How do you discharge a capacitor safely.

Usually just short it with a screwdriver. CRTs can store a charge too.

You may get a big spark from a large capacitor, don't jump when it happens, you can do as much damage to yourself from ripping a hand on nearby sheet metal as from the actual electrical charge. I know from experience over manyyears and a few seconds of carelessness.

H. R. (Bob) Hofmann

the safe way to discharge capacitor are through resistor, use the resistor in minimum 10 watts with resistor 10megaohms for AC capaitor from 110v to 440v

for DC from 5 to 220v capacitor us 5 watts and 150kilo-ohms above 220v DC use 10mega ohms resistor value, use two long lead with insulation and one end ground to chase or PCB ground, and other lead to discharge the capacitor. take more care when discharging more than 110v capacitor which will produced high sparking when discharging.

I wouldn't argue with using a resistor to discharge caps, and also, on occasion, just whacking a screwdriver across them - although this can leave a bit of a metal deposit 'splat' on the PCB. However, the suggested 10 meg

10 watt resistor, or even the 150k, seems rather high to me. I think that you might struggle to even find 10meg at 10 watts.

I have a metal film resistor of 2k at probably 3 or 4 watts, that has lived for years on the bench screw-magnet, and serves to discharge any caps that I ever come across that remain charged due to fault conditions. That includes up to 500 volts in some PA amps. This resistor has never even got warm during a discharge. If you work out the math, yes, at first the resistor may be massively overloaded from a high voltage cap, but it is for such a short period of time that the resistor never has a chance to get hot, which is what its power rating is all about. A higher value resistor than this ( though not as high as 150k even ) will do a gentler job of it, with less of an initial spark, but will take correspondingly longer to do it.

Note also that many high voltage cap circuits have bleeder resistors, or voltage sharing resistors across them when they are stacked, and these should discharge the caps for you. You should always check that they are doing their job though, as they do have a tendency to go open. You can also discharge caps safely with an older analogue multimeter, which has a much lower input resistance than its modern digital counterpart. This has the added advantage that you can see the voltage dropping, but is somewhat slower than using a resistor of just a few k.

In general, the following circuitry should discharge the caps for you, and caps remaining charged will usually indicate a fault condition such as a switch mode power supply not starting up, and leaving the main primary filter cap charged to peak line volts, or a filter resistor between two caps open circuit, leaving the first one in line, charged.

Arfa

Just how much wattage do you think you are going to get out of a capacitor?

See the capacitor FAQ at the site below.

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This is generally frowned upon both due to the pits it will produce in the screwdriver, the noise, and potential damage to some types of capacitors. See the info at the site below.

--- sam | Sci.Electronics.Repair FAQ:

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Rightly or wrongly, I use a 240volt BC pygmy lamp with two leads fitted with prods soldered to the pads on the bulb. Works for me.

Ron(UK)

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xcellent choice, lamp provides indication and nice slope for current rate.

According to my calcs, the maximum power needed for a 10M0 resistor is

0.01936 watts.

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