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Capacitor Protection?

Thread starter #1
Hello,

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?

Pictures

Thanks so much for any insight.

Alex
 

dknguyen

Well-Known Member
Most Helpful Member
#2
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.
 

unclejed613

Well-Known Member
Most Helpful Member
#3
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.
 

unclejed613

Well-Known Member
Most Helpful Member
#4
a 9V battery can't source enough current to smoke those caps anyway.
 

MichaelaJoy

Active Member
#6
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.
 

unclejed613

Well-Known Member
Most Helpful Member
#8
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...
 

unclejed613

Well-Known Member
Most Helpful Member
#10
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
 
#12
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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