Car Relay Guide: What it is & How A 12V Relay Works

Car Relay

Do you know, that sometimes a relay warms up due to constant usage, causes electrical short circuits inside the relay, and stops the electrical flow?

When the relay cools, it starts the electrical flow again. A relay is a simple switch used to connect and disconnect the electrical circuit.

If you want to gather information about car relay. Or If you are an automotive technician or a non-technical guy.

You came to the right place.

In this powerful guide, you will know what the relay is in a car, its construction, working, and many more things that are explained very easily.

You need to know that automotive relays have a big market. The demand for safety, luxury, and comfort in a car makes the automotive relay market grow bigger and it is estimated that by 2027 its market will grow up to 21.8 billion USD from 14.2 billion USD in 2019.

Automotive Relay Market
Automotive Relay Market

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What Is A Car Relay


In most simple words we can say that a relay is a switch. It is the same switch we people have on our home walls turning the fan and bulb ON and OFF. Both switches are the same, they both make and break the circuits.

But, listen there is one difference and that is, the home switch is operated manually by hand while in automobiles, it is operated electrically. So, a car relay is also called an electric switch or electromagnetic switch.

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Who Invented the Relay

An American Scientist Joseph Henry is considered to be the inventor of the relay in 1835.

Construction Of Car Relay


The basic theme and construction of all electro-mechanical relays are the same. But, the design, pin arrangement, and size differ from one another.

A relay is made up of a small coil of wires wrapped around the central iron core. The coil is fixed on a small metal piece frame called Yoke. And a relay also has a movable contact which is supported by a spring. This spring brings back the movable contact to its original resting position when the relay is de-energized.

Moreover, there are two separate circuits in a relay. One circuit is for a coil circuit, which has two wires positive and negative power sources. And the second circuit of a relay is for a high amperage circuit.

When a relay’s coil circuit is supplied with voltage, it generates a magnetic field.

How A 12V Relay Works


Car relay works on the principle of electromagnetism. When the current flows to the relay, it produces an electromagnetic field to attract the movable rod to make the connections.
Now here I am explaining the science behind how an electromagnetic field is produced in the relay.

Consider a relay as two halves. One circuit is called the Coil circuit (also called a low voltage circuit, a low amperage circuit, controlling circuit. The second half circuit is called a load circuit or high amperage circuit.

Remember that the load circuit is controlled by the coil circuit. When a current is applied to the relay’s coil circuit, a magnetic field is produced which attracts the movable rod and makes a physical connection to the high-amperage circuit.


On the other hand, when the current is removed from the coil circuit, the movable rod moves back to its original resting position, causing it to disconnect the high amperage circuit.

If you were to look at a coil circuit, you would see an electromagnetic coil. Now here is the interesting point why there is a magnetic coil in a relay?

Wait? I will come to it later, now I want to help you understand electromagnetism. Look, there is a rule when you bring a magnet closer to the conductor, electricity is produced.


This happens with electricity when you apply electricity to a conductor, the magnetic field is produced.


You might be wondering why a magnetic field is produced when a current goes in a conductor. The answer is, it is only known to Mother Nature.

However, scientist Michael Faraday discovered the relationship between the magnetic field and the electric field.

According to Faraday’s law, any change in the magnetic field will produce an electromotive force (Electricity), and any change inflow of electric current in a conductor, will produce a magnetic field around that conductor. When the current flows to the relay’s coil circuit, an electromagnetic field is produced, which attracts the movable rod to connect the high-amperage circuit of the relay.

To sum it up, the purpose of the coil in a relay is to bring the movable rod closer to the load wire and make a high-amperage circuit connection.

This is an Info
The Coil Circuit controls the high Amperage Circuit of the relay.

This is an Info
The purpose of the coil in a relay is to bring the movable rod closer to the load wire and make a high-amperage circuit connection.

Car Relay Case Labeling


Some relays are marked on the case for identification purposes.

1. Voltage

A relay is marked up with 6, 12, and 24 volts.

2. Current

A relay is labeled with a current rating of 30/40 amp. Remember this current rating is for a high amperage circuit.

3. Relay Terminals

The numbers 85, 86, 30, 87, and 87a are marked on the plastic case of the relay along with circuit schematics.

Related Post: SPST Relay Vs SPDT Relay, Symbol, Wiring Diagram Explained

12V Relay with Diode

Diode In Relay
Diode In Relay

In an automotive relay, a diode has a special purpose that prevents high voltage spikes produced by the relay’s coil circuit to protect the sensitive electronic circuits.

When an energized relay is switched OFF, the magnetic field collapses, which produces high voltage spikes in opposite polarity across the coil circuit’s windings. This voltage is several hundred volts, but with a very low amp current, and will cause arching on switch contacts or can damage sensitive electric devices.

A diode is installed across the coil in a reverse-biased position and prevents damage by absorbing the high-voltage spikes and dissipating them. Thus, no current will flow through the diode, and electrical components are protected from damage.

Resistor in Automotive Relay Circuit

Resister In Relay
Resister In Relay

Sometimes a resistor is used in place of a diode. The function of the resistor in a relay is the same as in a diode to dissipate high voltage spikes. It absorbs high voltage spikes produced by the relay’s coil circuit due to the magnetic field collapse.

But, here is a problem, this configuration is not as effective as a diode in stopping high voltage spikes. A little bit of voltage can leak through a resistor and can cause electrical damage.

But, here is also a piece of good news for you. The resistor has one advantage over the diode. It is not sensitive to polarity and you can connect any power source (Ground or Hot) to any terminal of the coil circuit (Terminal 85 or 86).

Some Helpful Tips

Openable and Waterproof Relay
Openable and Waterproof Relay
  1. Mostly moisture can get inside the relay making it prone to failure.
  2. Sometimes testing a relay, power is touched accidentally with the ground forcing it to blow the fuse. Be careful.
  3. Sometimes, a relay in the fuse box vibrates out due to being loosely fitted, which does not work properly, when the car goes on a bump the accessory loses work.
  4. Most relays come with a holder grip. Make sure to have clean and tight holder terminals.
  5. Sometimes too much current passing through a relay causes electrical connectors to “Stick” with each other due to heat. Power still goes on while the ignition is turned off.
  6. Some relays are openable while some are fully waterproof packed.

Frequently Asked Questions (FAQs)

What does a relay do in a Car?

A car relay connects and disconnects the electrical circuit. An automotive relay is used to unite and separate the circuit. It uses a low-current circuit to make and break a high-current circuit.
The magnetic field attracts a movable rod of high-amperage circuits to connect with each other. When the relay is de-energized, the spring pulls back the movable rod to its original rest position making the circuit disconnect.

When to use a relay in a Car?

This is a very common question clients often ask me when to use a relay. Car users should know when to use a relay as it saves the wire from burning.
Some components need a high amount of current such as a headlamp, air conditioning, fans, wiper, etc. Such kinds of components can overload the circuits causing overheating or sometimes burning out the whole circuit. So, we usually put a relay in-line to handle a high amount of current by using a low amount of current.
For example, turning the headlamp ON and OFF from the dash. The current will flow in direction of the battery to the dashboard switch to the headlamp. In this way, the length of a wire increases, and also voltage drops.
Therefore, by putting a relay in in-line, we reduce the length of the wire as well as its resistance. So, we use a relay when the component needs a high amount of current or a circuit of lengthy wire to reduce the voltage drop.

How to choose a relay for a Car?

Whenever you choose a relay make sure its pin number, pin arrangement and relay size exactly match the old one. Also, make sure you have the same voltage rating and current rating.

Where to install a relay in a Car?

Whenever a load exceeds the limit of wire capacity, a relay should be installed. It will be better to install the relay near the component.
Also, be careful not to install the relay in an open area because rainwater or a body wash can damage the relay, and the water can get in and begin to form rust. Install the relay in a safe and closed area, so that it does not shake on a bumpy road.

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