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How to deal with a potentially dangerous charge on a capacitor.

Summary

Power supplies usually contain a reservoir capacitor which can hold a dangerous charge long after disconnection from the mains. You need to know how to discharge this safely.

Safety

Always treat charged cpacitors with great respect.

Main section

Before doing any fault finding, disconnect from the mains. Note also that even so, the reservoir capacitors on the mains side may retain a dangerous charge for a long time, which could potentially give you a lethal electric shock.

There may be several reservoir capacitors on the high-voltage side - you must make sure they're all discharged. Sometimes, most of the charge in the capacitors will have drained away through the circuitry attached to it or by leakage, but you don't want to discover it hasn't discharged by getting a shock, so start by assuming they haven't discharged.

First of all, taking great care not to touch any connections at all, locate the reservoir capacitors on the high voltage side and one-by-one discharge them using a capacitor discharger consisting of a suitably prepared high-power resistor with well-insulated wires (apart from the tips) - for example see [1] for more info about constructing a capacitor discharger.

Using a capacitor discharger won't reduce the voltage instantaneously - to discharge a capacitor value C (farads) to 5% of its initial voltage using a resistor R (ohms) will take 3*R*C seconds - for example a 100uF (100*10^-6 farads) capacitor charged to 300V being discharged through a 10Kohm (10^4 ohms) resistor takes about 3 seconds to leave 15V on the capacitor. A further 3 seconds would reduce that 15V to 5% of 15V, i.e. 0.75V.

Always double-check that the capacitors are in fact discharged using a multimeter - apply the capacitor discharger for longer to reduce the voltage further, aiming for 10V or less.

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