Under rated electrolytic cap. Should I expect immediate or delayed failure? - Page 1
Forum section: electronics / beginners

author: Dangermouse
author_rank: Newbie
timestamp: January 12, 2018, 07:13:42 am
content: Hello. I was looking for a little experienced wisdom here. I figure there's probably no exact answer without more specific details, so I'm just looking for experienced guesstimates.I just replaced a number of bad caps off a power supply board. Two of the larger ones (not LARGE, but larger - 100uf 200v) I don't have on hand and have to wait a week for replacements (or drive across town). I started to wonder... how quickly would say a couple of cheap 50v rated caps likely last before failing in those spots???The closest 100uf electrolytics I happen to have are some much smaller 50v ones. If I installed the 50v caps temporarily in order to power the board up and make sure nothing else needs replacing, would it likely work for a while (minutes? hours? days?) ? Or would the 50v caps in a 200v application likely go up in an immediate puff of smoke?Thanks for your time,Dan

author: tautech
author_rank: Super Contributor
timestamp: January 12, 2018, 07:23:14 am
content: Poooooof.Don't do it.

author: Brumby
author_rank: Unknown
timestamp: January 12, 2018, 07:26:31 am
content: Quote from: Dangermouse on January 12, 2018, 07:13:42 amIf you only get a puff of smoke, I would be surprised.I would not recommend trying it - unless you want to video it and show us what happens. (Keep your distance.)It'll be a short video.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 12, 2018, 07:29:25 am
content: Quote from: Brumby on January 12, 2018, 07:26:31 amI agree. They won't go up in an immediate puff of smoke.They'll probably go up in an immediate blast of shredded former capacitor. It will be a late capacitor.

author: Dangermouse
author_rank: Newbie
timestamp: January 12, 2018, 07:38:01 am
content: Seem to have a consensus. Bad idea. Thanks for the responses. Guess I'll just set it on a shelf for a week.

author: tautech
author_rank: Super Contributor
timestamp: January 12, 2018, 08:18:29 am
content: Quote from: Dangermouse on January 12, 2018, 07:38:01 amWise move.When you're close to one that blows its clogs you'll know to use them wisely and always respect them.Those that are placed across rails can really go off with a bang.....and filling your pants is the result.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 12, 2018, 09:10:33 am
content: Electrolytics typically are surge rated on the order of 10%. More than that and they start to heat up, boil and explode.Film caps vary. The low cost, small size parts are higher density than ever, with clear sacrifices in ESR and surge voltage rating. (Note that a X2 type film cap may still be rated for some kV, surge -- this is a destructive test however, inducing self-healing. Extended time at high voltage will eventually self-heal the electrodes apart, and the ESR and capacitance go to crap.) Higher performance parts will be rated for typically 1.5 x nominal, and tested for a second or a minute, without significant damage.Supposedly, some aluminum polymer caps are self-healing; last ones I tested just failed shorted (and very smelly), though.Tim

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2018, 04:18:09 pm
content: 150V on a 50v electrolytic, and 100uf at that, will result in nothing for the first half second, then there will be a loud bang, a very fast moving capacitor case moving away from the board, and a shorted set of terminals, all the time shedding little bits of boiling hot caustic electrolyte and bits of boiled capacitor foil and paper insulation.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 12, 2018, 04:33:41 pm
content: Most modern electrolytics made byreputable vendorshave safety vents which allow a non-explosive failure to occur. Although it may still vent violently.Nevertheless, there will be plenty of paper and aluminum-foil shreds littering your work place, and the foul, acrid smell of vaporized electrolyte will linger for a while.