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A Comprehensive Guide About Relay And How To Test A Relay With A Multimeter?

how to test a relay with a multimeter


Relay is one of the prominent components in electronics technology. The application of relay is extensive. It is used in such things where controlling is required of very high-power equipment with the help of a low-power controlling unit. The correct function of the relay gives you a lot of benefits, but if the relays malfunction or encounter faults, this could be a big headache for you. This article will teach you how to test a relay with a multimeter.

Before knowing about testing with the help of a multimeter and other things, you must know about the relay, its type, and its functioning. So let gets started exploring the relay.


A relay is an electronic device that is used for switching purposes. Relay is working on the principle of electromagnetic induction. According to electromagnetic induction, the coil becomes an electric magnet due to the current flow when you provide an electrical supply to a coil. The coil remains an electric magnet until the supply provided to the coil is disconnected. Once the power to the coil is disconnected, the coil’s magnetic field disappears.

A relay consists of the following components.

  • Coil
  • Armature
  • Contacts (Normally Open contact ‘NO,’ Normally close contact ‘NC,’ Common contacts)

Working on a relay

Understanding the working of a relay is very simple once you know about electromagnetic induction.

  • In the normal position, the relay stays in Normally open condition. In this state, the high-power load is turned off.
  • When you want to turn on the high-power load, you must give a low-power dc signal to the common point of the relay.
  • Once you apply low-power dc volts to the relay, the coil inside the relay will be energized and pulls armature towards Normally closed contact.
  • In this situation, the high power load will be switched on with the help of low dc controlling volts.
  • It would help if you disconnected low dc voltage when you want to switch off the load. The coil will get de-energized upon cutting off low dc voltage, which will cause the armature to revert to its typically open state. The load will be terminated as a result.

In the relay, low-power and high-power contacts are independent of each other, and there is no chance of a short circuit between low and high voltages. A high level of protection can be gained between actual load and controlling circuits.

What kind of faults can be happened in relays?

To know how to test a relay with a multimeter and other methods, you must first know about the failure that could appear in a relay. In relays, there could be two types of failure that can be happened.

  • Electromagnetic component failure
  • Contacts components failure

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The failure of electromagnetic components

A relay could be faulted due to the failure of electromagnetics components. Relays consist of two electromagnetic components, which are mentioned below.

  • Common coil failure
  • Iron core failure

In relays, the common coil failure happened due to multiple factors. These factors are

  • High temperature
  • High voltage
  • Wrong voltage connected to it
  • Coil burning

The relay emits a noise when an electromagnetic component’s core develops a malfunction; as a result, the armature cannot be securely fastened to the electromagnet.

The failure in contact component failure

The failure that could happen in the contact component is

  • Loose contacts
  • Cracked contacts
  • Contact position deviation

How to test a relay with a multimeter?

The testing of a relay with a multimeter is a straightforward process. For testing purposes, you need a relay and a digital multimeter. After getting these two things, follow the steps below to determine how to test a relay with a multimeter.

Testing of relay coil with a multimeter

Step number 1:

In this step, you should set your multimeter on the “OHM” or “RESISTANCE” measurement setting.

Step number 2:

Now connects both leads of the multimeter and see the meter’s reading. If in the multimeter screen the “00” value is shown, then your meter is in proper working condition, and you are good to go for testing of the relay.

Step number 3:

Place the relay on the workbench or smooth surface and locate the coil pins according to the relay’s datasheet. After locating the coil pins, connect the multimeter leads with these two coil pins.

Step number 4:

If the relay’s coil properly functions, then, after connecting the leads to the coil terminal, you will see the reading of “40 ohms” to “120 ohms” on the multimeter’s screen.

Step number 5:

The meter shows out-of-range or infinity resistance if the relay’s coil is faulted. This infinity resistance indicates that your relay is faulty and needs replacement.

Testing of contacts of the relay with a multimeter

Another test you can perform with the help of a multimeter is checking the high voltage contact terminal of the relay.

Step number 1:

For this purpose, pull out the relay from the circuit and connects the leads of the multimeter with “common contact” and the ordinarily close contact “NC” of the relay.

Step number 2:

If the multimeter displays the least resistance after being connected to the contacts (described in step one), then the close “NC” contact is likely operating properly. And if the multimeter shows infinity or out-of-range resistance, then this means the ordinarily close contact is faulty

Step number 3:

For testing of normally open contact of the relay, you should energize the relay coil with the help of a power supply. After energizing the coil, connect the leads of the multimeter with the “Common Contact” and “Normally Open Contact” of the relay. If the meter shows minimum resistance, then “NC” is ok, and if the meter shows infinity resistance, then the “NC” contact of the relay is faulty.


How can you tell if a relay is damaged?

Set an Ohms multimeter to Ohms. Measure resistance by touching the leads across the magnetic coil pins.

Even if a relay clicks, may it still be bad?

If you hear or feel the relay click, the fault is not with the relay or its wiring. However, if it isn’t clicking, there can be an issue with the wiring or the relay itself.

What are some common relay faults?

Contamination and mechanical wear of the internal switching components are the two most frequent failure modes of relays, as follows: A significant contributor to failures in early life is contamination.