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Capacitor Protection?

Thread starter #1

Sorry to ask such a small and simple question here guys but its a potential safety risk.

Recently I applied 9v in reverse bias to a moderately large presumably Electrolytic Capacitor. My question is, did the black and metal small device at the top of the capacitor (it's a diode I belive) save the 2 capacitors? I applied 9v @ 5A for roughly 4 seconds.

They are large enough I would rather not use them at risk of failure however did the diode properly protect the capacitors?


Thanks so much for any insight.



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If it is a diode and was shorting the current across the capacitor in the direction you applied the reverse voltage, then yes. But those don't look like diodes to me. I don't know what they are, but diodes seems unlikely. I would normally expect something like that to be a bleed resistor which would not have provided any protection, but those don't look like resistors either. Somehow else probably knows.

I think electrolytics can handle a little bit of reverse voltage though and survive.


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looks like it could be a pair of diodes. even if it's a TVS to prevent too much forward voltage, when reversed, it acts like a regular diode, so all that appears across the cap is 0.7V.


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a 9V battery can't source enough current to smoke those caps anyway.


Active Member
They really don't look damaged to me. Check for swelling.

Since you have them apart, make sure they're discharged and check them with a meter.
Also check those diodes. See if they're shorted.
Thread starter #7
Thanks everyone for the help, I've decided to charge them in a vented area and nothing happened. They hold a charge well higher than expected so I'm going to assume they are likely okay. Ill just put them on a DC pulse all night with some restive load, if they blow ill hear them.

Again I appreciate the help.


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They really don't look damaged to me. Check for swelling.
actually, if they had smoked, they wouldn't swell, there's a little rubber vent plug on the bottom of the cap near the screw terminals. i would point it out, but the pictures have been deleted from imgur...


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having seen caps like that smoke before, it is quite spectacular, a lot like a fog machine turned all the way up or a military smoke grenade where you have this high velocity jet of smoke spewing out through a small hole.
Thread starter #11
Good points, yea im not sure why 2 capacitors would be removed from Imgur :/

And I'll take the stink and the smoke over a small hand grenade haha
Electrolytics can take a lot of reverse voltage without damage for a fairly long time (minutes) they heat up and blow. If they got damaged they would have let you know. 4 sec is no problem. The solid state parts blow first to protect the fuse. hehe. or in this case the caps.
I wouldn't even worry about them.

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