Understanding Relays

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OPERATIONAL CHECK WITH TESTLIGHT
Now start the second part of the test. Energize the relay (control side) by supplying B+ to pin 1 and a ground to pin 3. A click should be heard. With the relay still energized, supply B+ pin 2 of the load circuit. The test light will be on. De-energize (remove B+) the control circuit at pin 1; the test light at pin 4 should go off. A test light is preferred because a test light will draw current through the switch.

CAUTION
Testing relays with built in clamping diodes require a special procedure. These relays are polarity sensitive; placing B+ to the wrong pin (backwards) while performing a practical test will forward bias the diode and damage the diode, thus destroying the protective quality of the diode.