How do I replace the fan in my power supply without killing myself?

The bearings on the fan in my PC power supply are going, and it’s starting to make nasty noises. Problem is, power supplies are expensive, too expensive to be worth it for this particular computer. OTOH, fans are cheap. I don’t want to spend the money to buy a new power supply if I don’t have to, and I figure even I can handle the oh-so-complicated job of replacing a 5 V DC fan. Can probably even put in a nicer one than what was installed in Taiwan.

The problem is, I have this nagging memory that power supplies are nasty, nasty things. Something about the parts still being able to kill you even when it’s unplugged. Just hazy enough to scare me I suppose.

So does anyone know anything more about this? Are there certain parts I should watch out for? Is there something I can do to make the process safer?

Power supplies (and monitors) contain capacitors which can store an electric charge long after they are unplugged. There are tools for safely discharging a capacitor, but it would be cheaper to buy a new power supply. If you go poking around in there, you do so at your own risk.

Computer power supplies are “switching” supplies, and they work as follows:

  1. 120 VAC (nominal) enters the power supply.
  2. The 120 VAC gets full-wave rectified (AC to DC). The output of the rectifier is around 150 to 170 VDC; the actual voltage depends on output load, actual AC input voltage, size of filter caps (see below), etc.
  3. The output of the rectifier gets filtered with electrolytic capacitors. Without the capacitors, the output of the rectifier would look like a train of half sine waves. With the caps, the output of the rectifier is a steady-state DC voltage with some ripple.
  4. Using a solid state switch (transistor), the DC voltage is “pulsed” through a transformer at a high frequency.
  5. And so on.

1 through 4 pretty much does it for the “high voltage” section of a switching power supply. If you turn off the power switch and/or unplug the power supply from the wall, there is a high probability the 150 to 170 VDC is stored on the electrolytic capacitors. I can’t say for certain because the voltage on the caps could bleed off due to a) a “bleed off” resistor placed across the caps by the manufacturer, and/or b) the switching transistor left in the “on” position. But assuming neither is the case, then yes, I think there’s a good chance 150 to 170 VDC is stored on the caps.

So it would be a good idea to first discharge the caps if you’re going to open up the power supply. This can be done by placing a 100 ohm resistor across the one of the caps (using insulated test leads) for a few seconds.