A capacitor is a device that stores electricity, but unlike a battery that slowly discharges, it discharges immediately when a threshold charge is reached. This behavior has applications in a variety of electronic environments.

Around the house, many appliances with electric motors have motor start capacitors to provide the extra "motion" needed to start the motors. Many devices (such as air conditioners) also have running capacitors that work in conjunction with compressor contactors to produce a stable electrical shock to keep the compressor and fan running.

Capacitor wear. Symptoms include the buzzing of the garage door openers, inability to start work or the inability of the air conditioner to cool the room. You can use megometers to test capacitors. This is an all-or-nothing test that tells you if the capacitor is exhausted, but there is no way to diagnose a still weak capacitor that is about to stop working.

Capacitance measuring tool

A capacitor consists of two conductors separated by an electrical insulator that prevents current from flowing between the conductors until a threshold charge is established. This threshold is inversely proportional to the applied voltage, so the capacitance (C) of each capacitor is given by a simple equation: C = Q/V, where Q is the maximum charge accumulated before discharge.

Some megometers measure capacitance directly by providing a voltage at each end of the capacitor and measuring the discharge current. Although most megometers do not have this feature, you can still test capacitors using voltmeter or megometer Settings. If you suspect the capacitor is dead, a Megabyte test is the easiest way to find out.

Disconnect and discharge capacitors

To check the capacitor with a Megabyte, you must do two things. The first step is to remove the capacitor from the circuit. It is often easy to remove the start or run capacitor - you simply remove it from the harness and disconnect the wire. However, be careful not to touch the capacitor terminals. If the capacitor is not depleted, it may be fully charged, and if so, you may receive a severe shock.

To discharge the capacitor, wear a pair of rubber gloves and use a screwdriver with an insulated handle. Touch both capacitor terminals simultaneously with a screwdriver, and if the capacitor is charged, an impressive spark can be seen. By the way, that spark tells you that the capacitor may not be exhausted. However, if you don't see a spark, that doesn't mean the capacitor is broken. It could just be that the capacitors aren't charged. Now that the capacitor has discharged, it is time for the Megabyte test.

How do I do megohm tests

You can use analog or digital megabytes. Set it to its highest ohm (Ω) Settings, at least 1 k Ω (1000 ohms). Under this setting, a small current is generated when the instrument wire is connected to the capacitor terminal. After connecting the wire, hold it there for a few seconds. If you use an analog meter and the capacitor is in good condition, the meter pointer will start at a low reading and steadily climb to infinity as the charge builds up in the capacitor. If the capacitor is broken, the needle won't move at all.

If a digital megometer is used and the capacitor is in good condition, the number on the digital display will steadily increase until the capacitor discharges, then it will return to zero and begin climbing again. If the capacitor is broken, you'll get a very low resistance reading - probably zero - and it won't change. This means that the insulation inside the capacitor has worn away.

Multiple tests are best to verify the results.