How to safely discharge capacitors

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 it safely.

Safety

Always treat charged cpacitors with great respect.

Where you will find them

Switch mode mains adapters (i.e. those without a heavy iron-cored transformer) use a reservoir capacitor to smooth the mains after it has been rectified. This will be charged to over 300V.

Cameras containing a discharge tube flash (rather than a LED one) and separate flash guns contain a circuit which generates a voltage of hundreds of volts in order to create the discharge. This is stored in a reservoir capacitor until needed.

Microwave ovens contain a reservoir capacitor which is charged to thousands of volts. Needless to say, this is extremely dangerous, and our advice is to leave any repair to a professional.

A reservoir capacitor is usually recognised quite easily as an aluminium cylinder with a plastic sleeve marked with its value and rating. The bare aluminium end is usually scored with a cross and two leads enter the other end through a rubber plug. Sometimes the second lead is welded to the exposed aluminium end.

How not to do it!

The quick way to discharge a capacitor is with an insulated screwdriver across its leads or the circuit board traces they are soldered to.

This will result in a loud crack and a nice spark and may startle you and any bystanders, even if you are expecting it.

Apart from startling people, this is not a good idea because the discharge current is likely to be many amps and may well damage the capacitor.

How to do it safely

Before doing anything else, disconnect from the mains.

There may be several reservoir capacitors on the high-voltage side of a switch mode power supply. You must make sure they're all discharged. It may be that most of the charge in the capacitors will have drained away through the attached circuitry or by leakage, but start by assuming they still hold a dangerous charge. You don't want to discover your mistake by getting a shock!

You will need a capacitor discharger which you can very easily make out of a suitably prepared high-power resistor with well-insulated wires (apart from the tips) - for example see Constructing a Capacitor Discharge Tool for more information about constructing a capacitor discharger.

Taking great care not to touch any connections at all, locate the reservoir capacitor or capacitors and one-by-one discharge them using a capacitor discharger.

This won't reduce the voltage instantaneously - to discharge a capacitor value C (uF) to a twentieth of its initial voltage using a resistor R (kΩ) will take roughly R*C/330 seconds. For example a 100uF capacitor charged to 300V discharged through a 10kΩ resistor takes about 3 seconds to reduce the voltage to 15V. A further 3 seconds would reduce that 15V to a twentieth 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.