|Guide To Computer
is a Power Supply?
A computer power supply unit
(Computer PSU) is the component that supplies power to a computer.
More specifically, a power supply is typically designed to convert
100-120 V (North America and Japan) or 220-240 V (Europe, Asia and
Australia) AC power from the mains to usable low-voltage DC power
for the internal components of the computer. Some power supplies
have a switch to change between 230 V and 115 V. Other models have
automatic sensors that switch input voltage automatically, or are
able to accept any voltage between those limits.
The most common computer power supplies are built
to conform with the ATX form factor. The most recent specification
of the ATX standard is version 2.2, released in 2004. This enables
different power supplies to be interchangeable with different
components inside the computer. ATX power supplies also are designed
to turn on and off using a signal from the motherboard (PS-ON wire),
and provide support for modern functions such as the standby mode
available in many computers.
Computer power supplies are rated based on their
maximum output power. Typical power ranges are from 300 W to 500 W
(lower than 300W for Small form factor systems), although units used
by gamers and enthusiasts usually range from 500 W to 1000 W, with
the highest end units going up to 2 kW for extreme performance
computers with multiple processors and graphics cards (ATI CrossFire
or NVIDIA SLI).
Most computer power supplies have the appearance
of a square metal box, and have a large bundle of wires emerging
from one end. Opposite the wire bundle is the back face of the power
supply, with an air vent and C14 IEC connector to supply AC power.
There may optionally be a power switch and/or a voltage selector
switch. A label on one side of the box lists technical information
about the power supply, including safety certifications maximum
output wattage. Common certification marks for safety are the UL
mark, GS mark, T‹V, NEMKO, SEMKO, DEMKO, FIMKO, CCC, CSA, VDE, GOST
R and BSMI. Common certificate marks for EMI/RFI are the CE mark,
FCC and C-tick. The CE mark is required for power supplies sold in
Dimensions of an ATX power supply are 150 mm
width, 86 mm height, and typically 140 mm depth, although the depth
can vary from brand to brand. Before replacing a power supply, be
sure you know the capacity of the original case, so you buy a PSU
that will fit.
How To Buy A Power Supply?
Not Just Any Power Supply Will Do
Most people tend to overlook the importance of
their power supplies. Thatís because your power supply is far more
than just an unassuming gray or black box that you plug into an
outlet before you crank up your computer. Now that gaming machines,
extreme PCs and fast, powerful workstations crave increasing amounts
of reliable power, you can no longer afford to overlook the role
that your power supply has in keeping your PC healthy and running to
its full potential. Few spend any time considering the power supply.
Yet, when you build a system for the lowest possible price, the
manufacturer is most likely to cut corners on components like the
PSU. As a result, you need to be cautious when you buy a PC system.
If you are paying attention to the power supply, there is a decent
chance you are concerned mostly with how many watts of power it is
rated to generate, even though there is little chance you will be
able to verify those power ratings. Unfortunately, many do not take
the time to consider whether the power the supply produces is clean
and stable, whether it is noisy, or if it is prone to
system-jeopardizing spikes and surges, or just as important is the
MTBF (mean time between failure) life span of the power supply!
The Increasing Demands on Power Supplies
It seems that only the PC connoisseurs, those
putting together or purchasing extreme PCs or top-flight gaming
systems, consider the power supply to be the heart and soul of the
system and who always seek the best power supplies they can afford.
The power supply is important simply because it
supplies electrical power to every other component in the system.
Unfortunately, itís historically one of the components most likely
to fail, especially because so many manufacturers cut corners on
quality when adding power supplies to their systems. Itís sad to say
that a power supply failure can cause components to malfunction as
well as damage them beyond repair, by delivering improper or erratic
voltage. As you add more and more peripherals to your PC system -
extra hard drives, CD drives - anything that you plug into your
motherboard - you put increasing strain on your power supply. The
more strain the power supply must bear the more likely it is that
you will have problems with inconsistent levels of power. When your
system is deprived of a consistent stream of reliable power it will
be prone to random hardware failures, bizarre errors that you may
never have seen before (and which you certainly do not want to see),
or even the untimely demise of your personal computer. Yet despite
the increasing importance of power supplies they are far from being
the sexiest products in the computer supply catalogue or website. So
when purchasing a new PC or Bare-Bones Chassis, pay careful
attention to the power supply that is built-in, and chose the best
power supply you can afford.
Get Familiar with Power Supply
When you buy a PC or replacement power supply you
should familiarize yourself about the power supply itself. Learn
everything you can about it. The average consumer, however, tends to
be intimidated by the vocabulary and complex statistics found in the
specs. Here are a few common terms that can help make sense out of
- Mean Time Between Failures (MTBF) or Mean Time
To Failure (MTTF).
This value represents the calculated average hour interval that
the power supply is expected to perform before it fails, or simply
put, the time between failures. Although power supplies will have
MTBF ratings of 100,000 hours or more, be aware that these figures
were not always discerned from actual real-time testing. Most
manufacturers glean these totals based upon the comparative
failure rates of the power supply's individual components.
Regardless of the MTBF, a better quality power supply will use
quality components, and be a more reliable predictor of actual
- Overvoltage Protection.
Overvoltage Protection prevents a signal from being received if
the voltage exceeds a certain limit. This helps prevent an
electrical device from being overloaded and destroyed. Make sure
the power supply you purchase includes overvoltage protection.
- Maximum Load Current.
Maximum load current is the greatest amount of current, expressed
in amps, which can safely be delivered through a particular
output. Maximum load current values are represented as individual
amperages for each output voltage. With these figures, you can
calculate not only the total amount of power the power supply can
supply, but also how many devices using those various voltages it
- Minimum Load Current.
The converse to maximum load current, minimum load current is the
smallest amount of current (in amps) that must be drawn from a
particular output for that output to function. If the current
drawn from an output falls below the minimum, the power supply
could be damaged or automatically shut down.
- Load Regulation.
When the current drawn from a particular output increases or
decreases, the voltage changes slightly as well, usually
increasing as the current rises. Load regulation is the percent
change in output voltage as the load is changed from minimum to
maximum, at constant line and constant temperature. The load
change may be specified for other than no load to full load, such
as 20% load to full load.
The ratio of power that goes into a supply as opposed to how much
goes out if it. This value is expressed in percentile, with the
most common being within the range of 65%-85% in todayís power
supplies. Be sure to purchase a power supply that falls within
this range - the higher, the better! However, although greater
efficiency translates to less heat inside the computer, which is
something you should always strive for, precision, stability, and
durability are more important factors.
How Much Power Do I Need?
Obviously, we do not expect everyone to have all
the terms of art down pat when it comes to power supplies. As you
check out our power supply web pages you will notice these products
come in a number of wattage designations. How much power is
necessary for the ďaverageĒ PC system, if there is such a thing
nowadays? Take a look at what youíve had in the past. If you bought
a standard power supply from one of the top computer manufacturers,
chances are you purchased a 250- to 300W power supply of average
quality. Now that is more than enough for your run-of-the-mill
system consisting of a hard drive, an optical drive and a
fair-to-middling graphics card. However, if you are going to add
more peripherals you need to seriously consider upgrading your power
For your reference, following is a chart that
illustrates approximately how much wattage you will need to run
various common components in a PC system:
|Midrange to high-end CPU
||RAM 7 per 128MB
|PCI add-in card
|Low to midrange graphics board
|IDE hard drive
The Advantages of a Higher End
The More Watts the Merrier
When you purchase a better power supply you will
markedly decrease the amount of noise your computer makes when itís
on. The fan that cools your power supply has to be a heavy performer
to achieve its appointed task. Therefore, itís one of the biggest,
noisiest fans in your PC. And itís strategically located in front of
the vents that lead to the outside of your computer case. So, it
makes a noticeable noise, one that can become irritating over time.
A new, more efficient, more powerful power supply will deliver more
power, regulate it at a more even level and at a reduced noise
level. For you PC builders, be aware that some of the inexpensive
cases often come with cut-rate power supplies that may not be up to
the task of powering a high-end PC. Some of the more expensive
models do not come with a power supply, which give you the
opportunity to choose your own. If you've added a lot of new
components to your PC, you may be overtaxing your existing power
supply, so look at getting a bigger, better one. Power supplies can
cause problems--including random crashes or even component
failure--if they are asked to produce more power than they are
designed to generate. Reputable manufacturers will typically include
a chart of acceptable components.
Finally, Make Sure Your New Power Supply is Compatible with
Beware - you must make sure your power supply is
compatible with your system before you plug it in. Some computers
use a special plug that links the power supply to the motherboard.
If you use the wrong plug you can wind up literally torching your
Power Supply Connections
Used to power the motherboard
|Used for specific processor
For SATA Hard Drives
For Most Peripheral Devices
Floppy Disk Drive Connector
Keeping Your Power Supply & Your Computer Cool
One of the most overlooked aspects of
a PC Power Supply is that is also provides most of the PC's cooling.
The fan in the power supply delivers most of the airflow inside your
computer chassis. Therefore, not only is it important to have
reliable power, but you also need a reliable fan, and one capable of
producing the airflow needed to keep your components cool.
With today's higher speed processors,
video cards, and drives, thermal regulation is critical.
Components fail when over heated. So buying a power supply
with a high capacity fan can literally save your system.
Many PSU manufacturers also offer
variable speed fans that increase fan speed when heat builds up
inside your computer. This is an important feature, and one
that should be looked for. Also, look for the lowest noise
level, since the Power Supply fan is responsible for 90% of the
noise produced by your PC.
A Standard Box Fan and
Newer Open Fan Designs That Produce More Airflow.
PC Power Supply,