While working on high voltage power supplies – several hundred volts – I often found myself needing to discharge capacitors which had retained thier high voltage after power had been removed. Note that it is definately not a good idea to just short out the terminals, you may get a satisfying bang but it will damage your capacitor – it may not be immediatly obvious but it will reduce it’s capabilities. The method I used was to place a resistor across the connections. This had disadvantages: You had to hold the resistor on the high voltage terminals somehow and you didn’t know when the voltage had reduced to a safe level. So I designed a circuit which would make both these easier.

Note that this circuit is not suitable for low voltages which can still cause damage to your circuitry; I recommend an LED in series with a suitable resistor for these; make sure you’ve got it the right way round!

I put the wand in a small box with a fixed probe on the box and a probe on a flexible lead. There is an LED which illuminates as long as the voltage exceeds 24V; as 30V and below is considered safe voltage I think this is adequate. It is designed to go up to 500V but may go a little higher; especially if the capacitor is small in value. The voltage that mine will take is a bit limited as it is built on stripboard, if you build it on a better material it could go much higher, especially if you increase the value of R1 – R4. These resistors have to have a sufficiently high power rating for the voltage as it may take several minutes to discharge a large capacitor and the circuit takes up to 50mA.