Discharging electrolytic capacitors

Question

I believe that capacitors are safe to discharge through a short, if

• the voltage is not high enough to cause a dangerous spark; and
• the energy stored in the capacitor (E = V × Q = V² × C, if I'm not mistaken) which becomes heat almost instantaneously, is not enough to damage the most sensitive parts of the circuit, which are probably the plates inside the capacitor.

What are ballpark figures for V, Q and/or E where it's still safe to short the component leads of electrolytic caps (the common variety, which I believe goes up to 4700µF 50V) saving the time to compute time constants and resistor values?

Answer 1

I would not suggest deliberately directly shorting anything much bigger than 1000uF/16V ~= 0.1J (1/50 or so of your example), though I sometimes do it when I'm in a hurry and "pretty sure" 99.5% it's discharged through circuitry or whatever but would rather risk taking a nick from a screwdriver tip than my finger or meter. You could damage the shorting element or the capacitor doing that if it's fully charged. I have some fat 5W and 10W wirewound cement resistors around to do it properly, when necessary. If you misjudge the surge energy badly compared to what the resistor is capable of it's possible to have them explode or silently open up leaving the capacitor with a charge on it.

Very large capacitors can hold a lot of energy and demand serious precautions. Sometimes just the dielectric absorption can result in a big enough charge to give a significant jolt. That happens when you properly discharge a large capacitor and then remove the resistor- the capacitor appears to partially re-charge itself due to the way the dielectric behaves.