How to Know if a Relay Defective

Relay is an electromechanical part though nowadays there is already a so-called solid-state relay. An electromechanical means a combination of electrical and mechanical actions or functions. For hobbyist, it is important how to know if a relay defective. But before going to that, let us discuss the basics of a relay for better understanding.

As mentioned above, a relay is an electromechanical part. Its control circuit is electrical while the switching contacts are mechanical. Below is a symbol of relay. The coil is the one being controlled by an electrical action while the contact is just a mechanical assembly that closes and opens when the coil energizes and de-energizes. The term energizes mean that the coil is being supplied by a voltage.

Relay Schematic Symbol
Relay Symbol

The coil part is actually an inductor with a series resistance. You may often hear about the term coil resistance of the relay. Below is the expanded model of the relay coil with the series resistance R and inductance L.

Relay coil model
Relay Coil Model

The value of the series resistance is specified in the datasheet at a rated temperature. The inductance is as well specified. The series resistance is the one responsible in setting the current flowing to the coil when there is an applied voltage. The coil is only rated for small currents unlike the contacts that are designed to handle big currents.

Some Techniques on How to Know if a Relay Defective

1. Visual and Physical Check


1.1. Observe carefully the relay coil pins, contact pins and or the entire appearance. Observe if there is no burnt marks, mechanical deformations or unusual smell.

1.2. If there is a doubt based on the observation, then replace the relay with a new one.

1.3. Check initial state of the relay contact. A relay is can be a normally open and a normally closed contact. A normally open contact means the contact state is open when the relay is not energized. On the other hand, a normally close contact means the contact state is close or having a zero resistance when the relay is not energized. Take note of the relay contact state.

1.4. A fully close contact will have a resistance of zero ideally. On the other hand, a fully open contact will have infinite resistance ideally. If the contact resistance is not falling to either conditions, then the relay is maybe defective. Replaced with another one.

2. Check and Measure Coil Resistance

2.1. Get the relay to be tested and identify the coil pinouts. If you know the part number of the relay, you can search the net for the datasheet and drawing. If you have no information of the part number, the coil pins are often smaller than the contact pins. The relay body itself may have a guide.

2.2. Get a DMM and set it to resistance meter. Do not apply power to the relay coil yet.

2.3. Connect the DMM probes same as below. It does not matter if the red and black probes are reversed as we are only measuring the resistance.

Checking relay resistance
Relay Resistance Check

2.4. Read the DMM display and it is the coil resistance

2.5. How to know if a relay defective indeed? Simply, the measured resistance is not the same to the datasheet resistance. But what about the relay datasheet is not available as there in no body marking on the relay? There are some norms that can be used in the judgement. First, if you have a known good relay part (the same part) then you can compare the coil resistance to it. Second, if there is no good part to compare with, then an intelligent guess can be done. Like for automotive applications, the relay coil resistance is mostly in the 50 ohms – 200 ohms. For 24V systems, the coil resistance can be as high as 200 ohms to 600 ohms.

3. Energizing the Relay Coil

Energizing the relay coil
Energizing the Relay


3.1. If you know the relay part number, then you will know the required voltage to energize the relay coil. If so, get a voltage source and apply across relay coil pins. If you hear the relay contact clicking, then it perhaps working. But to be sure, measure the contact resistance. It must be ideally zero when the relay contact changes from open to close and a very high resistance when a relay contact switches from close to open.

3.2. If you don’t know the relay part number and no way to get it, then use trial and error method. Common relay coils are rated at 5V, 12V, 24V, 36V or 48V. If you know the application of the board where the relay is mounted, then you may have idea what is the voltage source of the circuit. For example, the application is automotive, then the voltage source of the circuit is surely between 12V and 24V. So, start using a 12V voltage source. Connect to the relay coil pins and observed if there is clicking and changing of contact resistance. Increase to 24V if the 12V source is not working.

3.3. If there is no idea about the circuit or the board, then start using 5V, then 12V, 24V and so on. I suggest not to connect the voltage source within a long period of time so that the coil will not stress if ever the voltage source used is higher than the coil rating.

3.4. A fully close contact will have a resistance of zero ideally. On the other hand, a fully open contact will have infinite resistance ideally. If the contact resistance is not falling to either conditions, then the relay is maybe defective. Replaced with another one.

Common Relay Failure Mode

Relay Failure Mode

Relay is not common to get damage especially in a small power or small signal circuit. But it does in high current circuit more often. The relay failure mode is can be melted/burnt contact, latched contact, burnt coil, open coil or even shorted coil.

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