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Understanding an automotive relay

Understanding the purpose of relays in automotive is very important as the small portable component safe life of many bigger electrical appliances in our vehicle. after publishing an in-depth post on relays for general purpose, I realized writing about the automotive types will be helpful.

Today you’ll get to know the definition, applications, components, diagram, types, and working of relays in automotive. You’ll also get to know the advantages and disadvantages of these automotive relays.

Read more: Understand the various types of relays

automotive relay

Definition of automotive relays

An automotive relay is an electronic or electromechanical controlled switch that is designed for DC voltages in passenger comfort and retainment systems. It also controls power levels in harsh environments.

Just as other switch, relays are operated by electrical power, computer, or control module. The purpose of an automotive relay is to automate the power to switch electrical circuits on and off at specific times. However, the functions of a relay are more than just automation; it provides the ability to switch multiple circuits, including different voltage types. This will happen within the same relay at the same time.

Generally, relays are an electric switch that works with electromagnetism to convert small electrical stimuli to larger currents. This conversion takes place when an electrical input activates electromagnets to form or break existing circuits.

Leveraging weak inputs to power stronger current, relays can effectively act as a switch or an amplifier for the electric current. These depend on the desired applications.

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The applications of automotive relays are found in various parts of vehicles to power the specific component. One or more appliances in a car can depend on a relay system so that they can work accordingly and stop when one of it develops a fault. Parts in vehicles that use relays include dashboard indicator lights, headlights, braking, and trafficator lights. The engine fan also works with a relay. windscreen, wiper, etc. in fact, along with a fuse almost all vehicle electrical system has a relay for their safety.

Automotive relays are used to provide switching functions that support modern electrical conveniences and safety features in vehicles. These relays provide support to all types of vehicles – cars, trucks, vans, trailers, boats, etc.

12V DC relay switches are suitable for full voltage application, as a low current flow circuit can control a high current flow circuit such as a vehicle’s horn, headlights, auxiliary lamps, fan motors, blower motors, etc.

Read more: Understanding automotive braking system

Components of automotive relays

All relays have the same primary components but some types may contain additional parts depending on their features on applications. Below are the major components that will be found on all types of relays:

Frame – is a container or heavy-duty frame that contains and supports the various parts of the relay.

Coil – is a wire wounded around a metal core. It’s the part that causes an electromagnetic field

Armature – is a moving part that opens and closes the contacts. There is an attached spring that returns the armature to its original position.

Contacts – it’s the conducting part that causes the relay to make (closes) or breaks (opens) a circuit.

Relays have two circuits; energizing circuit and contact circuit. The energizing side has the coil while the relay contacts have the contact side. A relay coil is energized when the current flow through the coil and creates a magnetic field. In an AC unit, the polarity changes 120 times per second, polarity is also fixed in a DC system.

Automotive relay diagram:

Types of automotive relays

There are many types of automotive relays on vehicles to meet the requirement of various applications. Below are the various types of automotive relay and their functions:

Normally open relays:

Normally open relays only have four pins and can be wired only one way – normally open. This is unlike the changeover types of relays that can be wired into different forms including N.O.

Change over relays:

These are common types of relays in vehicles, they have five standard pins. They can be wired to be Normally Open (N.O.) using pins 30 & 87, Normally Closed (N.C.) using pins 30 & 87a. these relays can also be wired as a changeover (both N.O. & N.C.) using 30 & both pins 87 & 87a.

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Whenever the relay is used as a changeover, it switches current from one circuit to another and back again. This depends on the state of its coil (ON or OFF).

Dual open contact relays:

A dual open contact relay has five pins like a change-over relay, but have a dual set of contacts instead of a single contact. Because of this, pins 87 & 87a are Normally Open (N.O.) inside the relay as opposed to 87a which is Normally Closed (N.C.).

Potted relays:

A potted relay is a special relay that functions like other types of relays. Its difference with others is that its bottom part is sealed with an epoxy to ensure water cannot penetrate inside. The limitation of the potted relay is that it cannot be repaired.

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LED flashers:

Led types of flashers are electrically controlled by a small solid-state integrated circuit (I.C.) board. It’s designed to work with a low current draw of LED lighting without causing issues. Well, several connected lights do not affect its output.

Time delay relays:

time relays are popular types of relays that are adjustable to maintain and control current flow through its contacts. Duration of current flow can also be adjected from as little as 2sec to as long as 3min after power is switched off to the coil.

These types of relays contain a solid-state timing circuit with a variable resistor that controls the time delay effect. A time delay relay is commonly used for powering vehicles parking lights for an extended time after the vehicle is exit so that you around the vehicle.

Flasher relay:

Flasher relay has two or three pins, unlike the standard relays that have four to five pins. In a two-pin flasher, one pin is connected to power and the other to the light circuit. While three-pin flasher with an addition pin that would be connected to an indicator light that lets the user know flasher is on.

Not all flashers relay actually operate as a relay, some act more like a circuit breaker. But they are still group as relays.

Thermal (standard) flasher:

Most flasher relays are thermally controlled like a circuit breaker. Current flows through the coil of the flasher relay create enough heat. This heat causes the contacts inside the relay to deflect causing the contacts to open and stop the current flow.

Once enough heat has dispelled, the contacts will return to their original position and a current will flow again. Because these contacts constantly “Make and break”, it produces a flash pattern of the turn signals. Well, numbers of light connected to a thermal flasher will determine its output performance.

Electromechanical flasher:

Electromechanical flashers work with a circuit board that contains a capacitor, a couple of diodes, and a coil to create the same flash pattern as a standard flasher. These types of flasher relays are capable of handling higher loads with improved performance over thermal flashers. The numbers of light connected to these types of flashers have less effect on its output.

Wig-wag flashers:

Wig-wag types of flasher are also known as alternating flashers that uses a solid-state flash circuit. This circuit helps to alternate between two separate lamp circuits.

Skirted relays:

Skirted types of relays have an extended skirt around its bottom to seal around a socket. This avoids water and contaminants from entering the relay contacts. These relays are often used in exposed areas of automotive.

Read more: Understanding the working of electronic ignition system

Working principle

The working of relays is less complex and can be easily understood. If a relay is open, you’ll see an electromagnet coil, a switch, and a spring in it. This spring holds the switch in position until a current pass through the coil. The coil then generates a magnetic field that moves the switch that turns on and off.

diagram of automotive relay

In the above diagram, there is a pinout of a typical 12V relay. These pins are numbered; 85 and 86 are the coil pin while 30, 87, and 87a are the switch pins.  87 and 87a pins are the two contacts that 30 will connect. That is, if the coil is not activated, 30 will always be connected to 87a which is when the switch is OFF. But if the current is applied to the coil, 30 will then contact pin 87.

Well, depending on how you want a relay to work, you can set 87 and 87a to be either open or closed. If your equipment requires a closed relay, you will want to wire it to 87a, it can also be reversed if you want a Normally open relay. most relays are labeled at the bottom to easily identify the power source.

working of automotive relay

Watch the video to learn how automotive relays works:

Advantages and disadvantages of automotive relays


Below are the benefits of relays in all kinds of automotive:

  • Switching of a high current circuit using a low current circuit.
  • Multiple circuits can be activated using a single switch.
  • Execution of logic functions.
  • It saves money, due to its low cost.
  • Automotive relays can switch AC and DC. Transistors can only switch DC.


Despite the advantages of an automotive relay some limitations still occur. below are the disadvantages of relays in vehicles:

  • Relays are too big for switching small currents.
  • They cannot switch rapidly compared to transistors except reed relays.
  • More power is used due to the current flowing through the coil.
  • A transistor may be required to switch the current for the relay’s coil.


In this article, we’ve gone in-depth to explain the definition, applications, components, types, and working of automotive relays. we’ve also seen their advantages and disadvantages.

I hope you enjoyed the reading, if so, kindly comment on your favorite section of this post. And please don’t forget to share with other technical students, it might be helpful. Thanks!

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