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

Dec 22nd, 2021 at 09:37   Automobiliai   Skuodas   19 views Reference: 57
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Kaina: Susisiekite

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.

Another important function of the PCB 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.

How to Understand the Electrical Specifications of a Relay

Basic 5pins Car Relays feature electrical ratings for both the coil and the internal switching contacts. The coil voltage rating is the voltage required for the coil to operate correctly. The switching circuit of the relay also features a voltage and ampere rating. This is the maximum rating of the switch contacts and should NOT be exceeded. Double throw relays often have 2 x switch electrical specifications. One for the normally open terminal, the other for the normally closed terminal. ie. N/O: 35A at 14vdc, N/C: 20A at 14V

Relays are part of a car’s electrical system. Without them, electrical components would not work. Several electrical devices in a car run off relays.


As its name suggests, it relays current from the battery to the device. It may seem superfluous as any device would still function if wired directly from the battery. But there is a sound reason for using relays. A relay is essentially a switch with an electromagnet inside. When energised, it triggers a lever to close the contacts of a circuit.


Let’s use the car horn as an example. When you press the steering-wheel boss to activate the horn, a small current – less than 0.5 amps – is sent from the horn switch to the Auto Relay. The relay’s electromagnet then activates an internal switch, which bridges the main 12-volt supply to the horn’s terminal. Most horns require at least 10 amps to work. If it is a twin-horn set-up, you are looking at 20 amps at least. A relay’s switch typically handles 30 to 40 amps.

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