Help! Almost killed by exploding capacitor! What did I do?

[konduct]

Hello...I exaggerated little. Not really killed, however a capacitor exploded on me, shooting me in the hand with EXTREME FORCE!  I am mostly a mechanical experimenter so electrical circuits are new to me and I would like to know if some of the electrical oriented folks here can help me understand capacitors better so I don't hurt myself.

What I was trying to do was to test how much voltage was stored in two capacitors connected in parallel.

I hooked up a 12V  ac>dc power supply to a VERY SIMPLE circuit with two capacitors in parallel. One cap was 220V and the other was 16V (They are both in the trash now so I don't know the uf rating? small though...) The smaller 16V cap is the one that exploded...the casing flew straight up into my hand, leaving a nasty mark. I had a multi meter hooked up to both ends of the wire coming from the caps and was tapping the positive of the power supply to the "input"? of the circuit when the cap exploded.

What did I do wrong? Is it not wise to hook up two different voltage capacitors? Will a single capacitor charge HIGHER that the supplied input voltage? How does a 16V cap explode from 12V input!?!?  I would think you could even short out a 12V cap and it wouldn't explode like that?!?

Thanks in advance for any help with this.

[broli]

Either you did nothing wrong and the cap as just waiting to commit suicude any minute. Or you misread the power supply stating 120V instead of 12V  . If you can confirm with certainty that the provided voltage is 12V then I would suggest to repeat the same experiment with the same cap this time also shielding it to protect yourself. But I think you just had a bad cap with some faulty dielectric or something.

[Thaelin]

 First check the open voltage of your power supply. I have a 12v that is actually 18v.

Second, the cap may have been faulty and did an internal short.

  Or, it was connected backwards which will definately blow them up. We used to
do that in tek school for fun. Some go off like a shotgun.

Make sure you do not breathe the fumes from them, really bad for you. Be very sure
of the hook ups to them. Hopefully you had your safety glasses on too, right?

thaelin

[TinselKoala]

Sounds like you had a cheap electrolytic capacitor hooked up with backwards polarity, and you gave it a surge, and it blew. Most electrolytic caps have pressure relief features (that "K" mark, for example, is a designed failure area) so that when they heat up they just fizz violently and let the smoke out. But sometimes there isn't a pressure relief or it fails to work properly, and the can explodes.
When working with electrolytic caps be sure to respect the voltage rating (including potential surges!) and polarity.

Now, I once blew up a big Maxwell oil-filled pulse capacitor, from overvolting and back EMF--it was charged to well over its 30 kV rating...it caught on fire, blew oil all over the place, and nearly really killed me with its residual charge after I put the fire out...

[konduct]

Okay...that helps a little...and no....didn't have any safety glasses on!    The outside casing blew off straight up into my hand thankfully only leaving a welt. The voltage is actually a little lower than 12V...11.88 on the multimeter.

I think I've been hooking them up backwards the whole time by the sound of it.  Is the side with the dashes on it the negative side?  I didn't think they were so much polarized as they were in and out.