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Voltage Rails

Power Supply 101: A Reference Of Specifications
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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 VDisk 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.

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