Hardware Reference
In-Depth Information
expensive power source and generates a minimum amount of heat. It also maintains a
small size and low price.
Voltage Rails
ThePSUnormallysupplies+3.3V,+5V,and+12Vtothesystem.Thesevoltagesareoften
called
rails
, referring to the fact that although there are multiple wires carrying a specific
voltage, they are normally tied to a single rail (or tap) in the PSU. Multiple wires are used
because, if all of the current were carried over a single wire, the wire and the terminals,
connectors, and even the traces on the circuit boards would all have to be extremely large
andthicktohandletheload.Instead,itischeaperandmoreefficienttospreadtheloadout
among multiple smaller and thinner wires.
The digital electronic components and circuits in the system (motherboard, adapter cards,
and disk drive logic boards) typically use the +3.3V or +5V power, and the motors (disk
drive motors and any fans) use the +12V power. In addition, voltage regulators on the
motherboard or in other components convert these standard voltages to others as neces-
sary.
Table 18.1
lists the devices typically powered by the various voltage rails.
Table 18.1 Voltage Rail Usage in a PC
You can think of each rail as a separate power circuit, kind of like a power supply within
the power supply. Normally each rail is rated for a specified maximum amount of current
in amperes. Because the extreme amount of 12V current required by newer CPU voltage
regulators and high-end video cards can exceed the output of common 12V rails, some
power supply designs use multiple +12V rails. This means that essentially they have two
or more separate 12V circuits internally, with some wires tapping off of one circuit and
otherstappingoffofanother.Unfortunately,thiscanleadtopowerproblems,especially if
you fail to balance the loads on both rails or to ensure you don't exceed the load capacity
ononeorthe other.Inother words,it isfarbetter tohave asingle 12Vrail that can supply
40ampsthantwo12Vrailssupplying20ampseachbecausewiththesinglerailyoudon't
have to worry which connectors derive power from which rail and then try to ensure that
you don't overload one or the other.
