Electronics, Engineering Projects

How To Test a Relay with a Multimeter Step by Step guide

In this post, we are going to learn How to Test a Relay with a Multimeter, Step by Step

How to Test a Relay with a Multimeter?

The main reason for testing a relay is to find, whether it is good or defective one. The simplest way of finding this is to use a multimeter with an ohmmeter setting and then measuring the resistance values by checking its terminals.

In this article, we will see what resistance readings are required to declare that the relay you are using is the good or defective one.

To do this there are only two easy steps to follow:

Step.01: Check the relay coil Terminals using Multimeter.

Step.02: Test the Relay contacts using Multimeter.

Relay
Relay

Step.01: Check the Relay’s Coil Terminals Using a Multimeter:

To check the relay whether it’s good or faulty one, we need to check the resistance of a coil.

In the Relay, there is a coil and that coil has some resistance, using that resistance knowledge we can find that the relay is a good one or not.

This is how we can do this: First of all take the relay and search that particular relay on the internet you will find the datasheet of that particular relay. Now in that datasheet, you will find the coil resistance and tolerance value. Note that values and move on the second step, which is finding the resistance value of a coil with a multimeter.

Let’s say A relay has a coil resistance of 400Ω then when you measure the actual relay’s coil terminals with a multimeter you should get the values in between of 350Ω to 450Ω.  otherwise your relay is faulty and in this case, you have to replace it.

Example of Datasheet of Relay
Example of Datasheet of Relay

The procedure of Checking the Coil’s Resistance of a relay with a Multimeter:

To check the coil’s resistance with a multimeter, take a multimeter and put it on the ohmmeter setting then connect the multimeter probes on any two terminals of relay’s coil you will find the resistance of relay coil. The polarity of probes on terminals doesn’t matter because we are finding resistance.

How to test a relay with a multimeter

Now if you found values near to the datasheet values of relay’s coil then it is perfect. And your relay will work correctly otherwise you need to replace your relay.

 

Step.02: Test the Relay Contacts using Multimeter:

Beside that technique of finding the relay functionality, you can use this method also. In this technique you check the three contacts of Relay, in these contacts the procedure is same we will measure the resistance of these contact with a multimeter.

  1. Normally Closed Terminal (NO)
  2. Normally Open Terminal (NC)
  3. Common Terminal (COM)

Normally Closed Terminal (NC) Contact:

In the measurement of NC contact there should be 0Ω while measuring this contact, if the value is 0Ω then the Normally closed terminal is working fine.

Procedure:

  • Take a multimeter and put it on the ohmmeter settings.
  • Place the multimeter one probe on NC terminal and other on COM terminal of Relay
  • Note the resistance values
  • If you get the resistance value of 0Ω, congratz this terminal is working correctly.

Illustration:

Checking the terminals of relay with a multimeter
Testing Normally Closed Terminal of a relay with a Multimeter

Normally Open Terminal (NO):

In the measurement of NO contact there should be the resistance of MΩ, if you find the value of MΩs then this NC terminal is working fine, and the reason behind this is that when NC and COM are not connected there is a huge amount of impendence.

Procedure:

  • Take a Multimeter and put it on the ohmmeter settings.
  • Place the multimeter’s one probe on NO terminal and other on COM terminal.
  • Note the resistance values.
  • If you get resistance values of MΩs, congratz this terminal is also working correctly.

Illustration:

Check the contacts of relay with a multimeter
Testing Normally Open Terminal of a relay with a multimeter

 

Conclusion:

In this article, we learned that there are two ways of finding the functionality of relay whether the relay is working well or not. Both ways have one thing common which is checking the resistance of the relay’s terminals. The first method of checking the terminal resistance was useful when you have datasheet available, and another method was also useful when you are making a project based on a relay or you do not have any datasheet available then you can use that method.

 

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