Relay Testing

1. When current flows through the coil of a relay, its core is magnetized to attract the iron piece, closing (ON) the contact at the tip of the iron piece. When the coil current is turned off, the iron piece returns to its original position by a spring, opening the contact (OFF).




2. By using a relay, a heavy current can be turned on and off by a switch using much less current. For example, in the circuit shown here, when the switch is turned on (closed), current flows to the coil of the relay. Then, its contact is turned on (closed) and the light comes on. The current flowing through the switch is much less than that for the light.




3. The relays may be classified into the normal open-type and the normal close-type by their contact construction.

NOTE: The deenergized state means that no current is flowing through the coil and the energized state means that current is flowing through the coil.




When a normal close-type relay as illustrated here is checked, there should be continuity between terminals 1 and 2 and between terminals 3 and 4 when the relay is deenergized, and no continuity between terminals 3 and 4 when the battery voltage and ground are applied to the terminals 1 and 2. A relay can be checked in this manner. It cannot be determine if a relay is okay or faulty by checking its state at rest.