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 edition of
How Stuff Works,
we'll learn how PC power supplies work and what the
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.
This is a power supply removed from its PC case. The
small, red switch at right, above the power-cord connector, is for
changing line voltages in various countries.
The interior of a power supply.
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
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.
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
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
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.
In this photo you can see three small transformers
(yellow) in the center. To the left are two cylindrical capacitors. The
large finned pieces of aluminum are heat sinks. The left heat sink has
transistors attached to it. These are the transistors in charge of doing
the switching -- they provide high-frequency power to the transformers.
Attached to the right heat sink are diodes that rectify AC signals and
turn them into DC signals.
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.
Personal computer power supply label.
VSB is the standby voltage provided to the power switch.
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
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).
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.
A PC power supply removed from its PC case. Cables
and connectors at right supply DC voltages.
Advanced Power Management
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
input/output system (BIOS),
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.
PC Power & Cooling, Inc., some power consumption values (in watts) for
common items in a personal computer are:
|Accelerated Graphics Port (AGP) card
||20 to 30W
|Peripheral Component Interconnect (PCI) card
|small computer system interface (SCSI) PCI card
||20 to 25W
|network interface card
|50X CD-ROM drive
||10 to 25W
||10W per 128M
Electronics (IDE) hard disk drive
||5 to 11W
|7200 RPM IDE hard disk drive
||5 to 15W
CPU or RAM)
||20 to 30W
|550 MHz Pentium III
|733 MHz Pentium III
|300 MHz Celeron
|600 MHz Athlon
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.
Power Supply Problems
The PC power supply is probably the most failure-prone item in a personal
computer. It heats and cools each time it is used and receives the first
in-rush of AC current when the PC is switched on. Typically, a stalled
cooling fan is a predictor of a power supply failure due to subsequent
overheated components. All devices in a PC receive their DC power via the
A typical failure of a PC power supply is often noticed as a burning
smell just before the computer shuts down. Another problem could be the
failure of the vital cooling fan, which allows components in the power
supply to overheat. Failure symptoms include random rebooting or failure in
Windows for no apparent reason.
For any problems you suspect to be the fault of the power supply, use the
documentation that came with your computer. If you have ever removed the
case from your personal computer to add an adapter card or memory, you can
change a power supply. Make sure you remove the power cord first,
since voltages are present even though your computer is off.
Recent motherboard and chipset improvements permit the user to monitor the
revolutions per minute (RPM) of the power supply fan via BIOS and a Windows
application supplied by the motherboard manufacturer. New designs offer fan
control so that the fan only runs the speed needed, depending on cooling
Recent designs in
include power supplies that offer a spare supply that can be exchanged while
the other power supply is in use. Some new computers, particularly those
designed for use as servers, provide redundant power supplies. This means
that there are two or more power supplies in the system, with one providing
power and the other acting as a backup. The backup supply immediately takes
over in the event of a failure by the primary supply. Then, the primary
supply can be exchanged while the other power supply is in use.