Blowing up caps....

[eliktronik]

Ok, so I finally wired together a Little Gem w/ a power supply ckt and all. I put 2 1000uf 25v in parallel across the +/- 11v supply. Interestingly enough, I had a ammeter connected in series w/ the ckt (in place of the fuse that I burned earlier w/ another ckt) and slowly watched it go from 30mA to 110mA, and see one of the caps start to smoke. This is a cap that I "recovered" from another ckt and maybe I damaged it while desoldering it, but I'm interested if anyone else has some opinions. Is this caused by a defect, since only one cap blew up? Any thoughts?

[drew]

No idea on the topic subject (my apologies) but I wanted to say that 120VAC is perfect for exploding electrolytic caps with a loud pop and a shower of bits of aluminum foil and paper.


drew
www.toothpastefordinner.com

[smoguzbenjamin]

Try a new cap and triple check if you orientated it correctly. Also check that the cap's been rated at the right voltage. If it still blows, you've made an error in your ckt somewhere.  

[Ge_Whiz]

Why would you want TWO 1000uF capacitors for that PSU anyway? Methinks you should have put one capacitor from ground to +11V, and one from ground to -11V, carefully observing the polarity (ground is positive wrt -11V, +11V is positive wrt ground).

[Peter Snow]

Whilst on the subject of capacitors rescued from older equipment, here is a piece of info that I picked up somewhere along the way - I think it was over on the synth DIY forum a few years ago. I haven't verified it myself but it is worth keeping in mind when using "previously enjoyed" capacitors:

If a capacitor rated at a certain voltage (say, 50V) is placed in a circuit where it is subjected to a lower voltage (say, 30V) for a long period of time, it seems to adapt itself to the lower voltage and the rating effectively drops to 30V. No big deal right?... so long as you do not now take that cap, see the marking is for 50V and place it in a circuit running 50V, because in essence the cap is now only a 30V unit. The results can be explosive  :?
Anyone else heard of this?  

Cheers,

Peter

[Ansil]

me...

[ExpAnonColin]

I suppose that the cap getting lower is perfectly possible, but I've never heard of it before... iiiinteresting...

-Colin

[eliktronik]

Thanks for the replies guys. My original idea was that since caps add up in parallel, having more capacitance would somehow provide a smoother voltage source. Now I realize that the caps must have been originally oriented across each individual +/- supply and ground. I guess this evidence supports the theory Peter brought up, since they were used for a while w/ a potential difference of around 11v, and exploded when I gave it a v of 22.... Interesting stuff.