Power for the processor comes from a device called the voltage regulator module (VRM), which is built in to 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 5V or 12V for input power. Most have used 5V over the years, but many are now converting to 12V because of the lower current requirements at that voltage. In addition, other devices might have already loaded the 5V, whereas, typically, only drive motors use the 12V.

Whether the VRM on your board uses 5V or 12V depends on the particular motherboard or regulator design. Many modern voltage regulator ICs are designed to run on anything from a 4V to a 36V input, so it is up to the motherboard designer as to how they will be configured.

For example, I studied a system using an FIC (First International Computer) SD-11 motherboard, which used a Semtech SC1144ABCSW voltage regulator. This board design uses the +5V to convert to the lower voltage the CPU needs. Most motherboards use voltage regulator circuits controlled by chips from Semtech or Linear Technology. You can visit their sites for more data on these chips.

That motherboard accepts an Athlon 1GHz Cartridge version (Model 2), which according to AMD has a maximum power draw of 65W and a nominal voltage requirement of 1.8V. 65W at 1.8V would equate to 36.1A of current at that voltage (volts x amps = watts). If the voltage regulator used +5V as a feed, 65W would equate to only 13A at +5V.

That would assume 100% efficiency in the regulator, which is impossible. Therefore, assuming 80% efficiency (which is typical), there would be about 16.25A actual draw on the +5V due to the regulator and processor combined. When you consider that other circuits on the motherboard also use +5V power—plus ISA or PCI cards are drawing that power as well—you can see how easy it is to overload the +5V lines from the supply to the motherboard.

Although most motherboard VRM designs up through the Pentium III and Athlon/Duron use 5V-based regulators, a transition is underway to use 12V-powered regulators. This is because the higher voltage will significantly reduce the current draw. So, modifying the motherboard VRM circuit to use the +12V power feed would seem simple.

Unfortunately, the standard ATX 2.03 power supply design has only a single +12V lead in the main power connector. The auxiliary connector has no +12V leads at all, so that is no help. Pulling up to 8A more through a single 18ga. wire supplying +12V power to the motherboard is a recipe for a melted connector because the contacts in the main ATX connector are rated for only 6A.

To augment the supply of +12V power to the motherboard, Intel created a new ATX12V power supply specification. This adds a third power connector, called the ATX12V connector, specifically to supply additional +12V power to the board. This connector has two +12V power pins, each rated for 8 amps total, allowing for up to 16A of additional 12V current to the motherboard, for a total of 22A of +12V. This connector is shown in Figure below.

If you are replacing your motherboard with a new one that requires the ATX12V connection for the CPU voltage regulator, and yet your existing power supply doesn't have that connector, an easy solution is available. Merely convert one of the peripheral power connectors to an ATX12V type.

PC Power and Cooling has released just such an adapter that can instantly make any standard ATX power supply into one with an ATX12V connector. This connector works because the issue is not whether the power supply can generate the necessary 12V—that has always been available via the peripheral connectors. The ATX12V adapter shown in Figure below solves the connector problem quite nicely.