Relay Case: How to Use Relays and Why You Need Them – OnAllCylinders

An electrical system is only as reliable as its components. One easy way to increase system reliability and performance is by using relays to switch devices (lights, fuel pumps, fans, etc.) on and off. A relay is an electro-mechanical switch. An electro-magnet (also called a coil) is used to pull a set of contacts, or pins, together. You can read our earlier post on How Relays Work for a more detail description of how relays operate.

Why not just use an ordinary on/off switch, you ask? Here are some reasons why relays are better than switches:


  1. A correctly wired relay will provide the shortest electrical path (i.e. shortest wire length) between the battery and the device(s) controlled by the relay. Combined with the proper gauge wire, this will minimize the voltage drop between the battery and the device, allowing it to function at peak performance levels.
  2. Using relays allows you to control a number of devices with a single switch (a master ignition switch on a race car, for example). Having only one switch to turn off is safer in an emergency, and more convenient as well. If you’re into clean-looking systems, you can use one switch and several relays instead of a bank of bulky switches.
  3. Relays allow you to use the proper size fuse for each device, and to place the fuses closer to the battery.
  4. If you use a vehicle’s stock wiring and switches to control aftermarket devices like high-output lighting, relays will not overload or stress the OEM components. The average automotive relay can also handle a much higher current load than a switch (about 30 amps vs. 3-20 amps).

Relay Types

It’s important to know a relay’s pin configuration and function before connecting devices to it. Many automotive relays are similar in appearance and pin configuration and will plug into the same relay socket, but are completely different in the switching duties they perform.

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The most common type of relay used in automotive applications is the single pole/double throw (SPDT). Also known as the Bosch relay, the SPDT has a common, movable contact that moves between two fixed contacts, termed Normally Open and Normally Closed. When the relay is off, the common and Normally Closed contacts are connected. When the relay is engergized the common is switched over to the Normally Open contact.

Another type of relay is the single pole/single throw (SPST). The SPST relay is often found in the wiring harnesses for aftermarket lighting; it has a common contact and two Normally Open contacts that are internally connect. When the switch is activated, the contacts are connected.

When power is removed from a relay’s electro-magnet, a high-voltage spike occurs. This spike can hurt on-board computers or other sensitive electronics. If your system has such devices, it’s a good idea to use replays with an internal shorting diode. The diode forces the voltage spike back into the electro-magnet, where it dissipates as heat.

If you move the battery to the rear of the vehicle, locate the relay/fuse bank near the battery and run 20-18 gauge wire to the cockpit to trigger the relays. If you have a master ignition switch controlling several devices (fans, ignition, water pump, etc.), but still want to use a switch for each device, you can wire the master ignition switch to those individual switches, and then to the device relays.

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Relays can help you make an electrical system perform better and run reliably. That’s why you’ll find them in most quality aftermarket lighting systems and wiring harnesses. Once you use them, you’ll wonder what you ever did without ‘em!

Author: David Fuller David Fuller is OnAllCylinders’ managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing. [external_footer]