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I have recently blown a capacitor on my home made emp. I have got the perfect replacement which is an electrolytic capacitor, 330v, 140 micro farads.

I have charged it but I want to learn how to safely discharge a high voltage capacitor.

I was going to short the capacitor but my friend said that high voltage capacitors can blow when you short them.

I don't have any resistors that I can discharge with.

Any ideas? Or can you short the capacitor without it blowing?

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    \$\begingroup\$ My answer, previous: "Drop it into a container of water", which I consider to be perfectly valid, was converted to a comment by some unknown moderator, for some unknown reason. If my answer was invalid for some technical reason I'd like to know what it was, otherwise I'll just chalk the downgrade up to the ignorance or the malicious mentality of the invisible moderator. \$\endgroup\$ – EM Fields Nov 17 '14 at 6:55
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    \$\begingroup\$ @NickAlexeev: There is no visible note and, consequently, the moderator's identity is masked. There is also no way to comment directly to anyone from the answer space. If you believe the premise of the question was flawed, then you should have taken steps to remove the question instead of supporting removing a valid answer. In addition, I don't believe it's within your purview to determine whether jury-rigging is proper or not, considering that it could save a life which, had your arbitrary "No jury-rigging allowed" guidelines been followed, might have snuffed it out. \$\endgroup\$ – EM Fields Nov 17 '14 at 7:44
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    \$\begingroup\$ @NickAlexeev: Well, since you've decided to come clean, why not go all the way and enlighten us all, technically, about why it's a bad idea to drop a capacitor charged to a moderately high voltage into a container of water to discharge it when there's nothing else around to do it? \$\endgroup\$ – EM Fields Nov 17 '14 at 9:48
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    \$\begingroup\$ Along the lines of @EMFields' idea- in a pinch you could stick wires in an open, but safely insulated, container filled with salty water. Salt water resistors were once used to control stage lighting. \$\endgroup\$ – Spehro Pefhany Nov 17 '14 at 10:09
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    \$\begingroup\$ @Mindrus, It's all about perspective :-) Saying "I don't have any resistors that I can discharge with." suggests you have run out of resistors and can not get any more. It might be an assumption on the readers part but you knew the answer was to get a resistor. So why ask the question if you can get some easily? This is where people jump in with an answer based on assumptions. This is to be expected as they are not mind readers. \$\endgroup\$ – Spoon Nov 17 '14 at 12:58
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A resistor is called for. If it's charged to 300V and you want to limit the current to 1-2A you need a 150-300 ohm resistor. DX has a suitable Dale-style 100W 200 ohm resistor for less than $4 including shipping, or you can get a genuine one for a bit more.

Peak power will be 450W

enter image description here

But a typical resistor of this type rated for 100W continuous can handle about 100J pulsed, and you have at most CV^2/2 = 7.6J.

enter image description here

As always, check that it actually is discharged before handling (and after that, maybe short it just to be double sure). Note that because of dielectric absorption, capacitors that are discharged briefly can appear to re-charge themselves somewhat. Probably not an issue with such a low voltage, but it can be enough to cause a jolt on high voltage capacitors.

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    \$\begingroup\$ Some capacitors can pick up a charge just from static electricity (or so I was taught in class). When in doubt, store large capacitors with a shorting wire across the terminals. \$\endgroup\$ – Alan Campbell Nov 17 '14 at 5:31
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    \$\begingroup\$ Are we in a hurry? I'd get a 100k resistor and clip it across the terminals and wait 60 seconds. And watch it decay on a voltmeter. It's about a watt at the very start but drops off quickly enough that I could get away with a 1/2W resistor. \$\endgroup\$ – gbarry Nov 17 '14 at 6:03
  • \$\begingroup\$ @gbarry: I had the same thought, except that I'd probably go a bit larger still (e.g., 470K), so the power would be low enough the resistor probably wouldn't even get slightly warm. Either way, even if you have to go buy one, a half watt resistor is about as close to free as a component can get. \$\endgroup\$ – Jerry Coffin Nov 17 '14 at 6:11
  • \$\begingroup\$ @JerryCoffin yes, just like the good old days. They sometimes put a bleeder resistor like that in there permanently. \$\endgroup\$ – gbarry Nov 17 '14 at 7:39
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Connect it across a hair dryer or toaster or soldering iron etc. I tend to use soldering iron.

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    \$\begingroup\$ I was thinking of answering with something like this, but a lot of modern soldering irons and other things with temperature control might be damaged. If you have a really cheap soldering iron from the dollar store it should work great (and not so great for soldering). +1 anyway. \$\endgroup\$ – Spehro Pefhany Nov 17 '14 at 10:11
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So your friend is correct as discharging by directly shorting it could be bad.

The previous answer has given a good solution already, discharge it through a resistor could do it.

For the last question, it is possible to discharge the capacitor by directly shorting it without blow up the cap. This is one of the safety demo my professor love to do. He will touch the two ends of a cap (which is the size of a bottle) with a metal rod. You will see a huge bright arc at the contact and a huge sound (worse then firing a gun). It is very very very frightening.

Also, if you accidentally touch the two ends of the cap while trying to short it. you are basically cooked.

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