Solid-State Devices Used in Industrial Logic Circuits: Relay Logic: Troubleshooting Relay Coils and Contacts

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It's important to understand that one can determine the condition of the relay coil and its contacts with an ohmmeter when the relay is out of the circuit or if it does not have voltage applied. Since the relay coil is made from a long piece of wire, the ohmmeter should indicate that the coil has some amount of resistance. The precise amount of resistance will vary from coil to coil, since the size and amount of wire used in the coil will vary. Generally, the amount of resistance will be between 20 ohms and 1500 ohms. You need to verify that the coil does not have a short circuit or an open circuit. If the coil is shorted, its continuity (resistance) test will indicate 0 ohms or near zero resistance, and if it has an open circuit, its continuity test will indicate an infinite reading (infinity ohms). If the coil shows any amount of resistance other than infinite and zero, you can move to the second part of the troubleshooting test and apply power to the coil. When the proper amount of voltage is applied to the coil, the contacts should move. If you find that the coil is open when you perform the continuity test, or if it won't pull in the contacts when power is applied, you must replace it.

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The contacts can also be tested with an ohmmeter if no power is applied to them. Normally open (NO) contacts should show a reading of infinity, and normally closed (NC) contacts should show a reading of zero or near zero ohms. You can manually move the contact carrier to move the contacts from their open to their closed position. You should always test the open and the closed position of the contacts because sometimes the contacts will weld together in the NC position. If the test of the contacts indicates high resistance when they are closed, it means they are damaged or pitted and they must be replaced.

IMPORTANT SAFETY NOTICE...

When you have a relay disassembled and the coil is removed from its armature you should never apply power to it because it will burn out from overcurrent When the coil is mounted correctly in the relay its current is normally limited by the metal in the armature If you have the coil out on a bench by itself there won't be anything to limit the current in the coil and it will act like a simple piece of wire instead of a coil The low resistance of the wire will allow the coil current to become exceedingly high which will burn it out.

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TYPICAL LOGIC CIRCUITS

Monday, August 25, 2008 1:40