How to Test Relays

How to Test Relays

A relay is a component that switches on or off power to one or more devices in a circuit. It works by allowing control circuits, usually working with much lower voltages and currents, to effectively amplify a switch signal and turn it on or off at a much higher power level. Relays are commonly used in automotive, telecommunications, and industrial applications, among others.

A typical electromechanical relay has two main parts: its contacts and the electromagnet. The contacts work the same way as a mechanical switch does, connecting or disconnecting an incoming wire from an outgoing wire by using a set of terminals or contact points. Depending on the type of relay, these contacts may have more or less than one set.

When current passes through a relay’s coil it creates a magnetic field that changes the position of its movable armature, which hinges on an iron yoke. The yoke provides a low reluctance path for magnetic flux and is mechanically linked to one or more sets of contacts (there are two in the relay pictured). The armature moves down onto the contact and completes the 'high' current circuit, which is said to be energized. If the voltage on the coil is removed, the spring pulls the armature back to its 'at rest' position and breaks the 'high' current circuit.

Relays are available in a variety of designs, and different types are used for specific applications and purposes. Some examples include latching, mercury-switched, and reed relays. They are also used to perform logical functions, such as turning on the headlights and audible alarms of an automobile when a key is turned on or to switch off a heating system after a car has been turned off.

To test a relay, put your multimeter in continuity or resistance mode and find where the circuits enter and exit the relay. Often, these points are marked with the letters "coil," "C," "NO," and "NC." Test for voltage by applying rated coil voltage; you should hear a click as the relay switches on. If it doesn't, check the associated fuse or switch.

Another way to test a relay is to look for the "normally open" (NO) and "normally closed" (NC) contacts. Normally, NC contacts are opened when the coil is energized and NO contacts are closed. Unless there is a gross mechanical fault, you should be able to reliably guarantee that any NO contacts will close and all NC contacts will open when the relay is energized.

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