Bad capacitors -- how fast do they rot after testing out OK?

[larrymoencurly]

If an electrolytic capacitor tests out OK with an ESR meter, is it
normal for it to go bad (high ESR, bulging) just two weeks later, even
if it's one of those Taiwan brands that was made with defective
electrolyte?

I noticed that my computer's 2-3 year old 300W Antec power supply had
a leaking electrolytic, so I replaced it and checked ESRs of all the
other caps. They all measured less than 0.05 ohm (out of circuit --
too many in parallel), with no leaks or bulges, the bad one about 0.6
ohm. Two weeks later, I just happen to have the power supply apart
(no problems with it) and see that one of the capacitors that tested
OK is now bulging, and its ESR is 0.15 ohm. This computer isn't a
power hog but draws only 50W or so (1A @ +12V, the rest from +5V), and
I don't think the power useage or temperature have changed recently.

 

[Tim Wescott]

If an electrolytic capacitor tests out OK with an ESR meter, is it
normal for it to go bad (high ESR, bulging) just two weeks later, even
if it's one of those Taiwan brands that was made with defective
electrolyte?

I noticed that my computer's 2-3 year old 300W Antec power supply had
a leaking electrolytic, so I replaced it and checked ESRs of all the
other caps. They all measured less than 0.05 ohm (out of circuit --
too many in parallel), with no leaks or bulges, the bad one about 0.6
ohm. Two weeks later, I just happen to have the power supply apart
(no problems with it) and see that one of the capacitors that tested
OK is now bulging, and its ESR is 0.15 ohm. This computer isn't a
power hog but draws only 50W or so (1A @ +12V, the rest from +5V), and
I don't think the power useage or temperature have changed recently.

I don't know, but it sure bears out the general rule that if you have a
group of components with one bad, you should replace the whole batch
whether they be brake shoes or capacitors. After all, the time to do it
is when everything is ripped apart anyway.

 

[Leonard Caillouet]

I find leaky caps that test fine for ESR all the time. If they are mounted
so that you can't see the legs, which most larger caps are, you have to
perform the "smell" test or remove them to inspect them. The smell test
consists of heating the legs and noting whether you smell bad tuna or not.

When in doubt replace. A few caps is not worth the time to have to go back
into the unit a second time.

Leonard

 

[Dan Whelan]

Ok, to answer your questions...no. It's not ok for capacitors to
bulge.

A polarized electrolytic capacitor will start to bulge IF:
- it's in a circuit backwards (i.e. wrong polarity)
- rated voltage is exceeded

It will likely NOT bulge if there is excessive heat...however it'll
either short or go open.

By the way, this was a problem with every IBM Netvista PIII computer.
All the capacitors near the CPU would bulge, and the computer would
freeze. Rogers Video uses these computers (which is why they have so
many problems while processing transactions).

Dan

 

[larrymoencurly]

I find leaky caps that test fine for ESR all the time. If
they are mounted so that you can't see the legs, which most
larger caps are, you have to perform the "smell" test or
remove them to inspect them. The smell test consists of heating
the legs and noting whether you smell bad tuna or not.

When in doubt replace. A few caps is not worth the time to have
to go back into the unit a second time.

Thanks for the information. I wrongly thought that if a capacitor had
low ESR and wasn't shorted that it was OK. That's interesting about
the smell test.

I was going to replace all the caps, but the only ones available
locally are either too fat, mystery brands, or old enough to have been
made at the same time as all those defective Taiwan caps.

 

[Leonard Caillouet]

Actually, there are other failure methods. Some caps will test fine for
capacitance and ESR but have high dielectric absorption and/or dc leakage.
These are hard to test for without the proper equipment. Like I (and
others) have said, when in doubt, replace electrolytic caps. They are cheap
and plentiful. You should be able to find the right caps from one of a
number of suppliers, most likely DigiKey or Mouser.

Leonard