workings of PC power supplies
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
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.
The basics about how a
power supply works.
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 newest
Pentium 4 motherboards also require a 12 volt adapter in addition to the typical
5 volt lead.
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
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.
At vs. ATX form factors
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.
Connectors and Leads
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. The instructions are straight forward.
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 POWER SUPPLY UNPLUGGED WHEN ATTACHING ITS CONNECTORS!
- 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
- If you have a green
wire, sometimes with a stripe, it is a ground wire, and is attached to the
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.
What you can do to make
your power supply last longer.
- 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