Power Supply

The power supply is a very
important part of your computer, and also the most likely to fail.
Also, if you are planning a big upgrade, a weak power supply can keep
your new upgrade from working.

How do you know your power supply is bad? Well, beyond the obvious
problem of not turning on at all, there are some things you can look
for. These can include overheating, periodic boot-up failures or
errors, parity errors, lots of noise, and even electrical shock when
the case or connections are touched.

If your power supply is showing any of these signs, it probably
needs to be replaced. Never attempt to repair one yourself. Always
take it to a technician. Or just buy a new one. Replacement
is probably the most economical choice.

Power supplies are rated by the wattage output. An old computer
will have a 70 watt power supply. Today, they go as high as 500 watts.
The average computer today uses 230 watt or 250 watt. If you are
buying a new power supply, buy one with this rating, and you will be
set to go with numerous attachments.

How They Work – The Basics

Your power supply converts the standard 120 volt power from your
wall into a form usable by your computer. In converts the AC current
from the wall into a DC current or 5 volts and 12 volts. It then
distributes this power to all the devices in your system. Usually, the
motherboard, adapter cards, and diskette drives get the 5 volt power,
while the disk motors and cooling fans get the 12 volt leads.

The power supply also gives the system the final go-ahead before it
boots. It sends an electric signal to the motherboard assuring that
the system has enough power to operate correctly.

The main fan of your power supply is very important. It cools the
power supply and keeps a constant and bearable temperature inside the
rest of the computer’s case. Obviously, the fan depends on a nice
supply of air to operate. It pulls air through the vent at the front
of the case, circulates it, and blows it out of the hole at the back
of the power supply. Therefore, it is best to keep the case on most of
the time while the PC is turned on to ensure a nice flow of air. Also,
make sure you have dust plates covering each hole on the back of the
computer that is not being used by an expansion card.

Form Factor – ATX

Baby AT power supplies are the most common. They are plain and
simple power supplies. The ATX form factor requires a special power
supply. This supply is much more advanced. It contains special
circuitry which allows software control of the power on signal. It
supplies a steady 5 volt power to the motherboard even when the rest
of the system is off, thus giving the system the ability to boot
itself through software. The fan on the ATX power supply is reversed.
This blows air into the case instead of out. The air blows right over
the CPU, eliminating the need to a CPU fan. It also pressurizes the
inside of the case, keeping it clean.


When working with the power supply, you need to know about some of
the connectors. Most importantly if you are using a new case, you
might have to connect the supply to the power switch on the front of
the case. Some people have a hard time doing this, since there are
four wires, and four tabs, and finding which wires goes to what tab
can be difficult. So here I show you.

You will usually have four color-coded wires. Some cases provide a
fifth that is simply attached to the case for grounding. The wires are
color-coded as follows:

  • The brown and blue wires are the feed wires from the 110V
    supply. They bring the full 110V to the switch. They are always
    hot when the power supply is plugged in, meaning ALWAYS HAVE THE
  • The black and white wires carry the AC current from the switch
    to the power supply. These wires are only hot when the system is
    on, because the switch actually closes the circuit.
  • If you have a green wire, sometimes with a stripe, it is a
    ground wire, and is attached to the case somewhere.

When connecting, place the blue and brown wires on the tabs that
are parallel and next to each other. Place the black and white wires
on the tabs that are angled and next to each other. I’ve heard you can
actually mix these up a bit without problems, but ALWAYS make sure the
black and brown are right next to each other, and the same with the

The drive connectors are all about the same, except for size. In
response to some requests for a pin-out, here it is.

Wire Color



+12 V






+5 V

Preventive Measures

There are some things you can do to help prolong the life of your
power supply:

  • House the computer in a friendly environment. Since the fan in
    the thing is working to keep things cool, it needs to have cool
    air to blow. The fan is ineffective in hot air. Therefore keep the
    things in a air conditioned room.
  • Try to minimize the amount of dust and pollution in the air
    around your computer. The fan draws air in, so you don’t want it
    to blow in smoke or something that will just make your computer
    cough and hack. You may notice that the fan hole in your power
    supply is covered with dust. The little furry kind. You may want
    to clean this off, but make sure you don’t stick anything into the
    power supply.
  • Use a surge protector. This is a device that will keep electric
    surges from entering your computer through the cord and frying
    your computer. Make sure the surge protector has phone line
    protection too so that you don’t get a surge through your phone
    cords into the modem.
  • You might want to use a UPS, or Uninterruptible Power Supply.
    This is a little box with a built in surge protector plus a
    battery capable of running your computer for 10-15 minutes if the
    power shuts off. This will give you enough time to save your work.

Leave It On Or Turn It Off?

This seems to be a bit of a debate in some circles. Should you turn
the computer off when you are done using it, or does this harm the
machine? Here’s my $0.02!

Turning a computer on and off is hard on it. This is mainly due to
thermal expansion. When you turn it on, the parts in the computer
start to heat up, and they expand. Then, when you turn it off, they
contract again. This cycle repeats each on/off cycle you put the
system through. This movement causes the system to deteriorate over
time. Chips may split, wires may break, boards may crack, etc. You
will also run into chip creep, causing intermittent problems until you
finally find it.

So, it is best to keep your system at a constant temperature at all
times. Leaving the computer on all the time is the best way to do

That said, you need to weigh this with some other factors. A system
left on all the time can be a fire hazard. If your system is in an
office building, there could be a security risk in leaving the system
on. But, there is also the question of power. Obviously, leaving the
computer on at all times hogs a lot of power, which you must pay for.

The best way, in my opinion, to balance this out is to leave your
computer on until you are completely done using it for the day. If you
are working on a project, and you take a break for lunch, leave the
computer on. Even if you must leave for a couple hours, it might be
best to leave the system on. Then, when you are done for the day, you
turn it off. This is a happy medium, and reduces wear and tear on your

But, then again, you must work out your own habits.=)


  1. i always connect the wires to the disk drive, dvd rom/writer etc..that is the yellow wires is on the right and red is on the left..is this correct? how do i know that my power supply on the case is still working? i’ve turned on my pc and its not working..even the fan on the processor is not moving..is it the switch(power on/off) or the power supply which is not working? thanks

  2. Surge protectors and keeping my computer out of dusty environments are how I keep my computer running smoothly. Also as this article states, only turning it off every once in a while. Not daily.

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