Hey guys, I have a 650 watt Computer PSU that never worked from factory. I took it apart and pulled the 2 big capaciators out. They are rated 820uf at 200v. I tested both of them and one of them is showing 680uf whiles the other is showing 655uf. IS this normal or is that out of the tolerance range and a sign that they are bad.

Also if it helps, I did resistance test on both of them. One shows 3.5mega ohms while the other shows about 32 kiloohms. I dont want to simply buy them as they are about 6 dollars a piece.

Thanks, Kuljot Dhami

Caps are very difficult to manufacture to the actual specs labeled on them. Plus or minus 10% is doing good. 20% is typical from my experience, especially when larger voltages are concerned. These appear to be within that 20% range, or close to it. Resistance is not a good way to check a cap. It's like checking the resistance of a battery. If you swap the leads on a cap, you should see widely different values and it will change as you leave the leads connected.

If you temporarily short the leads and ensure the cap is discharged, you should read a high resistance once you remove the short. It should come down as you continue reading.

The best way to test them is to install them in an oscillator circuit and compare the frequency between these and a known good cap. (Unless you have an actual tester for that purpose, which most people do not.) Even though it is rated for 200V, lower voltages in a test circuit will act the same for the value, regardless of the rated voltage.

@Mongo Thanks for the reply. On one of the caps I saw a tiny tiny hole that looks like a needle poke. And there caps arent the traditional aluminium vent preforated. So that tiny little hole only being on one of the caps made me suspicious.

Depends a lot on the physical size or the cap. Some are vented through one end but if it looks like a puncture, I would shy away from it. That could be damage and not manufactured into it.

I have had caps explode in my time, usually in high power circuits and capacitive braking of motors. Fortunately, they have all been inside control cabinets and nobody got hurt. But they sure make a mess in the cabinet. ribbons of foil all over the place.