How PC Power Supplies Work

If there is any one component that is absolutely vital to the
operation of a computer, it is the power supply. Without it,
a computer is just an inert box full of plastic and metal.
The power supply converts the alternating current (AC) line
from your home to the direct current (DC) needed by the
personal computer. In this article, we'll learn how PC power
supplies work and what the wattage ratings mean.

In a personal computer (PC), the power supply is the metal
box usually found in a corner of the case. The power supply
is visible from the back of many systems because it contains
the power-cord receptacle and the cooling fan.

Power supplies, often referred to as "switching power
supplies", use switcher technology to convert the AC input to
lower DC voltages. The typical voltages supplied are:

* 3.3 volts
* 5 volts
* 12 volts

The 3.3- and 5-volts are typically used by digital circuits,
while the 12-volt is used to run motors in disk drives and
fans. The main specification of a power supply is in watts.
A watt is the product of the voltage in volts and the current
in amperes or amps. If you have been around PCs for many
years, you probably remember that the original PCs had large
red toggle switches that had a good bit of heft to them. When
you turned the PC on or off, you knew you were doing it.
These switches actually controlled the flow of 120 volt power
to the power supply.

Today you turn on the power with a little push button, and
you turn off the machine with a menu option. These
capabilities were added to standard power supplies several
years ago. The operating system can send a signal to the
power supply to tell it to turn off. The push button sends
a 5-volt signal to the power supply to tell it when to turn
on. The power supply also has a circuit that supplies 5 volts,
called VSB for "standby voltage" even when it is officially"
off", so that the button will work.

Switcher Technology

Prior to 1980 or so, power supplies tended to be heavy and
bulky. They used large, heavy transformers and huge
capacitors (some as large as soda cans) to convert line
voltage at 120 volts and 60 hertz into 5 volts and 12 volts
DC.

The switching power supplies used today are much smaller and
lighter. They convert the 60-Hertz (Hz, or cycles per second)
current to a much higher frequency, meaning more cycles per
second. This conversion enables a small, lightweight
transformer in the power supply to do the actual voltage
step-down from 110 volts (or 220 in certain countries) to the
voltage needed by the particular computer component. The
higher-frequency AC current provided by a switcher supply is
also easier to rectify and filter compared to the original
60-Hz AC line voltage, reducing the variances in voltage for
the sensitive electronic components in the computer.

A switcher power supply draws only the power it needs from
the AC line. The typical voltages and current provided by
a power supply are shown on the label on a power supply.

Switcher technology is also used to make AC from DC, as found
in many of the automobile power inverters used to run AC
appliances in an automobile and in uninterruptible power
supplies. Switcher technology in automotive power inverters
changes the direct current from the auto battery into
alternating current. The transformer uses alternating current
to make the transformer in the inverter step the voltage up
to that of household appliances (120 VAC).

Power Supply Standardization

Over time, there have been at least six different standard
power supplies for personal computers. Recently, the industry
has settled on using ATX-based power supplies. ATX is
an industry specification that means the power supply has the
physical characteristics to fit a standard ATX case and the
electrical characteristics to work with an ATX motherboard.

PC power-supply cables use standardized, keyed connectors
that make it difficult to connect the wrong ones. Also, fan
manufacturers often use the same connectors as the power
cables for disk drives, allowing a fan to easily obtain the
12 volts it needs. Color-coded wires and industry standard
connectors make it possible for the consumer to have many
choices for a replacement power supply.

Advanced Power Management (APM) offers a set of five
different states that your system can be in. It was developed
by Microsoft and Intel for PC users who wish to conserve
power. Each system component, including the operating system,
basic input/output system (BIOS), motherboard and attached
devices all need to be APM-compliant to be able to use this
feature. Should you wish to disable APM because you suspect
it is using up system resources or causing a conflict, the
best way to do this is in the BIOS. That way, the operating
system won't try to reinstall it, which could happen if it
were disabled only in the software.

Power Supply Wattage

A 400-watt switching power supply will not necessarily use
more power than a 250-watt supply. A larger supply may be
needed if you use every available slot on the motherboard or
every available drive bay in the personal computer case. It
is not a good idea to have a 250-watt supply if you have 250
watts total in devices, since the supply should not be loaded
to 100 percent of its capacity.

According to PC Power & Cooling, Inc., some power consumption
values (in watts) for common items in a personal computer are:

PC Item Watts

Accelerated Graphics Port (AGP) card 20 to 30W

Peripheral Component Interconnect 5W
(PCI) card

small computer system interface 20 to 25W
(SCSI) PCI card

floppy disk drive 5W

network interface card 4W

50X CD-ROM drive 10 to 25W

RAM 10W per 128M

5200 RPM Integrated Drive 5 to 11W
Electronics (IDE) hard disk drive

7200 RPM IDE hard disk drive 5 to 15W

Motherboard (without CPU or RAM) 20 to 30W

550 MHz Pentium III 30W

733 MHz Pentium III 23.5W

300 MHz Celeron 18W

600 MHz Athlon 45W

Power supplies of the same form factor ("form factor" refers
to the actual shape of the motherboard) are typically
differentiated by the wattage they supply and the length of
the warranty.