How To Test A Relay With A Multimeter

How To Test A Relay With A Multimeter

PCBWay

In this post, I am going to teach you how to test a relay with a multimeter .

DRex Electronics

The purpose of testing a relay is to find, whether it is good or bad . And one of the best ways to do this is to use a multimeter with an ohmmeter setting and then measuring the resistance values.

The First Step Is To Test The Coil of Relay

Refer to the datasheet to get the coil’s resistance . In the datasheet you will also get the tolerance value of the coil. For example  if the resistance is 320Ω and the tolerance value is ±10% , then we should get resistance values somewhere between 288Ω to 352Ω.

Bring your multimeter and put it in the ohmmeter setting. After this connect the multimeter’s probe leads to both the terminals of the ohmmeter. Don’t worry about which probe should be connected to which terminal. We are trying to find resistance, so we don’t have to bother about polarity.

Now, read the resistance. If the resistance falls with the range mentioned in the datasheet . Then the coil is good. However, if you get readings that are either very high or very low, then there is a problem with the coil. The only thing that you can do now is to replace the relay.

The Second Step Is To Test The Terminals of Relay

Now, that we have tested the coil, its time to test the various terminals of the relay. Again, the best way is to measure the resistance between them. We shall be testing the following :

  • Normally Closed Terminal (NO)
  • Normally Open Terminal (NC)
  • Common Terminal (COM)

Testing The Normally Closed (NC) Terminal of The Relay

When there is no voltage through NC, then the resistance of NC-COM should be nearly 0Ω . If this reading is around 0Ω, then the NC terminal is working fine.

I am providing below, the step by step guide to test the NC terminal.

  • Put the multimeter in the ohmmeter setting
  • Place one of the multimeter probes on the NC terminal and the other on the COM terminal
  • Read the resistance
  • If the resistance value is around 0Ω , the NC terminal is working fine.

Testing The Normally Open (NO) Terminal of The Relay

The step by step guide to test the NO terminal is given below.

  • Put the multimeter in the ohmmeter setting
  • Place one of the multimeter probes on the NO terminal and the other on the COM terminal
  • Read the resistance
  • If the resistance value is around several MΩ , the NO terminal is working fine.

If you get very high resistance , to the tune of several MΩ, then the NO terminal is also working fine. That’s because when the NO and COM are not connected, the impedance is very high.