How to test electrical relay | methods of testing relay-multimeter

How to test relay? Methods for testing electric relay

A relay is defined as an electrically controlled device that opens and closes electrical contacts with help of electromagnetic switching, and performs ON and OFF operation of other devices in the electrical circuit. various methods are used to test relay like visual inspection, test with the multimeter 

To test electrical relay working or defective one, the easiest way to test is to use a multimeter and set it on ohmmeter settings and measure the resistance of the relay coil.

In this article, we will learn how to test relay using multimeter by measuring the resistance of the relay coil and different Relay testing methods. 

From obtain reading we can distinguish relay is in good condition or not. 

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Methods for finding relay conditions

Do a visual inspection of relays:

Most of relay have a clear plastic transparent body containing coil and relay contacts.

From visual inspection we can find Melton, blackening of contacts and this will help narrow down the issue.

‌Testing electrical relay coil using Digital Multimeter:

PLA relay 8 circuit diagram

To test relay coil using multimeter put a multimeter on ohmmeter settings, and measure resistance between coil contact.

For that, we need a relay datasheet, in which all relay detail described. eg. it's pin configuration like a number of pins, on which pin relay coil is connected. NC and NO  pin.

Typically these data will be printed on the larger relay case.

If the measure value of relay coil resistance lies between describe rage then relay is in good condition. If the coil resistance shows very high or very low than the described value, the relay needs to replace.

For example, the relay coil may have resistance of 520 ohms +_ 10% tolerance. then coil resistance must have resistance within 468-572 which indicates its good condition.

If it shows different than this value, then relay consider as not in good condition. and relay needs to be replaced because we can't repair relay coil. 

‌Testing other pins of relay (NO  and NC)  contact:

Pin diagram printed on the case of large size relay or also available in manufacture's datasheet, some relay has one or more pin, like shown in the above image, it has 8 pins, and coil in between 2 and 7 pins. 

Each pin has normally open NO and normally closed NC contact. The pin diagram indicates this contact.
Relay drawings will show each pole as either touching the pin indicating an NC contact or nontouching the pin indicates NO contact.

Testing normally closed NC contact:

NC contact will show 0 ohms result on Digital multi-meter if relay in good condition otherwise shows different value in ohms.

Testing normally open  NO contact:

NO contact will show high resistance in the range of Mega ohms, the digital multimeter will show OL (overload)

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