Hardware Reference
In-Depth Information
Chapter 18. Power Supplies
The Power Supply
The power supply is not only one of the most important parts in a PC, it is unfortunately
one of the most overlooked. Although most enthusiasts who build their own systems un-
derstand its importance, the mainstream PC buyer generally does not. Some that do pay
any mind seem concerned only with how many watts of power it is rated to put out (even
thoughnopracticalwayexiststoverifythoseratings),withoutregardtowhetherthepower
being produced is clean and stable or whether it is full of noise, spikes, and surges.
Ihavealwaysplacedgreatemphasisonselectingapowersupplyformysystems.Iconsider
the power supply the foundation of the system and am willing to spend a little extra to get
a more robust and reliable unit. The power supply is critical because it supplies electrical
power to every other component in the system. In my experience, the power supply is also
one of the most failure-prone components in any computer system. Over the years I have
replaced more power supplies in PCs than any other part. A malfunctioning power supply
not only can cause other components in the system to malfunction, but it also can damage
theothercomponentsinyourcomputerbydeliveringimproperorerraticvoltages.Because
of its importance to proper and reliable system operation, you should understand both the
function and limitations of a power supply, as well as its potential problems and their solu-
tions.
Primary Function and Operation
The basic function of the power supply is to convert the electrical power available at the
wall socket to that which the computer circuitry can use. The power supply in a conven-
tional desktop system is designed to convert either 120V (nominal) 60Hz AC (alternating
current)or240V(nominal)50HzACpowerinto+3.3V,+5V,and+12VDC(directcurrent)
power. Some power supplies require you to switch between the two input ranges, whereas
others auto-switch.
Technically, the power supply in most PCs is described as a
constant voltage switching
power supply unit (PSU)
, which is defined as follows:
•
Constant voltage
means the power supply puts out the same voltage to the computer's
internal components, no matter the voltage of AC current running it or the capacity
(wattage) of the power supply.
•
Switching
refers to the design and power regulation technique that most suppliers use.
Compared to other types of power supplies, this design provides an efficient and in-
