(^322) PC Hardware: A Beginner’s Guide
wiresattachedtotheconnectorrepresentedinFigure14-4hasaspecificpurposeandvoltage.
When the soft switch is activated, the power supply is signaled to provide each wire with
its voltage.
One word of caution about always-on or momentary-on motherboards: unplug the PC
before working inside the PC. This may seem like common sense, but many earlier PC models
advised you to leave the power cord in the wall socket for grounding purposes. However,
your best bet these days is to turn it off and unplug it before opening the system case.
Voltages
As indicated on numerous occasions in this chapter, the devices inside the PC are designed
for a specific voltage level. This is why the PC’s power supply generates multiple voltage
levels. Here are the voltages required from the power supply in a typical PC: –12V This is a holdover from earlier systems. It was used primarily for serial
ports. This voltage is still common on nearly all power supplies for backward
compatibility to older hardware.
–5VDC This voltage level is no longer used. It was used on some of the
earliest PCs for floppy disk controllers and ISA bus cards. It was available on
many power supplies strictly for backward compatibility purposes, but it is
generally not used.
+/–0VDC A circuit that carries zero volts of direct current (DC) is a
grounding circuit that is used to complete circuits with other circuits using
another voltage level. A circuit with 0VDC is also called a common or earth
ground circuit.(By the way, it really doesn’t matter if the circuit measures out
at plus or minus zero volts.)
+3.3VDC This is the voltage used on most newer PCs, especially those on the
ATX and NLX form factors, for powering the CPU, memory, AGP ports, and
the other motherboard components. Prior to the ATX form factor (on the Baby
AT, for example) and the second generation of Pentium processors (Pentium
Pro, Pentium II, etc.), voltage regulators were located on the motherboard and
used to reduce a +5VDC circuit to +3.3VDC.
+5VDC Prior to the Pentium processor, +5 volts was the primary voltage on
motherboards for CPUs, memory, and nearly all devices attached or connected
to the motherboard. For the Baby AT form factor and the power supplies that
preceded it, this is the standard voltage.
+12VDC After +3.3V, +12V is the workhorse voltage on the PC. It is used for
powering the devices that directly connect to the power supply, including hard
disks, floppy disks, CD-ROMs, DVDs, and the cooling fan. This voltage is
passed through the motherboard to the expansion bus slots to provide power
to any expansion and adapter cards installed.