Stressing E-caps....

[mook]

At what point to you feel you've exceeded the limits of an electrolytic? Say if an e-cap was rated at 500v, would you set the limit at 485v? 500v? 520v?

I've had 500v e-caps working fine with a load of 525v on them, but I'm hesitant to keep this the way it is. I feel like doing the caps-in-series-with-bleeder-resistors trick.

what's your max stress voltage for an electrolytic?


Mook

[Albert Kreuzer]

usually electrolytics can take 10% above their rated value for a short time (startup).
But consider that your mains voltage may vary +/- 10%, in some areas even more.
So I would stay at 10% below the rated value, i.e. 450V for a 500V cap.

Cheers,
Albert

[tedmich]

lifespan of the EL decreases only geometrically with rated voltage over actual voltage, while its an exponential decrease with temperature; so keep them cool and you can get reasonable lifespan above their rated voltage, see:
Life Calculators - Illinois Capacitor

CDE 381LX have a 12,000h lifespan at their rated voltage, and still pretty damn good at voltages 2x that! (IFF KEPT COOL!)

[Enzo]

e-caps have a "working voltage" rating and a surge voltage rating. A 450WV cap - 450 working volts - is made to run all day with 450v on it. Most caps today don;t have WV printed on them, but look at a lot of old can caps and you will see that.

SO making up an example your 450WV cap might have a surge rating of 520v. SO run it in a 450v circuit, but be wary of the 520 for situations like when the B+ comes up high before the tubes warm up. That would be your surge.

[Bruce / Mission Amps]

Just an observation

Just an observation... I have a 350ma 500vdc variable power supply and when messing around one boring day, I found it amazing that most import 50v and 100v E-caps would not overheat and blow up at 100vdc-150vdc but would blow up instantly with 120vac.
Other cheap import 450v Ecaps sat there seemingly forever at 500vdc and never really even got warm... while some old replaced Illinois 350v Ecaps started to really warm up at 450vdc and would eventually make a hissing sound .... but they didn't blow open either... except that I have opened reissue amps using them and seen them with their pooped out wet/oily toilet paper.
What does this all mean?
Maybe I didn't let them cook long enough... possibly it takes hours.

I don't know, but it was the start of me questioning the use of grossly over priced Sprague 475v and 500v filter caps much anymore.
I've been using hundreds of the import TAD 500v caps and haven't had a bad one yet... but I have had a handful of bad Sprague 475v and 500v caps and quite a few bad sections in the AES/CE Dist multicaps.

[R.G.]

DC electros start leaking exponentially as the applied voltage exceeds their formed voltage. Electros are formed to a voltage where the leakage is below X at the specified voltage. This voltage is selected so that after five years of storage, the UN forming of sitting on a shelf does not make the leakage exceed Y when it's put into service at its rate voltage. The use at its rate voltage re-forms the insulator to the working level.

The surge voltage is the voltage where the leakage does not exceed Z. Z is a higher leakage than X, and does not cause excessive dissipation over a short period, that being the time specified for the "surge" to exist. Z is usually just below the knee of the exponential increase, and does not cause the cap to overheat from leakage if the surge is short.

Aluminum oxide on aluminum foil is a kind of metal oxide diode. One direction it blocks current flow, the other it lets current flow through. Remember copper oxide rectifiers? Same thing. DC electros blow semi-instantly on AC because one polarity is unopposed by insulation, so very large currents flow on the reversed direction and the resulting heat destroys the cap quickly. Reverse polarity on an electro makes it a sloppy shorted diode.

AC electros have insulator formed on both foils, in opposing directions, so there is one insulator stopping current flow for each polarity.

[jmaf]

I found it amazing that most import 50v and 100v E-caps would not overheat and blow up at 100vdc-150vdc but would blow up instantly with 120vac.

Probably dumb question but, it wasn't a polarized electrolytic, was it?

[Bruce / Mission Amps]

Probably dumb question but, it wasn't a polarized electrolytic, was it?

Well of course they were polarized Electrolytic caps... I just wanted to blow stuff up.

[jmaf]

Well of course they were polarized Electrolytic caps... I just wanted to blow stuff up.

Hihi. Well now I've found something interesting to do on bored sunday afternoons.

[R.G.]

Hihi. Well now I've found something interesting to do on bored sunday afternoons.

But it smells terrible when they vent.

[Chuck H]

So do I, but it's still fun

Chuck

[jmaf]

So do I, but it's still fun

Chuck

OK so I just burst out laughing at the computer again.

But it smells terrible when they vent.

I've gotten the computer one in the wrong socket more than once here in 220 voltland.

You know what's worse: it venting through central air conditioning. It's like you started a restaurant busines inside the computer room.