Understanding Relays

26 of 27

OPERATIONAL CHECK WITH AN OHMMETER
An ohmmeter can also be used to test the load circuit, but the same problem as the voltmeter comes into play. Energize the relay (control side). Supply B+ to pin 1 and a ground (neg.) to pin 3. A click should be heard. Place the leads on an ohmmeter to across pin 2 and pin 4. Assuming it is a normally open relay the ohmmeter will indicate a complete circuit (close to zero -0 ohms). De-energize the control circuit at pin 1(remove B+). The ohmmeter should indicate OL (an open circuit - infinite). Re-energize the relay and the ohmmeter should return to "zero" ohms. Note: some manufactures provide a maximum ohm value when the switch contacts are closed, example 5 ohms max.

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.