Understanding Relays

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OPERATIONAL CHECK WITH VOLTMETER
A voltmeter can be substituted in place of a test light; however be aware if the contacts are partially burned, the voltmeter will show voltage indicating good contact even when bad. Remember high impedance digital voltmeters draw almost no current. 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+ to pin 2 of the load circuit. Connect the RED lead to pin 4 and the BLACK lead to ground. The voltmeter will indicate source voltage (12V). De-energize (remove B+) the control circuit at pin 1; the voltmeter should now read "zero". Re-energize the relay and the voltmeter should return to 12 volts.

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.