connector.
| PCI Express Graphics Power Connector Maximum Power-Handling Capabilities | ||||
|---|---|---|---|---|
| Connector | No. +12V Pins | Using Std. Terminals (W) | Using HCS Terminals (W) | Using Plus HCS Terminals (W) |
| Six-pin | 2 | 192 | 264 | 288 |
| Eight-pin | 3 | 288 | 396 | 432 |
| Only two +12 V pins are used in the six-pin connector, even though most power supplies include three. Standard terminals are rated eight amps. HCS terminals are rated 11 amps. Plus HCS terminals are rated 12 amps. All ratings assume Mini-Fit Jr. connectors using 18-gauge wire under standard temperature conditions. | ||||
Even though the specification allows for a delivery capability of 75 W (six-pin connector) or 150 W (eight-pin connector), the total power-handling capacity of these connectors is at least 192 and 288 W, respectively, using standard terminals, and even more using the HCS or Plus HCS terminals.
These two auxiliary power connectors are sometimes called PCI Express Graphics (PEG), Scalable Link Interface (SLI), or CrossFire power connectors because they are used by high-end PCI Express boards with SLI or CrossFire capabilities. SLI and CrossFire are Nvidia and AMD’s methods of using two video cards in unison, with each one drawing half of the screen for up to twice the performance. Each card can draw hundreds of watts, with many of the high-end cards using two or three auxiliary power connectors. This means that most power supplies that are rated as SLI- or CrossFire-ready include at least two or more of the six/eight-pin PCI Express graphics power connectors. Using two video cards drawing 300 watts each means that even if you have a 750-watt power supply, you will have only 150 watts of power left to run the motherboard, processor, and all the disk drives. With high-powered processors drawing 130 watts or more, this may not be enough. For this reason, systems using two or more high-end video cards require the highest-output supplies available, and some of the current ones are capable of putting out up to 1000 watts (1 kilowatt) or more.
Note: Nvidia has trademarked the term SLI as meaning scalable link interface, but its primary competitor, AMD, uses similar dual-graphics card technology called CrossFire to achieve comparable performance improvements.
If your existing power supply doesn’t feature PCI Express auxiliary power connectors, you can use Y-adapters to convert multiple peripheral power connectors (normally used for drives) into a single six-pin or eight-pin PCI Express auxiliary power connector. Note, however, that these adapters will not help if the power supply is not capable of supplying the total power actually required.
Power supplies have several specifications that define their input and output capabilities as well as their operational characteristics. This section defines and examines most of the common specifications related to power supplies.
Power Supply Loading
PC power supplies are of a switching rather than a linear design. The switching type of design uses a high-speed oscillator circuit to convert the higher wall-socket AC voltage to the much lower DC voltage used to power the PC and PC components. Switching-type power supplies are noted for being efficient in size, weight, and energy compared to the linear design, which uses a large internal transformer to generate various outputs. This type of transformer-based design is inefficient in at least three ways:
- The output voltage of the transformer linearly follows the input voltage (hence the name linear), so any fluctuations in the AC power going into the system can cause problems with the output.
- The high current-level (power) requirements of a PC system require the use of heavy wiring in the transformer.
- The 60 Hz frequency of the AC power supplied from your building is difficult to filter out inside the power supply, requiring large and expensive filter capacitors and rectifiers.
The switching supply, on the other hand, uses a switching circuit that chops up the incoming power at a relatively high frequency. This enables the use of high-frequency transformers that are much smaller and lighter. Also, the higher frequency is much easier and cheaper to filter out at the output, and the input voltage can vary widely. Input ranging from 90 V to 135 V still produces the proper output levels, and many switching supplies can automatically adjust to 240 V input.
One characteristic of all switching-type power supplies is that they do not run without a load. Therefore, you must have something such as a motherboard and hard drive plugged in and drawing power for the supply to work. If you simply have the power supply on a bench with nothing plugged into it, either the supply burns up or its protection circuitry shuts it down. Most power supplies are protected from no-load operation and shut down automatically. Some of the cheapest supplies, however, lack the protection circuit and relay and can be destroyed after a few seconds of no-load operation. A few power supplies have their own built-in load resistors, so they can run even though there isn’t a normal load (such as a motherboard or hard disk) plugged in.
Some power supplies have minimum load requirements for both the +5 V and +12 V sides. According to IBM specifications for the 192-watt power supply used in the original AT, a minimum load of 7.0 amps was required at +5 V and a minimum of 2.5 amps was required at +12 V for the supply to work properly. As long as a motherboard was plugged into the power supply, the motherboard would draw sufficient +5 V at all times to keep those circuits in the supply happy. However, +12 V is typically used only by motors (and not motherboards), and the floppy or CD/DVD drive motors are usually off. Because floppy or optical (CD/DVD) drives don’t present +12 V load unless they are spinning, systems without a hard disk drive could have problems because there wouldn’t be enough load on the +12 V circuit in the supply.
To alleviate problems, when IBM used to ship the original AT systems without a hard disk, it plugged the hard disk drive power cable into a large 5-ohm, 50-watt sandbar resistor that was mounted in a small metal cage assembly where the drive would have been. The AT case had screw holes on top of where the hard disk would go, specifically designed to mount this resistor cage.
Note: Several computer stores I knew of in the mid-1980s ordered the diskless AT and installed their own 20 MB or 30 MB drives, which they could get more cheaply from sources other than IBM. They were throwing away the load resistors by the hundreds! I managed to grab a couple at the time, which is how I know the type of resistor they used.
This resistor would be connected between pin one (+12 V) and pin two (Ground) on the hard disk power connector. This placed a 2.4-amp load on the supply’s +12 V output, drawing 28.8 watts of power (it would get hot!) and thus enabling the supply to operate normally. Note that the cooling fan in most power supplies draws approximately 0.1–0.25 amps, bringing the total load to 2.5 amps or more. If the load resistor were missing, the system would intermittently fail to start.
Most of the power supplies in use today do not require as much of a load as the original IBM AT power supply. In most cases, a minimum load of 0–0.3 amps at +3.3 V, 2.0–4.0 amps at +5 V, and 0.5–1.0 amps at +12 V is considered acceptable. Most motherboards easily draw the minimum +5 V current by themselves. The standard power supply cooling fan draws only 0.1–0.25 amps, so the +12 V minimum load might still be a problem for a diskless workstation. Generally, the higher the rating on the supply, the more minimum load that is required. However, exceptions do exist, so this is a specification you should check when evaluating power supplies.
Some switching power supplies have built-in load resistors and can run in a no-load situation. Most power supplies don’t have internal load resistors but might require only a small load on the +5 V line to operate properly. Some supplies, however, might require +3.3 V, +5 V, and +12 V loads to work; the only way to know is by checking the documentation for the particular supply in question.
No matter what, if you want to properly and accurately bench test a power supply, be sure you place a load on at least one (or preferably all) of the positive voltage outputs. This is one reason it is best to test a supply while it is installed in the system instead of testing it separately on the bench. For impromptu bench testing, you can use a spare motherboard and one or more hard disk drives to load the outputs.
Power Supply Ratings
A system manufacturer should be able to provide you the technical specifications of the power supplies it uses in its systems. This type of information can be found in the system’s technical reference manual, as well as on stickers attached directly to the power supply. Power supply manufacturers can also supply this data, which is preferable if you can identify the manufacturer and contact it directly or via the Web.
The input specifications are listed as voltages, and the output specifications are listed as amps at several voltage levels. You can convert amperage to wattage by using the following simple formula:
watts = volts × amps
For example, if a component is listed as drawing 8 amps of +12 V current, that equals 96 watts of power according to the formula.
By multiplying the voltage by the amperage available at each main output and then adding the results, you can calculate the total capable output wattage of the supply. Note that only positive voltage outputs are normally used in calculating outputs; the negative outputs, Standby, Power_Good, and other signals that are not used to power components are usually exempt.
The following table shows the ratings and calculations for various single +12 V rail ATX12V/EPS12V power supplies from Corsair (www.corsair.com).
| Typical ATX12V/EPS12V Power Supply Output Ratings | |||||||
|---|---|---|---|---|---|---|---|
| Model | VX450W | VX550W | HX650W | HX750W | HX850W | TX950W | AX1200 |
| +12 V (A) | 33 | 41 | 52 | 62 | 70 | 78 | 100 |
| –12 V (A) | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
| +5 VSB (A) | 2.5 | 3 | 3 | 3 | 3 | 3 | 3.5 |
| +5 V (A) | 20 | 28 | 30 | 25 | 25 | 25 | 30 |
| +3.3 V (A) | 20 | 30 | 24 | 25 | 25 | 25 | 30 |
| Max +5 V/+3.3 V (W) | 130 | 140 | 170 | 150 | 150 | 150 | 180 |
| Rated Max. (W) | 450 | 550 | 650 | 750 | 850 | 95- | 1200 |
| Calculated Max. (W) | 548 | 657 | 819 | 919 | 1015 | 1111 | 1407 |
Virtually all power supplies place limits on the maximum combined draw for the +3.3 V and +5 V.
The calculated maximum output assumes the maximum draw from all outputs simultaneously and is generally not sustainable. For this reason, the (sustainable) rated maximum output is normally much less.
Although store-bought PCs often come with lower-rated power supplies of 350 watts or less, higher output units are often recommended for fully optioned desktops or tower systems. Unfortunately, the ratings on cheap or poorly made power supplies cannot always be trusted. For example, I’ve seen 650 W-rated units that had less actual power output than honestly rated 200 W units. Another issue is that few companies actually make power supplies. Most of the units you see for sale are made under contract by a few manufacturers and sold under a variety of brands, makes, and models. Because few people have the time or equipment to actually test or verify output, it is better to stick to brands that are known for selling quality units.
Most power supplies are considered to be universal, or worldwide. That is, they also can run on the 240 V, 50-cycle current used in Europe and many other parts of the world. Many power supplies that can switch from 120 V to 240 V input do so automatically, but a few require you to set a switch on the back of the power supply to indicate which type of power you will access.
Note: In North America, power companies are required to supply split-phase 240 V (plus or minus 5%) AC, which equals two 120 V legs. Resistive voltage drops in the building wiring can cause the 240 V to drop to 220 V or the 120 V to drop to 110 V by the time the power reaches an outlet at the end of a long circuit run. For this reason, the input voltage for an AC-powered device might be listed as anything between 220 V and 240 V, or 110 V and 12 0V. I use the 240/120 V numbers throughout this chapter because those are the intended standard figures.
Caution: If your supply does not switch input voltages automatically, make sure the voltage setting is correct. If you plug the power supply into a 120 V outlet while it’s set in the 240 V setting, no damage will result, but the supply won’t operate properly until you correct the setting. On the other hand, if you plug into a 240 V outlet and have the switch set for 120 V, you can cause damage.
In addition to power output, many other specifications and features go into making a high-quality power supply. I have had many systems over the years. My experience has been that if a brownout occurs in a room with several systems running, the systems with higher-quality power supplies and higher output ratings are far more likely to make it through the power disturbances unscathed, whereas others choke.
High-quality power supplies also help protect your systems. A high-quality power supply from a vendor such as PC Power and Cooling will not be damaged if any of the following conditions occur:
- A 100% power outage of any duration
- A brownout of any kind
- A spike of up to 2500V applied directly to the AC input (for example, a lightning strike or a lightning simulation test)
Decent power supplies have an extremely low current leakage to ground of less than 500 microamps. This safety feature is important if your outlet has a missing or an improperly wired ground line.
As you can see, these specifications are fairly tough and are certainly representative of a high-quality power supply. Make sure that your supply can meet these specifications.
You can also use many other criteria to evaluate a power supply. The power supply is a component many users ignore when shopping for a PC, so it is one that some system vendors choose to skimp on. After all, a dealer is far more likely to be able to increase the price of a computer by spending money on additional memory or a larger hard drive than by installing a better power supply.
When buying a computer (or a replacement power supply), learn as much as possible about the power supply. Many consumers are intimidated by the vocabulary and statistics found in a typical power supply specification. Here are some of the most common parameters found on power supply specification sheets, along with their meanings:
- Mean Time Between Failures (MTBF) or Mean Time To Failure (MTTF)—The (calculated) average interval, in hours, that the power supply is expected to operate before failing. Power supplies typically have MTBF ratings (such as 100 000 hours or more) that are clearly not the result of real-time empirical testing. In fact, manufacturers use published standards to calculate the results based on the failure rates of the power supply’s individual components. MTBF figures for power supplies often include the load to which the power supply was subjected (in the form of a percentage) and the temperature of the environment in which the tests were performed.
- Input Range (or Operating Range)—The range of voltages that the power supply is prepared to accept from the AC power source. For 120 V AC power, an input range of 90 V–135 V is common; for 240 V power, a 180 V–270 V range is typical.
- Peak Inrush Current—The greatest amount of current drawn by the power supply at a given moment immediately after it is turned on, expressed in terms of amps at a particular voltage. The lower the current, the less thermal shock the system experiences.
- Hold-Up Time—The amount of time (in milliseconds) that a power supply can maintain output within the specified voltage ranges after a loss of input power. This enables your PC to continue running without resetting or rebooting if a brief interruption in AC power occurs. Values of 15–30 milliseconds are common for today’s power supplies, and the higher (longer), the better. The Power Supply Design Guide for Desktop Platform Form Factors specification calls for a minimum of 16 ms hold-up time. The hold-up time is also greatly affected by the load on the power supply. The hold-up specification is normally listed as the minimum time measured under the maximum load. As the load is reduced, hold-up times should increase proportionately. For example, if a 1000 W PSU has a 20 ms hold-up time specification (measured under a 1000 W load), then under a 500 W (half) load I’d expect that to double, and under a 250 W load I’d expect it to double again. This is in fact one of the reasons I’ve always been a proponent of specifying higher output PSUs than are strictly necessary when building systems.
- Transient Response—The amount of time (in microseconds) a power supply takes to bring its output back to the specified voltage ranges after a steep change in the output current. In other words, the amount of time it takes for the output power levels to stabilize after a device in the system starts or stops drawing power. Power supplies sample the current being used by the computer at regular intervals. When a device stops drawing power during one of these intervals (such as when a floppy drive stops spinning), the power supply might supply too high a voltage to the output for a brief time. This excess voltage is called overshoot, and the transient response is the time that it takes for the voltage to return to the specified level. This is seen as a spike in voltage by the system and can cause glitches and lockups. Once a major problem that came with switching power supplies, overshoot has been greatly reduced in recent years. Transient response values are sometimes expressed in time intervals, and at other times they are expressed in terms of a particular output change, such as “power output levels stay within regulation during output changes of up to 20%.”
- Overvoltage Protection—Defines the trip points for each output at which the power supply shuts down or squelches that output. Values can be expressed as a percentage (for example, 120% for +3.3 and +5 V) or as raw voltages (for example, +4.6 V for the +3.3 V output and +7.0 V for the +5 V output).
- Maximum Load Current—The largest amount of current (in amps) that safely can be delivered through a particular output. Values are expressed as individual amperages for each output voltage. With these figures, you can calculate not only the total amount of power the power supply can supply, but also how many devices using those various voltages it can support.
- Minimum Load Current—The smallest amount of current (in amps) that must be drawn from a particular output for that output to function. If the current drawn from an output falls below the minimum, the power supply could be damaged or automatically shut down.
- Load Regulation (or Voltage Load Regulation)—When the current drawn from a particular output increases or decreases, the voltage changes slightly as well—usually increasing as the current rises. Load regulation is the change in the voltage for a particular output as it transitions from its minimum load to its maximum load (or vice versa). Values, expressed in terms of a +/– percentage, typically range from +/–1% to +/–5% for the +3.3 V, +5 V, and +12 V outputs.
- Line Regulation—The change in output voltage as the AC input voltage transitions from the lowest to the highest value of the input range. A power supply should be capable of handling any AC voltage in its input range with a change in its output of 1% or less.
- Efficiency—The ratio of power input to power output, expressed in terms of a percentage. Values of 65%–85% are common for power supplies today. The remaining 15%–35% of the power input is converted to heat during the AC/DC conversion process. Although greater efficiency means less heat inside the computer (always a good thing) and lower electric bills, it should not be emphasized at the expense of precision, stability, and durability, as evidenced in the supply’s load regulation and other parameters.
- Ripple (or Ripple and Noise, or AC Ripple, or PARD [Periodic and Random Deviation])—The average voltage of all AC effects on the power supply outputs, usually measured in millivolts peak-to-peak or as a percentage of the nominal output voltage. The lower this figure, the better. Higher-quality units are typically rated at 1% ripple (or less), which if expressed in volts would be 1% of the output. Consequently, for +5 V that would be 0.05 V or 50 mV (millivolts). Ripple can be caused by internal switching transients, rectified line frequency bleed-through, or other random noise.
Power Factor Correction
In order to improve power line efficiency and to reduce harmonic distortion generation, the power factor of PC power supplies has come under examination. In particular, new standards are now mandatory in many European Union (EU) countries that require harmonics to be reduced below a specific amount. The circuitry required to do this is called power factor correction (PFC).
The power factor measures how effectively electrical power is being used and is expressed as a number between 0 and 1. A high power factor means that electrical power is being used effectively, whereas a low power factor indicates poor utilization of electrical power. To understand the power factor, you must understand how power is used.
Generally, two types of loads are placed on AC power lines:
- Resistive—Power converted into heat, light, motion, or work
- Inductive—Sustains an electromagnetic field, such as in a transformer or motor
A resistive load is often called working power and is measured in kilowatts (KW). An inductive load, on the other hand, is often called reactive power and is measured in kilovolt-amperes-reactive (KVAR). Working power and reactive power together make up apparent power, which is measured in kilovolt-amperes (KVA). The power factor is measured as the ratio of working power to apparent power, or working power/apparent power (KW/KVA). The ideal power factor is 1, where the working power and apparent power are the same.
The concept of a resistive load or working power is fairly easy to understand. For example, a light bulb that consumes 100 W of power generates 100 W worth of heat and light. This is a pure resistive load. An inductive load, on the other hand, is a little harder to understand. Think about a transformer, which has coil windings to generate an electromagnetic field and then induce current in another set of windings. A certain amount of power is required to saturate the windings and generate the magnetic field, even though no work is being done. A power transformer that is not connected to anything is a perfect example of a pure inductive load. An apparent power draw exists to generate the fields, but no working power exists because no actual work is being done.
When the transformer is connected to a load, it uses both working power and reactive power. In other words, power is consumed to do work (for example, if the transformer is powering a light bulb), and apparent power is used to maintain the electromagnetic field in the transformer windings. In an AC circuit, these loads can become out of sync or phase, meaning they don’t peak at the same time, which can generate harmonic distortions back down the power line. I’ve seen examples in which electric motors have caused distortions in television sets plugged into the same power circuit.
PFC usually involves adding capacitance to the circuit to maintain the inductive load without drawing additional power from the line. This makes the working power and apparent power the same, which results in a power factor of one. It usually isn’t just as simple as adding some capacitors to a circuit, although that can be done and is called passive power factor correction. Active power factor correction involves a more intelligent circuit designed to match the resistive and inductive loads so the electrical outlet sees them as the same.
A power supply with active power factor correction draws low distortion current from the AC source and has a power factor rating of 0.9 or greater. A nonpower factor-corrected supply draws highly distorted current and is sometimes referred to as a nonlinear load. The power factor of a noncorrected supply is typically 0.6–0.8. Therefore, only 60% of the apparent power consumed is actually doing real work!
Having a power supply with active PFC might or might not lower your electric bill (it depends on how your power is measured), but it definitely reduces the load on the building wiring. With PFC, all the power going into the supply is converted into actual work, and the wiring is less overworked. For example, if you ran a number of computers on a single breaker-controlled circuit and found that you were blowing the breaker periodically, you could switch to systems with active PFC power supplies and reduce the load on the wiring by up to 40%, meaning you would be less likely to blow the breaker.
The International Electrotechnical Commission (IEC) has released standards dealing with the low-frequency public supply system. The initial standards were 555.2 (Harmonics) and 555.3 (Flicker), but they have since been refined and are now available as IEC 1000-3-2 and IEC 1000-3-3, respectively. As governed by the EMC directive, most electrical devices sold within the member countries of the EU must meet the IEC standards. The IEC1000-3-2/3 standards became mandatory in 1997 and 1998.
Even if you don’t live in a country where PFC is required, I highly recommend specifying PC power supplies with active PFC. The 80 PLUS certification for highly efficient power supplies also includes a requirement that the power supply has active PFC. The main benefits of PFC supplies is that they do not overheat building wiring or distort the AC source waveform, which causes less interference on the line for other devices.
SLI-Ready and CrossFireX Certifications
Both Nvidia and AMD have certification programs that test and certify power supplies to be able to power systems with multiple graphics cards in either a Scalable Link Interface (SLI) or CrossFire configuration. This type of configuration puts extreme demands on the PSU, because it not only has to power what would normally be a high-end motherboard, CPU, and multiple drives in a RAID configuration, but also up to three video cards, which may be capable of drawing 300 watts or more each.
The certification process involves PSU manufacturers sending PSUs in for testing, whereby they are verified to supply sufficient power (and the proper type and number of connectors) to run the desired graphics hardware as well as the system. Power supplies that have passed either of these certifications are virtually guaranteed to produce high output and use high-quality design, engineering, and manufacturing. For more information on these certifications, as well as lists of certified PSUs, visit the following links.
Certified SLI-Ready Power Supplies— www.slizone.com/object/slizone_build_psu
CrossFire Certified Power Supplies—http://support.amd.com/us/certified/power-supplies
I recommend checking the lists or looking for the “NVIDIA SLI-Ready” or “AMD CrossFireX Technology” logos on a power supply as an excellent indicator of a high-power, high-quality unit.
Safety Certifications
Many agencies around the world certify electric and electronic components for safety and quality. The most commonly known agency in the United States is Underwriters Laboratories, Inc. (UL). UL standard #60950—Safety of Information Technology Equipment —covers power supplies and other PC components. You should always purchase power supplies and other devices that are UL-certified. It has often been said that, although not every good product is UL-certified, no bad products are.
In Canada, electric and electronic products are certified by the Canadian Standards Agency (CSA). The German equivalents are TÜV Rheinland and VDE, and NEMKO operating in Norway. These agencies are responsible for certification of products throughout Europe. Power supply manufacturers that sell to an international market should have products that are certified at least by UL, the CSA, and TÜV—if not by all the agencies listed, and more.
Apart from UL-type certifications, many power supply manufacturers, even the most reputable ones, claim that their products have a Class B certification from the Federal Communications Commission (FCC), meaning that they meet FCC standards for electromagnetic and radio frequency interference (EMI/RFI). This is a contentious point, however, because the FCC does not certify power supplies as individual components. Title 47 of the Code of Federal Regulations, Part 15, Section 15.101(c) states the following:
“The FCC does NOT currently authorize motherboards, cases, and internal power supplies. Vendor claims that they are selling ‘FCC-certified cases,’ ‘FCC-certified motherboards,’ or ‘FCC-certified internal power supplies’ are false.”
In fact, an FCC certification can be issued collectively only to a base unit consisting of a computer case, motherboard, and power supply. Thus, a power supply purported to be FCC-certified was actually certified along with a particular case and motherboard—not necessarily the same case and motherboard you are using in your system. This does not mean, however, that the manufacturer is being deceitful or that the power supply is inferior. If anything, this means that when evaluating power supplies, you should place less weight on the FCC certification than on other factors, such as UL certification.







