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Discharging a Capitor? Answered

I have this camera I'm taking apart. The flash unit  has a HUGE capacitor for flashing. When I was taking it apart, I got a shock. I'm fine but I dont want it to happen again. How long will it take for the capacitor to discharge by itself if I leave it for a while?



Best Answer 7 years ago

It may take years for some capacitors to discharge, actually.

The next time you see a large capacitor like that one, it is a _very_ good idea to manually discharge it by connecting the two leads with a screwdriver.

For small caps, the 'short it with a piece of metal' method is fine.

For really big ones not.
First, most of the energy will be released immediatelly which might heat up the cap and damage it and/or weld the tool you use to the terminals.
Second, there are chemical processes after discharging going on in the cap, so that it seems to magically recharge itself.

Solution for big caps: Use tweezers/pincers to short the cap with a resistor. 10/100/1000 Ohm, depending the voltage. Just make sure, that the resistor can handle the current. Leave the resistor on the terminals for at least an hour, over night is better.

I like this answer except for the part about "at least an hour, over night is better".That part is kind of naive, because the transient voltage on a capacitor discharging through a resistor like that falls to 1/e its previous value every R*C seconds. No really, it does. You can pick R*C to be significantly less than an hour, or overnight.

My answer was for really big electrolytic capacitors.
The 1/e discharge is true for the energy that is stored in displaced charged parts (ions, electrons etc.).
But part of the energy is stored by (reversible) chemical processes. When discharged, these processes are reverted and produce new charge and therefore voltage.

Try it: Take a big elko (say 100000µF,25V), charge it completely for some minutes. Attach a high impedance volt meter to the terminals (should read about 25V) and short it (0V). Remove the short and watch the meter: It will go up from zero as magic! How good this works depends on the capacitor, the way it was loaded, the impedance of the volt meter, etc.

The R*C stuff is correct, but doesn't take into account the chemical part of caps.

Remember: In theory, theory and praxis are the same, in praxis, not.

+1 (outside of the quibble that I think you meant "practice" rather than "praxis"). Big electrolytics should be STORED shorted, both for this reason and because it's possible for them to pick up enough charge electrostatically to become a problem.

Also, slight quibble about picking R*C. Don't forget, faster discharge means more heat, which can do unpleasant things to the capacitor, the resistor, the wiring... Otherwise known as "Don't short the terminals of a big electrolytic with a screwdriver unless you want a screwdriver welded across them, and don't ask why we know this."

Well maybe I'm the one being naive, by ignoring this slow chemical process. Still it seems like a long time to wait.

I would have picked you for best answer.

Just short the terminals with a piece of (insulated) wire, or a small resistor.It will discharge immediately.

Thanks for the answers guys!