Chapter 14: Power Supply and Electrical Issues^323
Power Supply Form Factors
Nearly all form factors also specify a power supply that is compatible with the other parts
of the PC they define. A power supply must conform to one or more form factors to fit
and function with certain case styles and motherboard specifications.The Power Supply and the Case
Theform factorof a power supply defines its physical shape, how it fits into a case, and the
amount of power it produces. In most situations, the power supply’s form factor is the same
as that of the system case and the motherboard. Since the power supply is normally bought
already installed in the case, there are rarely any matching issues. It is only when you need to
replace a power supply, or when you buy a case without a power supply, that form factor
issues ever arise. There are newer power supplies that are compatible with several form
factors, and there are cases that can fit several different power supply form factors. However,
the most important part of matching the power supply to the system is to match the mother-
board’s power requirements to the capabilities of the power supply. Most AT class power
supplies, which include the AT, Baby AT, ATX, and a few others, differ only in their size and
mounting requirements; their power capabilities are roughly the same.
The capabilities of the power supply are directly related to the size and shape of it case.
Mid- and full-sized tower cases are typically larger (in both height and width) and use
morepowerforcoolingitsinteriorcomponents.Becauseofitssize,amid-orfull-sizetower
case usually has more components installed inside the case. On the other hand, a desktop
or a smaller tower case typically has fewer interior components, which means they need
less power from the power supply. The more demand on the power supply, the larger the
power supply needs to be. Therefore, the power supplies defined by smaller form factors,
such as the microATX and the LPX, are much smaller than those used for a full AT or ATX
formfactorPC.SeeChapter15formoreinformationonsystemcasesandtheirformfactors.Form Factors
Here is a quick overview of the most common power supply form factors: PC XT The IBM PC and its successor, the IBM PC XT, created the first form
factor for PC power supplies (see Figure 14-5). The power supply was placed
in the rear right corner of these desktop cases. The power switch was an
up-and-down toggle switch mounted directly on the power supply.
AT The power supply of the IBM PC AT (see Figure 14-6) was larger, had a
different shape, and produced about three times more power than the PC XT.
The AT standard quickly became the form factor of choice among clone
manufacturers, who built a wide variety of AT-compatible systems. The AT
form factor was the foundation of several form factors that followed.