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%|
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|
+12 V Rating
+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.
The pinout of the +12 V power connector is shown below.
|+12 V Four-Pin CPU Power Connector Pinout (Wire End View)|
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)|
|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.
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.
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)|
|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.
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.
- Power Supplies
- Voltage Rails
- Power Supply Form Factors
- Modern Form Factors: ATX And SFX
- Modern Form Factors: EPS, TFX, CFX, LFX, And Flex ATX
- Power Switches
- Motherboard Power Connectors: AT/LPX And ATX
- Motherboard Power Connectors: Six-Pin Auxiliary And 24-Pin Main
- CPU Power Connectors
- Compatibility Issues
- Additional Power Connectors: Peripheral, Floppy, And SATA
- PCI Express Auxiliary Graphics Power Connectors
- Power Supply Specifications
- Other Power Supply Specifications And Certifications