Automotive Relays – What Are They And Why Do You Need Them?

What is a relay?

A relay is an electromechanical or electronic device through which a heavy load

can be   switched  ON & OFF with a nominal input of current & voltage. This article

focuses on electro-mechanical relays used in automotive applications.

How does a relay work?

The utility and function of a relay can be best understood by thinking of a lever.

A lever is inserted at a corner of a heavy load and by placing a wedge (fulcrum)

under the lever, the heavy load can be easily lifted by a small amount of effort.

Similarly, a heavy electrical load drawing heavy current can be  switched  on & off

by applying a small amount of current through a coil(solenoid). What happens is

this – the coil gets magnetized with the current and attracts or repels a

plunger(a rod passed through the coil). The attracting or repelling of the plunger

either connects two  switching  contacts in the relay (also known as NO or Normally

Open/Single Throw/Form A) or separates them (NC or Normally Closed/Double

Throw/Form C), as required. As soon as the energisation of the coil is stopped,

the main contacts go back to their original position.

In automobile applications relays of different configurations with single or

multiple contacts are used for different applications.

Why is a relay required?

Apart from  switching  a circuit with heavy load, on & off, a relay is required to

1. Conserve energy

2. Prolong the life of device

3. Improve the efficiency of a device.

For example let us take the  switching  of two headlamps of an automobile. If the

same is  switched  on through a normal  switch  at the dashboard, the voltage drop

from the battery to the lamps through the  switch  will be enormous and the heat

generated in the circuit due to resistance will be considerable. The heavy current

will be drawn by the lamps which will not only heat the circuit but also drain the

battery heavily.

To give an example of the effect of voltage drop, a chart is given below:

Operating Voltage. Luminous Intensity of a Lamp.
100% 100%
95% 83%
90% 67%
85% 53%

To  switch  on a 12V-60/100 Watt Halogen lamp, a typical 12V Relay allows a voltage

drop of only 0.02V and the coil current is only 2 milliamps.

Another important function of the relay is quick  switching  off(0.05Sec Max.). The

more time the contacts of a  switch  take to break at full load, the more sparks

will be generated due to electromagnetic effect on contacts in a  switched  on

state. Prolonged sparking will mean more wear of the contact points and the danger

of contacts melting and generating a fire. International standards have very stringent relay specifications. The endurance of a relay is generally specified in millions of  switching  operations.

Where should a relay be installed?

Wherever manual  switching  has been used for direct  switching  of a Lamp or Motor or

any gadget exceeding 10 Amps of load, a suitable relay must be put in the circuit.

It is best if the relay can be kept as near as possible to the load (but not in a hot area surrounding near the engine)

How should a relay be installed?

1. Wires connected to the manual  switch  should be connected to any Coil terminal

of the Relay.

2. The load should be connected to any one contact terminal of the Relay (unless

Positive/Negative are specified) and the other to the positive terminal of the

battery (for Negative Ground circuit), with an appropriate Fuse.