How to Check an Electrical Capacitor

Electrical capacitors when used in alternating current (AC) circuits can be likened to a battery.

The capacitor stores an electrical charge to give that circuit it is connected to, a boost of power. The most typical application for a capacitor is for starting and running single phased motors. These motors can be found on air compressors, air conditioning systems or any motor that requires a high torque value when in operation. Generally when a motor has a difficult time in starting, and a capacitor is used in the circuit, it is best to check that capacitor for problems.

Remove all electrical power from the motor circuit by disconnecting the power plug or shutting off the circuit breaker. Use the screwdriver and remove the two screws that hold the protective cover in place over the round capacitor. Generally the capacitor is mounted on top of the motor.

Pull the two power connectors from the end of the capacitor using the needle nose pliers. Set the capacitor on its end, so the connectors are facing upward, on a firm worktable surface.

Use either an analog volt ohmmeter or a digital volt ohmmeter with a capacitor testing function. Connect the leads into the meter where the red lead is in the volt connector and the black lead is placed into the common connector. Turn the meter to the volts position. Hold the two leads, one each, onto the two connectors. This will complete a discharge circuit. Hold the leads onto the connectors for one minute, reverse the leads and hold in place for another 60 seconds. The capacitor should now be discharged.

Check the capacitor using an analog volt ohmmeter. Switch the meter to "ohms". Move the red lead from the volts connector on the meter and into the ohms connection. Touch the leads, one each, to the capacitors connectors. Hold in place for 10 seconds. Reverse the leads. Observe the meter. As soon as the leads are touched to the capacitors connectors, the meter should "jump". This indicates that the capacitor is storing a charge and discharging on the reversal of the meters leads. Repeat the test several times. If the capacitor failed this test. Replace the capacitor.

Use the digital voltmeter for checking the capacitor. Move the voltmeters selector switch to the capacitor mode. It may also be shown as a symbol shown in quotation marks "--|( ". Touch the leads to the connectors on the capacitor. The meter will show either "good" or "bad". Follow the meters indication and replace the capacitor with a "bad" reading.

Things You Will Need

  • Screwdriver
  • Needle nose pliers
  • Analog volt ohmmeter (optional)
  • Digital voltmeter with capacitor testing function (optional)

Tip

  • Another method for discharging a capacitor is to use a 100-ohm, 20-watt resistor. Hold the resistor in the jaws of the needle nose pliers. Touch the ends of the resistor to the capacitor leads. Forming a complete electrical circuit. Hold in place for 10 to 15 seconds. The resistor will dissipate any electrical charge.