Electrolytic Capacitor Blown up

[wiredmonkey]

Hi all,

I recently re-assembled a gainclone amplifier basically as per the datasheet. I noticed I had some very weak bass transients which didn't quite make sense to me. Anyway, after listening for about 15 minutes an electrolytic cap went kaboom. It was one of the supply caps on the circuit (Cs), not the main supply filtering cap. The cap is meant to be rated at 50V and the supply is about 37V +/-.

I've only ever heard about capacitors blowing when being used above their rated voltage. Is it common for cheaper (this was a cheap cap) to have this issue?

I'm a bit reluctant to replace and try again until I get some other thoughts.

Cheers!

[Joris]

Maybe I'm totally wrong, but did you observer the polarity before mounting it on the pcb? Caps don't like wrong way currents... Caps, even cheap ones, can handle voltages of 10% up to their rating. Don't know what quality your caps are though...

[wiredmonkey]

The polarity was definitely correct as per the marking on the caps - The cap may have been printed on incorrectly? The quality is just your run of the mill 2200uF cap 50V from a local electronics shop.

I've used plenty of them in other applications with no problems so is it possible this was just a dodgy cap?

[Mooly]

If the DC voltage across the cap is within the rating, and the ripple current passing through the cap is also within limits (a duff or wrongly wired bridge would do that) then it has to work OK.
New faulty parts... it shouldn't happen if they are of reputable manufacture. Loads of fake stuff around.

[hybridfiat]

Ive only had electrolytic caps explode when connected back to front or fed too many wheaties

[Redshift187]

I've seen pictures of fake capacitors that were just a case over a lower voltage rating capacitor. If you got it at a local store, I would hope it wasn't a fake no-name capacitor

[AndrewT]

Hi,
an old electrolytic can have degraded to such an extent that it must be reformed before applying rated voltage to it.

If reforming is required and not done then the leakage current can be dangerously high and since these components are not designed to be self healing then explosion due to excessive heating could, in very rare occasions, occur.

[gfiandy]

It can happen with old electrolitics but normally you are talking valve amplifer old for this to be the case . I would guess it was incorrectly labeled, probably the wrong way round as caps can survive quite a high over voltage for a reasonable length of time especially if as you describe they are not the main smoothing cap so the ripple current is fairly low.

Regards,
Andrew

[wiredmonkey]

I'll just put my tail between my legs now.... Turns out I had connected it (in the correct polarity at least) from the positive to negative rail instead of positive to ground. Just a complete mental blank on my behalf when I put it all together.

Anyway, the poor 50V cap which I was so quick to judge had about 75V across it. Sorry cap!

[tomchr]

Well... Let's just say you took it out to the farm so it could run free...