Automotive Relay Guide - What is a Relay?

What is a Relay?

What is a car relay?  In the most simple terms an automotive relay is an electronically operated switch.  The type most commonly used in the automotive industry is an electro-mechanically operated switch.  They are found in all types of vehicles - cars, trucks, vans, trailers and boats.  They employ an electromagnet device to mechanically manoeuvre a switch to make or break an electrical circuit.  The type of relay most frequently used in the automotive industry is the small cube shaped unit known as a standard relay or a mini relay.


Application & Advantages of Automotive Relays:

1: Switching High Current Circuits:

The most common application of automotive relays, see collection, is to switch a high current circuit using a low current circuit.  This application comes into play whereby an in-line switch does not have the capacity to handle the current required to switch a high current electrical system.  For example, this scenario could be observed during the operation of a set of high powered worklamps.  If the worklamps are wired to light when the headlights are activated they could exceed the capacity of the existing loom set-up.  An automotive relay can solve this problem.

2: Activating Multiple Circuits via Single Switch

A key advantage of using a relay is that it allows the activation of multiple circuits from a single switch.  A single input within an electrical system can be used to activate and trigger multiple relays installed in an electrical circuit.  These relays can then complete/ break multiple circuits and therefore execute multiple functions given a single input signal.  A simple real life example of this multiple actions from a single switch is the central locking system in a car - a single button press results in all the door locks locking or unlocking.  Simple, time-saving and effective.

relay up close picture

3: Execution of Logic Functions

Although in recent times car relays have been largely replaced by OEM electronic module designs they can also be used to perform logical operations.  Auto relays can be used to perform both simple and more complex logical tasks such as - momentary inputs and time controlled functions, for example - the time control operation of wiper blades and the interior light time delay.  While manufacturers have largely moved over to programmable logic for these tasks, relays offer a simple and cheap alternative.  The likes of a car enthusiast may find it easier, cheaper and more appealing to install a relay to operate a simple logic function.

4: Save Money

High current electrical circuit components are expensive.  Low current components are much cheaper.  The use of an auto relay means you can limit the installation of high current circuitry to those parts of the system to which it is essential.  Then my installing relays at critical points the remainder of the circuit can be completed with more inexpensive low current components.  This helps drive costs down.  Everybody's happy!

How Does a Relay Work?

Electro Magnetic Relay Structure:

An electro-magnetic relay is constructed of a coil of wire that is wrapped around a soft iron core - known as a solenoid.  It has a movable iron armature, a low reluctance iron yoke and a set of contacts.  There can be multiple sets of contacts depending on the design and function of the relay.  The yoke has a hinged armature attached which is linked to the movable contacts.

how does a relay work



Operation of Relay Switch Mechanism:

A spring holds the armature in position and when the relay is de-energised an air gap in the magnetic circuit is created.  This condition delivers the the physical closing and opening of contacts.  In an automotive relay with two contacts this would mean that one contact is opened while the other is closed.  Different relays may have a greater/ fewer number of contacts depending on design.  When an electric current is delivered to the solenoid a magnetic field is generated.  This field triggers the armature and the resultant movement of the contacts either makes or breaks the circuit.


Types of Relay:

Although there are many different types of car relay out there in the market the two most commonly used in the automotive industry are 'Make & Break Relays' and 'Changeover Relays'.

picture of a changeover relay and a make or break relay

Make and Break Relays:

A make and break relay has four pin terminals and a single high current circuit.  The contact in a make or break relay is either normally open (NO), or normally closed (NC).  The open or closed position of the relay depends on its design and controlled by whether the relay is at rest or energised.  A normally open relay, (NO), at rest has a broken contact - it's open.  The reverse is true for a normally closed, (NC), relay.  A normally closed relay at rest has a closed contact.  Typically the relay is switched manually using push or toggle switch - an everyday example of this is the car ignition switch used to start the engine.

Changeover Relays:

A changeover relay is built with two circuits instead of one and five pins rather than four.  Instead of making and breaking a single high voltage circuit it alternates the closing and opening of two circuits - 'changing over' which circuit is open or closed.  A typical example of a change-over relay in use can be seen in the operation of vehicle headlights.  The relay can switch between circuits to activate the full beam or dipped beam as required.  Another everyday example is the car air-conditioning system.  A changeover relay is used to activate either the heating/ cooling system depending on where the thermostat is set.  One or the other will be running and never both at the same time.

Single Pole Single Throw v. Single Pole Double Throw:

Make & break relays are sometimes referred to as Single Pole Single Throw relays.  Changeover relays are sometimes referred to as Single Pole Double Throw relays.  While other types of relay exist - Changeover and Make & Break are by far the two most commonly used in the automotive industry.

Relay Coil & Contact Terminology:

The terminals of a relay are numbered using an identification system known as DIN 72552.  This system was developed in the German automotive industry and has since been adopted across the world as the standard method for labelling and identifying electrical terminals.  DIN 72552 assigns a numeric code that clearly defines the function of each pin/ terminal in a relay.  The table below shows the codes used in automotive relays.

  • 85 Relay Coil Negative
  • 86 Relay Coil Positive
  • 87 Common Contact
  • 87a Normally Closed Contact (NC)
  • 87b Normally Open Contact (NO)
  • 88 Common Contact 2
  • 88a Normally Closed Contact 2
  • 88b Normally Open Contact 2
  • 30 Feed/ Line In Positive

Diodes & Resistors in Relays

Some automotive relays are designed with diodes or resistors built in. These devices help suppress coil voltage spikes and protect the electronic circuit.

Diode Relay Protection:

When the voltage is removed and a relay is denergised the magnetic field collapses.  This can result in a voltage surge in the opposite direction.  These low current surges can have significantly high voltages, often up to 100 volts.  To prevent the damage of sensitive upstream electronic circuitry a diode can be installed across the coil.  The diode absorbs and dissipates the momentary voltage spike and protects upstream damage. 

Resistor Relay Protection:

Resistors can be employed to provide protection similar to a diode in the previous example.  They too can absorb voltage spikes which result from a sudden magnetic field collapse.  A resistor however allows a small current to flow and is not as effective as a diode in absorbing a voltage surge.  The advantage a resistor holds over a diode is that it is not sensitive to polarity and the 12v can be connected to either the 85 or 86 terminal, (as per DIN 72552).

Micro Relay v Standard Relay - What's the Difference?

A micro relay is a smaller version of the more popular standard relay and are used when space is at a premium.  Note, a standard relay is also sometimes referred to as a 'Mini Relay'.  Micro relays are more compact than the standard size and are an ideal choice when you need to save space - for example in an ATM fuse box set up with mini fuses.  They have a smaller rectangular shape compared to the square cubed shape of the standard relay.  Micro automotive relays use a different terminal number identification system compared to standard mini relays.  See the list below for micro relay terminal numbering:

  • 1: (4.8mm) Coil
  • 2: (4.8mm) Coil
  • 3: (6.3mm) Common Connection to Terminals
  • 4: (4.8mm) Normally Closed (NC)
  • 5: (6.3mm) Normally Open (NO) 

Relays in Cars: 

A typical car can contain up to twenty automotive relays and they can be located throughout the vehicle.  However common locations for their installation are in the engine compartment behind the kick panels and underneath the dash.  Fuse block covers often have an electrical schematic detailing the position and location of fuses and auto relays within the block.  

That should answer the question we have heard so many times - what is a relay in a car?

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Sarah – the switch that just initiated the relay would be upstream, like o2 sensor nomenclature, since it comes first in the system.



Hey, so, about these diodes, lol. The article says it protects from upstream damage. But I would take that as protecting the item that just had the load? Or are we talking about the switch that just initiated the relay, because that would be in the ‘opposite’ direction right? Thanks.

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