A capacitor is a device that stores electricity, but unlike a battery, which discharges slowly, a capacitor discharges in an instant when it reaches its threshold charge. This behavior has applications in all sorts of electronics contexts.
Around the house, many appliances with motors have an electric motor start capacitor to provide the extra "oomph" needed to get the motor going when it cycles on. Many appliances, such as air conditioners, also have a run capacitor that works in conjunction with the compressor contactor to send steady jolts of electricity to keep the compressor and fans running.
Capacitors wear out. Symptoms include the hum of a garage door opener and its failure to start working or the failure of an air conditioner to cool the room. You can use an ohm meter to test a capacitor. It's an all-or-nothing test that tells you if the capacitor is dead, but it won't diagnose one that is still functioning weakly but about to stop working.
Tools for Capacitance Measurement
A capacitor consists of two conductors separated by an electrical insulator that prevents current from flowing between the conductors until a threshold charge builds up. This threshold is inversely proportional to the applied voltage, so each capacitor has a capacitance (C) given by a simple equation: C = Q/V, where Q is the maximum charge that builds before discharge.
Some multimeters measure capacitance directly by supplying a voltage across a capacitor and measuring the discharge current. Most multimeters don't have this function, though, but you can still test a capacitor using the voltmeter or ohm meter setting. If you suspect the capacitor is dead, conducting an ohm meter test is the easiest way to find out.
Disconnect and Discharge the Capacitor
To check a capacitor with an ohm meter, you must do two things. The first is to remove the capacitor from the circuit. It's usually easy to remove a start or run capacitor – you simply unhook it from its harness and disconnect the wires. However, be careful to avoid touching the capacitor terminals. If the capacitor isn't dead, it might have a full charge, and if so, you could get a serious shock.
To discharge the capacitor, put on a pair of rubber gloves and get a screwdriver with an insulated handle. Touch the screwdriver to both capacitor terminals at the same time, and if the capacitor is charged, you'll see an impressive spark. That spark, by the way, tells you the capacitor probably isn't dead. However, if you don't see a spark, it doesn't mean the capacitor is bad. It may simply be because the capacitor isn't charged. Now that the capacitor is discharged, it's time for the ohm meter test.
How to Conduct an Ohm Meter Test
You can use an analog or digital multimeter. Set it to its highest ohm (Ω) setting, at least 1 kΩ (1,000 ohms). At this setting, the meter generates a small current when you connect the meter leads to the capacitor terminals. After connecting the leads, hold them there for several seconds. If you're using an analog meter and the capacitor is good, the meter needle will start at a low reading, and as charge builds up in the capacitor, the needle will climb steadily toward infinity. If the capacitor is bad, the needle won't move at all.
If you're using a digital multimeter and the capacitor is good, the number on the digital display will increase steadily until the capacitor discharges, then it will return to 0 and begin climbing again. If the capacitor is bad, you'll get a very low resistance reading – possibly 0 – and it won't change. That means the insulating material inside the capacitor has worn out.
It's best to conduct the test more than once to verify the results.