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Very Important Warning!

Warning! Read this first, even if
you are clever enough to remove all external power before you start to
open the PSU.
A switched mode power supply contains some large
capacitors which are charged to well over 300 V DC. Normally, when the PSU
is functioning properly, these capacitors will loose this voltage rather
quick when the power supply is switched off. But this is not a normal
situation and therefore the capacitors may not loose this very dangerous
voltage fast enough. Special care should be taken to discharge the
capacitors (by means of a normal light bulb for instance) before you start
touching the PCB all over the place.

Do NOT operate the PSU while it's open!
Not only can this cause a serious electrical shock, it can also cause
short circuits, destroying the PSU for ever!

Some basic knowledge about electronics and electronic components is
mandatory when you intend to fix your PSU.

PC PSU Repair Tips

I know, PC Power Supply Units are very cheap. Why not chuck it
away when it's dead and put in a new one? Simple because the new one is
not always at hand. Or the PSU is a very special model, tailored to this
particular brand of computer. What's more, sometimes you can't even get a
separate PSU without buying an entire PC case to go with it. So you'd have
to throw away at least one of them, which is of course a shame.
You
see, there are some reasons why one would bother about fixing a PSU
yourself.

This description is by no means intended to solve all your power supply
problems. I only cover a few basic tips so you'll know where to look. If
the golden tip is not among them, bad luck, throw it away and buy yourself
a new one. Please don't ask me for more tips, because these are all I can
give you.

One important tip in advance: "Never try to operate the PSU without a
load". Most switching mode power supplies expect some load on the
regulated output (usually the +5 V on a computer PSU). If you intend to
try out the PSU without a computer you can always connect a 12 V car light
bulb to the +5 V output as artificial load.

Is the main fuse alive?

This is a good point to start looking for the problem. If the main fuse
is dead, check the main switching component for shorts. Usually the main
switching component is a transistor or a FET and can be found on one of
the heat sinks on the primary side of the PSU. If it's shorted you're in
bad luck. It is very likely that the transistor killed part of the control
circuit as well.
My best advise is: "Get yourself a new PSU" because
it may cost quite a lot of effort to find all the broken parts and get the
required new ones from you're local component store.

The main fuse is still alive!

That's more like it! There's a big chance that we can fix it.
Well, there are a few possible causes why the PSU doesn't work anymore:

One or more Electrolytic Capacitors are leaking or have dried out,
causing them to loose capacity. Pay particular attention to capacitors
with a crack in the tin cap. Furthermore capacitors in close proximity
to heat sources are particularly susceptible for loosing their capacity.
Use only 105 °C capacitors as
replacements!
I've had a 47µ capacitor measuring just 43µ once,
which prevented to PSU from starting up! Just to give you an example
how critical some designs are.

Check all secondary diodes for shorts. Schottky diodes have a
typical forward voltage of approximately 0.2
V. Normal silicon diodes have a typical forward voltage of about
0.6 V.

Cold solder joints are also a common cause. They can especially be
found around the pins of "warm" components, particularly warm componants
which are fixed to heat sinks which prevents them from moving when their
legs extend due to the heat.

Finally the start resistor may have died. I'll explain this is in
more detail below.

There are probably dozens of other causes why a PSU has died. Don't
spend too much time and effort to find it if these tips don't solve your
problem though, considering the current prices of new ones.

The Start Resistor

It is quite obvious that a PSU supplies some low voltages on its
secondary side. But it also generates at least one voltage on the primary
side as well because the control circuit would like to have something to
eat too. Usually this voltage is simply derived from an extra coil around
the main switching transformer.
However the power supply must have a
way to get started. When it is switched on, this control voltage is not
available because the power supply is not operational yet.

One popular way to solve this problem is the use of a start
resistor. This is a relatively high ohmic resistor (usually in the range
between 390 kΩ to 680
kΩ) which is placed between the 300 V
DC primary voltage and the control circuit's power supply.
At
first the control circuit is switched off completely. This allows the high
ohmic start resistor to charge the capacitor C1 slowly, but steadily. Then
once the voltage on this capacitor is high enough to fire up the power
supply the control circuit kicks in and gives the first pulses to the
switching device. This will cause the control circuit power supply (D1) to
deliver enough current to keep the control circuit going until everything
reaches a stable operating condition.

Remember that most ATX power supplies are never really switched
off. Normally they remain in standby when the computer is not in use.
Therefore such a PSU can run forever, even with a dead start resistor.
Only when you have removed the power completely you will find that the
computer won't start up any more. This is a clear indication that the
start resistor has died.

The start resistor is usually a normal ½ W
or ¾ W carbon resistor and is always connected
to the +300 V line on the primary side of the
PSU.
The resistor in this picture should be 560
kΩ, but measured almost 1.2 MΩ instead!

Last Updated: 23 May 2011
© 2005, San Bergmans, Oisterwijk, The
Netherlands
http://www.sbprojects.com/