How Do Automotive Relays Work?

Relays are switches controlled by electrical power, like another switch, computer or control module. The purpose of an automotive relay is to automate this power to switch electrical circuits on and off at particular times. However, the real benefit behind a relay is more than just automation; they also provide the ability to switch multiple circuits, including different voltage types, within the same relay at the same time.

12V DC relay switches are the best solution for full voltage applications, as they allow a low current flow circuit to control a high current flow circuit, like a vehicle’s horn, headlights, auxiliary lamps, fan motors, blower motors and countless pieces of equipment existing on vehicles today.


How is a relay designed?

If you were to open a relay, you would see an electromagnet coil, the switch, and a spring. The spring holds the switch in position until a current gets passed through the coil. The coil then generates the magnetic field which moves the switch on and off.

Looking at the diagram below, you can see the pinout of a typical 12V relay. Note that each pin is numbered. 85 and 86 are the coil pins while 30, 87, and 87a are the switch pins.

87 and 87a are the two contacts to which 30 will connect. If the coil is not activated, 30 will always be connected to 87a. You can think of this as the switch in OFF. When current is applied to the coil, 30 is then connected to pin 87. The great thing about relays is that you can set 87 and 87a to be either open or closed, depending on how you need the switch to work. If you want a closed relay, you will want to wire to 87a. If you want a normally open relay, you will wire to 87.

Although most relays are labeled at the bottom, you can always find the 30 pin set perpendicular to the other pins for easy identification of the power source.

Realizing that 85 and 86 are the coil pins, these pins will be transferring the current through their coil. One of these will be used to ground your current, while the other will be connected to an accessory, or your point of current.

87 and 87a will be connected to your controlled accessories that you wish to turn on and off with your relay.

30 will then be the pin connected to your battery power.


Types of Automotive Relays

  1. Change Over 
  2. Normally Open
  3. Potted
  4. Flasher
  5. Skirted
  6. Time Delay
  7. Dual Open Contact

This list of type of relays is not exhaustive, but is a great start to understanding the options available.

To learn more about these different types of relays, check out our blog post on the types of relays.

Posted in Automotive, Relays & Power Distribution.