I’ve written before about checking train transformers for safety. There’s a similar process you can use for power supplies for computers and electronics. Here’s how to check a power supply for safety.
Checking the AC input
The first step to check a power supply for safety is to check the AC input. First, set your multimeter to ohms and measure the resistance across the AC prongs. How much resistance you see will vary of course depending on the design of the power supply. But you definitely don’t want to see a dead short. You also don’t want infinite resistance. In this case, as with many things, it’s the extremes that are bad. You’re looking for moderate resistance here.
If you see a dead short, that’s a good sign that a filter capacitor has gone bad. If you plug that in and turn it on, you’ll unleash a stink bomb. The RIFA filter caps in old computer power supplies are notorious for this, and aren’t something you want firsthand experience with.
Very high resistance is also a sign something is wrong. In that case, the most likely thing is the power supply just won’t work.
That covers the AC side. Nice, quick, and easy.
Checking the DC outputs
It’s perhaps even more important to check a power supply’s DC outputs for safety. For this, I like to put the multimeter on continuity mode. You shouldn’t see continuity between the voltage pin and the ground pin. If you have multiple voltages, check all of them against ground and against each other. You shouldn’t measure continuity across any dissimilar voltages or ground and any voltages. If you do, that’s a sign that one or more capacitors may be shorted. And if you plug it in, you can expect a loud pop and and maybe some smoke.
If you don’t find any shorts, it’s probably safe to plug the power supply in and check the voltages. Keep in mind most PC power supplies need a load in order to power on, so plug a hard drive you don’t care about into it before you try to turn it on if that’s what you’re checking.
Power supplies for many other computers don’t necessarily need a load. The voltages may be a bit off, but they will probably supply power. Plug the power supply in and turn it on and switch your multimeter to voltage. Measure each of the voltage pins to see how far off they are. It is very unusual for them to be perfect, but you’re looking for voltages to be fairly close. 5.1 volts is fine on a 5 vol t supply, but 5.35 volts is high enough to scare most people. If the voltages are too far off, you don’t want to use them as is, because it could damage your computer. Overvoltage on the 5V rail is a leading cause of death of Commodore 64s.