Safety

Large capacitors can store enough energy to be dangerous. Large capacitors in high voltage circuits must be discharged (through a suitable resistor to ground) after power is removed before working around them. You will usually see a high value resistor (e.g. 20k) across the terminals of such capacitors. The high resistance will not load the capacitor but will discharge it when power is removed. This is a safety feature but can fail. Never trust these “bleeder resistors”. One common way to discharge such capacitors is go use clip leads to connect one end of a 20k resistor to a screwdriver and the other end to ground. Make sure the screwdriver has a heavy insulated handle if you are working with high voltages. Once connected to ground through the resistor, touch the screwdriver to the positive terminals of each capacitor. Never connect the discharge probe (the screwdriver) directly to ground. You must have a high value resistor between the screwdriver and ground. With high voltages and big capacitors you will get big sparks. Many technicians have partially melted screwdrivers to remind them of what happens when they discharge capacitors to ground without the resistor. Some people have suggested that a voltmeter can be used to discharge capacitors. However, the impedance (resistance) of a typical digital voltmeter is too high and will take far too long to discharge a capacitor.