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When I put it in the outlet it "popped" with some oily gray smoke. Honestly I thought that the voltage rating on the capacitor meant you could feed at least that much voltage through it, but clearly I'm missing something fundamental.

I am glad I tried this with a rubber gripped wrench.

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In addition to @Barry 's correct answer, most electrolytic capacitors are polarized. That is, they have a positive lead and a negative lead. If you hook them up backwards, they can fail violently! When attaching a polarized cap to AC, this applies a negative voltage for half of the time, which may be another reason your cap failed. –  bitsmack Jul 25 at 3:03
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You know what, I'm just going to be blunt: Samwise, you're an idiot. This is like testing how far you can jump starting with "off the top of a skyscraper". Mains voltage does not fuck around, and it will kill you. What exactly did you think you might learn by plugging a capacitor into a mains socket? What behaviour were you trying to measure or observe that you could not have investigated in a far less dangerous manner, like with a 9v battery or something? –  John U Jul 25 at 11:22
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@OlinLathrop What's "Alternating Current voltage" then, and how is it different to "Alternating Current current"? –  OrangeDog Jul 25 at 14:31
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@AdamDavis - I was simplifying out of some possibly misguided attempt to save Samwise from himself. Best to assume mains voltage WILL kill you and avoid pratting about with it until you know enough to know better, than assume you will likely be OK - you only get one chance to be proved wrong. Usually I don't interfere with the best efforts of natural selection but it's Friday and my boss left me in charge of the donuts. –  John U Jul 25 at 14:54
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@JohnU I'm not going to disagree it was a stupid thing to do, however I'm more than willing to come forward and admit (post on a forum like this) to doing a stupid thing if it means others can learn from it. –  samwise Jul 25 at 16:07

1 Answer 1

up vote 15 down vote accepted

The voltage rating on an electrolytic capacitor is for DC, not AC. The impedance of a 10 uF capacitor at 60 Hz is 265 ohms so it would draw about 0.45 amps. The combination of too much voltage and too much current will result in destroying the capacitor. In most applications of such capacitors, they follow the output of an AC to DC rectifier so they see an AC ripple voltage on top of a relatively high DC voltage. The ripple voltage will cause current to flow through the capacitor but it will be much less than what you subjected your capacitor to because the ripple voltage is only a fraction of the full AC input voltage. Most electrolytic capacitors have a maximum ripple current rating to prevent overheating. The voltage rating gives the maximum of DC + AC ripple that the capacitor can withstand. In general this should be derated by at least 25% to protect the capacitor and increase reliability.

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This is all true, but not the cause of the cap exploding quickly after being connected to the AC line. The real reason is that half the AC cycle will drive the cap with significant negative voltage, which electrolytic caps can't handle. –  Olin Lathrop Jul 25 at 11:42
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@OlinLathrop What evidence do you have that it was polarized, or electrolytic (other than it exploding, of course)? This answer could be correct. It could also be because the capacitor was faulty. There are a number of ways it could have failed and not been electrolytic, or polarized. –  Adam Davis Jul 25 at 14:13
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@Adam: The circumstantial evidence is overwhelming. First the vast majority of 10 uF 450 V caps someone will find lying around will be electrolytic. Second, any other technology would be physically large and would be able to handle the ripple current. It's really not that high at 60 Hz. Even if not, it would heat up slowly, not explode within a few seconds after being connected to the power line. –  Olin Lathrop Jul 25 at 14:27

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