Let's talk about testing capacitors

[Axtman]

I have a bunch of new NOS and pulls of electrolytic capacitors. If they have obvious bulges, leaks, or are really old, then I toss them. What puzzles me is that sometimes a newish capacitor will test okay on ESR but lower than stated value on capacitance. Granted I am using a Fluke 87V to test the capacitance not a fancy LCR meter. At what point is the capacitance too low and I should toss them?

[nosaj]

I think the fluke is only reading the value and not testing it. You want to test get you an eico 950 or similar so you can test at rated voltage.
nosaj

[mauman]

Most new caps have a tolerance of +/-20%, and your Fluke 87V has an accuracy of +/-1%. So a 100 uF cap could read anywhere from 79 uF to 121 uF and still be within specs. Are yours reading farther out than that?

[Chuck H]

As mentioned, the Fluke will only measure value. It can't measure relative goodness @ voltage. I have the same meter and no additional cap tester. But I don't do repairs. My own proclivity is to avoid NOS caps. Though there are some that are claimed to reform and test fine sold here and there. Unless you're willing to equip yourself with a capcaitor testing system that qualifies relative goodness for reformed NOS caps you could buy caps from these guys that do. Otherwise my boiler plate advice would be DON'T. The draw of NOS caps is the internet mojo and lore of OEM components. Since this is almost unattainable anymore anyway I don't see the point in troubling with it. And I don't think there's enough money to be saved in the effort for the end game. JM2C.

[Justin Thomas]

Most new caps have a tolerance of +/-20%, and your Fluke 87V has an accuracy of +/-1%. So a 100 uF cap could read anywhere from 79 uF to 121 uF and still be within specs. Are yours reading farther out than that?

And that's NEW caps. Old Mallories were either -80/+20 or -20/+80, I can't recall which direction the 80 was.

Jusrin

[Helmholtz]

Main problem with long time ecap storage is leakage current.
This will not show in capacitance or ESR measurement.

[Chuck H]

I have a bunch of new NOS and pulls of electrolytic capacitors. If they have obvious bulges, leaks, or are really old, then I toss them. What puzzles me is that sometimes a newish capacitor will test okay on ESR but lower than stated value on capacitance. Granted I am using a Fluke 87V to test the capacitance not a fancy LCR meter. At what point is the capacitance too low and I should toss them?

Ok. To address the question directly, will you ever actually use those capacitors? I had the same dilemma. I had a bunch of old electrolytics I hadn't used. Then a project would come up and I would think "Alright, I have these caps." And then I would decide the project was more important than saving a few bucks for the risk of using the old caps and possibly having a problem. So then I'd buy new caps specific to the project. And the old caps remained in their caddy slots taking up space. I did finally toss them after rationalizing that I wouldn't really use them.

If I have leftover caps after a project I'll keep them for a couple of years and then toss them. Sometimes I use leftovers but I don't trust them after about two years of disuse. So...

Are you really going to trust those caps in a repair or project? Or will you decide to avoid the potential trouble and buy new ones anyway? If you're not going to use them they aren't getting any better sitting there.

[TomCarlos]

Regarding "leakage" ...

I came across a four video series on YouTube where a guy builds a Capacitor Leakage Tester. Using his build ideas and box design, I came up with my own version of this (in lieu of buying an old Heathkit Cap Tester IT-28 or similar). In addition to the text box, I use a Heathkit 2717a Power Supply to provide the DC voltage. So my cap testing includes using meters to check the Cap value and ESR. But I also have this test box to check for leakage.

Here is the video series on the Leakage Tester.


Capacitor leakage tester design & build DIY - Part 1

Capacitor leakage tester design & build - Part 2

Capacitor leakage tester design & build - Part 3

Capacitor leakage tester design & build - Part 4

[R.G.]

I'm with Chuck. Are you really doing yourself any good trusting old, salvaged or unused caps? Would you really trust them? Are they worth the debugging time spent when one is marginal and develops high leakage in that repair or new project?

Electro caps have a shelf life as well as a working life. With no voltage applied, the aluminum oxide insulator "relaxes" and thins down. The capacitance goes up and the stable withstanding voltage goes down. New caps are formed up to a voltage that is mildly bigger than the rating so that they all pass specification. The withstanding voltage is a little high on new ones, because the oxide is thicker to allow for a few years on the shelf. Thicker insulation means the capacitance is a little low, too. In use, the applied power voltage on the cap is in the direction to repair any spots that get a little thin. So there is a mild self-repair mechanism in use. The cap still degrades, but more slowly, until it's decayed/self repaired to nearly equal the applied voltage. Over time, the decay wins, of course. But in use is better than on the shelf. Since the insulator is thinner, the capacitance goes up a bit.

Cap manufacturers specify shelf life on many electros.

It is possible to carefully apply a current limited voltage to old electros to imitate the original insulation forming process. This is the basis of much tube amp mythos, and can work to some extent if you're careful, and lucky. The working electrolyte in caps is not the same chemical composition as the original optimized oxide forming bath, so it probably doesn't work as well as a re-forming bath.

Electro caps are made by forcing an electrical current, carefully set up and limited, through bare aluminum foils. The polarity is set so that it causes formation of aluminum oxide on the surface of the aluminum. The current is used to force the aluminum to grow thicker layers than would happen with no electricity forcing it. The thicker the layer, the more volts the layer can withstand.
The insulation is not a pure insulator. Below some voltage, it allows only trivial leakage; this is the nominal rated voltage. Above that, leakage increases more rapidly with voltage. At some voltage, the leakage is so big that it forms hot spots in some places, and these places go into thermal runaway. Eventually it shorts.

[Leo_Gnardo]

Old Mallories were either -80/+20 or -20/+80, I can't recall which direction the 80 was.

Jusrin

Bonus capacitance at no extra charge (?!?) maybe up to 80% extra! These are the caps to buy. Where do you get a deal like that nowadays?

-80% would be plain unacceptable.

[Justin Thomas]

Where do you get a deal like that nowadays?

I get them in all those "aLl OrIgInAl FeNdErS aRe ThE bEsTeSt EvEr!" Amps I buy on De Farcebook.

Jusrin