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Old 10th February 2004, 08:50 PM   #1
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Default how do you discharge a capacitor?

As you know I'm a beginner and I want to know how do you discharge the capacitor before working on it? Anyone have any safety tips when upgrading or working with power capacitor?
Do you measure the capacitors on the board with a dvm? and how do you use a resistor to drain the capacitor charge??? I don't want to get SHOCKED!
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Old 11th February 2004, 03:07 PM   #2
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You can use 1k/5w resistor (1 kilo ohm 5watt) I would put longer insulated leads on it so you're sure you won't touch anything accidently. In general, how bigger value in resistance how slower discharge.. Never discharge caps by shorting them.. They hate it... Hope this helps..
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Old 11th February 2004, 05:16 PM   #3
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I use two 40W / 230V light bulbs in series with 2 Hirshmann “Kleps” safety isolated grabbers mounted on the leads for discharging large caps in tube amps. For discharging PSU caps I leave it connected for more than a minute.

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Old 11th February 2004, 05:25 PM   #4
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<Mr. Unsafe> I always let 'em discharge, then short whatever's left.. otherwise... just short 'em.. </Mr. Unsafe>

Safe advice is yes, use a resistor. Pick a resistance such that 5 to 20 time constants pass in say 1 to 15 seconds (depends on how impatient you are ). If you want to discharge in 5 seconds, each T.C. is going to be 1 second... if the cap is 100uF, that's 1sec = R ohms * .0001 farad, uhm R = 1s / .0001F = 10kohm. Power rating adequate for releasing that energy in that time... 100uF at 450V is 10J (E = 1/2 V^2 C ), which is 2W average over 5 seconds. It's an intermittent thing so I wouldn't worry about using too small a resistor here.

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Old 11th February 2004, 05:51 PM   #5
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Lightbulb lightbulb

I always use light bulbs too, they can handle a lot of power and you can see when the cap is almost discharged.

best regards

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Old 11th February 2004, 05:57 PM   #6
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Originally posted by Sch3mat1c
It's an intermittent thing so I wouldn't worry about using too small a resistor here.

Hi Tim,

Depends, the resistor needs at least capable of handling the peak current, otherwise there is a risk the connection between the resistor element and the end caps will break. So don’t use film resistors for it. Best is to use a wire wound resistor of 10-20W.

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