Voltage Through Relay Coils

When checking circuits with relay coils, a very simple rule can be utilized. If the coil is energized, all input voltage is dropped through the coil windings. If the coil is relaxed (not energized) the input voltage can be read "downstream" from the relay itself.

The controlling system could be a digital LRU with a transistor or amp activating the ground to the relay. It could be a thermal switch, mechanical switch, or even possibly the closed/open contacts of another relay.

If the input cannon plug to the controlling unit is removed (or LRU removed from rack), the coil voltage of a relay can be checked at the plug. The relay can be activated by supplying your own ground with a jumper wire. (Caution should be used when supplying a ground to a relay circuit. Any possible damage to secondary systems should be ruled out before connecting a hard ground....... don't let the smoke out!!!)