﻿ How to Test the Relay Contacts

How to Test the Relay Contacts

In this article, we will explain how you can test the contacts of a relay.

Besides the coil terminals, you may also want to check to see if the the other terminals are good on a relay as well. This includes the COM terminal, the Normally Open Terminal, and if the relay is a double throw relay, the Normally Closed Terminal.

The best way to test if these contacts of the relay are good is by measuring the resistance between them.

Below is a diagram of a Single Pole Double Throw (SPDT) relay:

To Test the NC (Normally Closed) Terminal of a Relay

The relay's Normally Closed Terminal should read near 0Ω from the NC (Normally Closed) terminal to the COM (common) terminal, when the relay has no voltage going through it. This is because the NC terminal is normally closed, meaning there should be direct continuity from the NC terminal to the COM terminal when the relay isn't energized; thus, there is almost 0Ω resistance between the 2 terminals.

Take out a multimeter and place it in the ohmmeter (Ω) setting. Place one probe on the COM terminal and the other probe on the NC Terminal. Make sure that you read a resistance of near 0Ω. If you do, then the NC Terminal reads the correct resistance and should function properly.

To Test the NO (Normally Open) Terminal of a Relay

The relay's Normally Open Terminal (in double throw relays) should read a very high impedance (several megohms (MΩ)) from the NO (Normally Open) terminal to the COM terminal. This is because the NO terminal is normally open, meaning there's no direct connection from the NO terminal to the COM terminal when the relay isn't energized; thus, there is an extremely high impedance between the 2 terminals.

Take out a multimeter and place it in the ohmmeter (Ω) setting. Place one probe on the COM terminal and the other probe on the NO Terminal. Make sure that you read a high resistance of several megohm (MΩ). If you do, then the NO Terminal reads the correct resistance and should function properly.

Related Resources

How to Test a Relay