Lifespan of old NOS electrolytic capacitors.

I have a pair of giant 15V 130,000uF capacitors, each about the volume of a pint. I have had these for about 30 years and can't even remember where I got them from, possibly a surplus store. They are new, no oxidation or bulging or any obvious cosmetic problems. Are caps like this good for anything? Are they likely even good? I will hook it up to my LCR meter, but I wondered what you all though. What should I use them for?

Before you use them it is best to allow the capacitor's dielectric to reform. To do this connect each to a power supply through a 10K resistor and allow it to "charge" for 24 hours.

Grumpy_Mike: Before you use them it is best to allow the capacitor's dielectric to reform. To do this connect each to a power supply through a 10K resistor and allow it to "charge" for 24 hours.

Cool, I will try that. Now, for the dumbest question of my career as an electronics enthusiast, 10K on the positive terminal or negative terminal or it doesn't matter or both?

It doesn't matter, it is just limiting the current into the capacitor. If you stick it on full load first off then the capacitor might over heat and blow.

Here is the data if anyone cares:

MALLORY MADE IN U.S.A. :grin: TYPE CGS 130000 MFD 15VDC POS +85C MAX SURGE 20VDC 20-97563 235-7908B

the other one is the exact same but the last code is 235-7908*A* instead of B

I wonder if that last is a date code indicating 1979 vintage, which it feels like this could be. They are still in beautiful cosmetically new condition.

Screw terminals. Looks like quality. Hope it works.

Also if you want to be extra careful try charging them via the resistor first to 5V for a while and then measure the leakage current (calculate from the voltage drop across the resistor), take more readings until this stabilises.

Then up the voltage to 10V and repeat,

Finally 15V

Reforming involves re-building the Al2O3 oxide layer that tends to deteriorate over time, this oxide layer is the insulating layer in the capacitor.

I'd recommend wearing eye protection while doing this, a faulty cap may burst, spraying corrosive electrolyte steam and aluminium shreds out.

MarkT: I'd recommend wearing eye protection while doing this, a faulty cap may burst, spraying corrosive electrolyte steam and aluminium shreds out.

Good thing I have home insurance!

JoeN:

MarkT:
I’d recommend wearing eye protection while doing this, a faulty cap may burst, spraying corrosive electrolyte steam and aluminium shreds out.

Good thing I have home insurance!

You forgot to mention not only is the electrolyte nasty stuff, it smells horrible. Roughly like cat piss. The advice you received though is 100% correct. With some careful reformation, they should work just fine. Assuming of course that the electrolyte hasn’t dried out or any other physical failures don’t manifest themselves.

My Standard method for old electrolytics was to reform them as advised above.. But measure the drop across the resistor at the end of the 24 hour cycle and if not zero leave it in place for another 24 hours.. If at the second reform cycle the drop hand't improved it. It was leaky, I'd just toss them.. Devices that passed the DC check were measured for the actual capacitance and If that was within the typical +80/-20% then measure the ESR.. That's a big really important part and failing any of the test's effectively disqualified the part for anything but a door stop..

Doc

PedroDaGr8: You forgot to mention not only is the electrolyte nasty stuff, it smells horrible.

It smells mainly of aluminium, which is indeed pretty pungent(*), but its acidic too which is enough to cause nasal offence!

Aluminium dissolves in alkali, but in acid forms a protective oxide layer, which is handy for a capacitor (Al2O3 is about the best dielectric out there!). This same effect is used in anodizing.

(*) try filing or hacksawing some aluminium and small the freshly exposed surface

Selenium rectifiers burning, now there is a smell you will never forget.

Grumpy_Mike: Selenium rectifiers burning, now there is a smell you will never forget.

Ahh, selenium. One of the few things worse smelling than sulfur compounds:

http://pipeline.corante.com/archives/2012/05/15/things_i_wont_work_with_selenophenol.php