Relays are one of the essential electrical components used in cars, bikes, home automation systems, home appliances, and many other applications to easily switch high power circuits. Being electromechanical devices, relays are bound to a lot of wear and tear (as the relay contact switches between the contacts) and eventually get damaged or faulty. So, in this article, we will learn the importance of a relay, how a relay works, and if you encounter a faulty relay, then how to test a relay with a multimeter.
What is a Relay?
A Relay is essentially an electromagnetic switch. It consists of a coil, an armature, and a couple of contacts. When we apply a low voltage (and low current) signal across the coil, it becomes an electromagnet and pulls the armature towards itself. As a result, the armature switches from one contact to the other. If we remove the voltage across the coil, it de-energizes and stops being an electromagnet. The armature switches back to the other contact.
The contacts are known as Normally Open (NO) and Normally Closed (NC). When the electromagnet is inactive (coil is de-energized), the armature makes contact with Normally Closed (NC) contact. When the electromagnet is active (coil is energized), the armature makes contact with Normally Open (NO) contact. Since the armature is the common point for both NO and NC contacts, it is known as COM (short for Common).
All the three terminals i.e., COM, NO, and NC are independent of the coil and the COM terminals switch between NC or NO depending on the energization of the coil.
Where are Relays used?
Understanding the physical and mechanical aspects of a Relay is good but it won’t do much good if you don’t know how and where to use relays. Imagine a simple application in your car; its headlights. During evening and nighttime, you flick or twist a switch near your car’s steering wheel and the headlights turn on.
On the outside, it looks like a simple application and circuit. But if we dig a little bit deeper, we can understand how relays are useful in these scenarios. The headlights in your car are high-power devices that draw anywhere between 55W to 100W or more power. The car’s computer, which is responsible for reading the headlight turn on the switch, and providing power to the headlights, cannot directly supply the power as it is a low power and low voltage device and it will burn if we connect headlights directly to the car’s computer. We have to use an intermediary device, which can accept commands from the computer and provide power to the headlights. This device is Relay.
If the car’s computer can somehow energize the coils of the relay and we connect the headlights to NO terminal, then we can easily control the headlights without burning or damaging any components. This is exactly what happens in an actual car with its headlights.
When we turn on the headlights switch in the car, its computer reads this switch and activates the coil of the relay. The relay’s armature switches its position and the headlight turns on. When we turn off the headlight switch, the car’s computer will deactivate the coil of the relay, and the armature of the relay switches back to the default contact.
Before looking at the circuit for the above-mentioned example, we have to understand a couple of important specifications about the relay. First is the low-side voltage across the coils. We usually call the relays as “5V Relay” or “12V Relay”. What this means is that the coil of the relay requires 5V or 12V to be energized and become an electromagnet.
Next is the high-side voltage and current of the load (at COM, NC, and NO terminals). Relays usually have ratings for either AC or DC voltages and currents (or sometimes both). A commonly used 12V relay will have a rating such as 10A – 125VAC, 7A – 250VAC, or 10A 30VDC. These ratings represent the maximum allowed voltages at the terminals of the relay.
With that aside, here is a simple circuit typically used in cars to control the headlights. The car’s computer (Electronic Control Unit or ECU) controls the transistor, which is connected to the coil of the relay. Headlights are connected to the Normally Open (NO) terminal of the relay. COM is connected to the negative of the car’s battery.
How To Test a Relay?
It is clear from the above explanation that relays are extremely important electronic devices, which enable low-powered devices such as microcontrollers to control high-powered devices. As relays are mechanical devices, they can become faulty over time and it is essential to check whether a relay is working properly or not. If it is not working, then we have to immediately replace it with a new one.
But how to test a relay? There are different ways to check if a relay is working as expected or not. We will see a couple of methods to test a relay and importantly, we will learn how to test a relay with a multimeter.
How to Test a Relay with DC Power Supply?
In the first method, we will simply energize and de-energize the coil of the relay and see if the armature is switching between the two terminals. Assuming a 12V relay, connect the coil terminals to a 12V DC Power Supply or a battery with a switch.
If we close the switch, it completes the circuit and activates the coil. This should make the armature switch to Normally Open contact. If we open the switch, then the armature should go back to the Normally Closed contact.
This is a simple method for testing relays, especially automotive relays as you readily have access to a 12V battery.
How to Test a Relay with a Circuit?
For the next method, we will build a small circuit using simple components and test if the relay is working or not. This method is useful for relays used in circuits and PCBs but we can also test automotive relays as well.
Here is the circuit to test the relay.
Once again assuming a 12V relay, if we supply 5V to the base of the transistor, it turns on and activates the coil. If the relay is working properly, the armature contacts switches position and light up the LED.
How to Test a Relay with a Multimeter?
Finally, we will see how to test a relay with a multimeter. This is one of the easiest ways to test a relay. Take out the relay from the circuit or from the car. Put the multimeter in Resistance Measurement mode (Ohmmeter). If you have a manual ranging multimeter, put the range meter in minimum range (200Ω or similar value).
Take the leads of the multimeter and connect them across the coil terminals of the relay. For a normal coil, the multimeter should read anywhere between 40Ω to 120Ω. If the coil is damaged i.e., it is open, the meter shows out of range and you have to replace the relay.
Another test we can do with the multimeter is the continuity between the three high-power contacts of the relay. When the relay is in a de-energized state, the COM terminal must be connected to Normally Closed (NC) terminal. But as soon as we energize the relay, the COM terminal switches and makes contact with the Normally Open terminal.
So, to test this, put the multimeter in continuity mode and while the relay is not activated, connect the leads of the multimeter between the COM and NC terminals. The multimeter should start the buzzer if the relay contacts are okay.
Now, energize the relay and test for continuity between COM and NO terminals. If the buzzer is activated in both cases, then the relay is well and good. But if any test fails, then it might be a faulty relay.
A beginner’s guide on relays, what’s inside a relay, how a relay works, and also where relays are useful. Like any mechanical device, relays are also susceptible to damage. We can easily test a relay using several techniques and determine whether a relay is functioning or not. We also learned how to test a relay with a multimeter.