Power Supplies - Upgrading and Repairing PCs - page 855

and current PCI Express cards use only 0.8V differential signaling—all of which are supplied by
simple onboard regulators. Processors also require a variety of voltages (as low as 1.3V 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 +3.3V 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 +5V to +3.3V
for the processor. When other chips began using 3.3V as well, Intel created the ATX power
supply specification, which supplied 3.3V to the motherboard. You would think that having
3.3V direct from the power supply would have eliminated the need for onboard voltage
regulators, but by that time, processors, memory, and other components began running on
voltages lower than 3.3V. 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 +3.3V, +5V, or +12V.
See the Chapter 3 section, “ CPU Operating Voltages ,” p. 87 .
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 +3.3V, +5V, and +12V, but also -12V and possibly -5V. Although -12V and
(possibly) -5V are supplied to the motherboard via the power supply connectors, the motherboard
normally uses only the +3.3V, +5V, and +12V. If present, the -5V 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 -5V. The motherboard logic
typically doesn't use -12V either; however, in the past it was 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, -5V 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 -5V was required on the ISA bus for full backward compatibility. Because modern PCs no
longer include ISA slots, the -5V 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 -5V signal.
Although older serial port circuits used +/-12V outputs, today most run only on +3.3V or +5V.
The main function of the +12V 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 +12V 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 +12V supply is used by any cooling fans in the system—which, of course, should
always be running. A single cooling fan can draw between 100mA and 250mA (0.1-0.25 amps);