Will a capacitor automatically release its energy over time? Or will it stay in there until manually discharged?
So let's say I've had an old computer sitting around for a year and decide to take every piece apart... am I in danger of being shocked by the capacitors?
Answer
In theory it will. If an ideal capacitor is charged to a voltage and is disconnected it will hold it's charge.
In practice a capacitor has all kinds of non-ideal properties. Capacitors have 'leakage resistors'; you can picture them as a very high ohmic resistor (mega ohm's) parallel to the capacitor. When you disconnect a capacitor, it will be discharged via this parasitic resistor.
A big capacitor may hold a charge for some time, but I don't think you will ever get much further than 1 day in ideal circumstances. You should watch out if you have turned on the PC just 'a moment ago', but if you let it unplugged for a couple of hours and it will be fine.
The capacitors in the mains power supply are the most suspicious, these contain high voltages and high capacitance. If you don't know for sure, measure them. You can short them out if you find something, like the device Nick shows.. (it's probably a high voltage 1 kilo ohm resistor or something with some wires and isolation). But I suspect those are quite expensive and more designed for really high voltage situations (like kV's).
Or if you dare an old isolated screw driver (watch out for sparks though! :-) ). But I think it's obvious a very direct short will not enlarge the life of the components.
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