Understanding Relays

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OPERATIONAL CHECK FOR RELAY VOLTAGE SUPPRESSION DIODES
An ANALOG OHMMETER must be used. This test cannot be performed with a digital meter. The analog meter sends out a higher voltage which is required to forward bias the diode. Place the ohmmeter across the control circuit and record reading. Reverse the leads and check the control circuit again. A functioning diode will be indicated by have two different readings. A faulty diode will have the same reading in both directions.

Current from the ohmmeter flows through the control coil, in one direction. By reversing the leads, you send current in the opposite direction through the control coil. One of the two directions the diode will be forward biased(on), creating two paths for current thus lowering resistance. With the leads in the other direction, the diode in will be reversed biased (off) creating only one path, with higher resistance.