PCBs, or printed circuit boards, are flat sheets of fiberglass printed with metal tracks into which components -- including capacitors -- are soldered. Capacitors store electricity, and retain their charge even when no longer supplied with power. Before conducting any electronic repairs, discharge larger capacitors and test them to ensure they're discharged. Failing to do so can result in damage to other parts of the circuit or an electric shock from a capacitor. Small capacitors won't do much harm, but large ones can cause serious injury. Safely discharge capacitors using a suitable resistor.
Instructions
1 Calculate the resistance required to discharge the capacitor from its working voltage. Use a resistor rated between 5 and 50 ohms for each 5 volts of the capacitor's working voltage. A slightly higher or lower resistance will not cause any problems, but will affect the amount of time needed to fully discharge the capacitor.
2 Clip one end of the clip lead -- an insulated lead with alligator clips on either end -- to one of the resistor's leads. Clip the other end to an unpainted spot on the metal chassis of the item you're repairing. Using insulated gloves or pliers with insulated handles, hold the other lead of the resistor against each pin of the capacitor for a few seconds in turn. Alternatively, turn the whole PCB over, identify the pins of the capacitor where they protrude through the back of the PCB, and touch one lead of the resistor to one of these pins and the other lead to the other. Take care not to let the bare wires touch your skin as they may be carrying a high voltage and can get hot during the discharge process.
3 Measure the voltage remaining in the capacitor using a reliable voltmeter with a high voltage rating. Do not use a standard digital multimeter, or DMM, unless you have high-voltage probes; you can destroy the unit and cause an injury. Look for the negative marking -- minus sign -- on the capacitor's casing. Place the black probe of your meter against the pin on this side of the capacitor. Place the red probe against the pin on the opposite side. You should have a reading of zero or very close. If a voltage is still present, the capacitor is not discharged, in which case repeat Steps 1 to 3.
