This page is to help familiarize you with relays which can be used to control hi-current devices (headlights, etc.) that cannot directly draw from the existing wiring harness. In the example of factory headlight wiring harnesses; some hi-current aftermarket headlights can fry the wiring if you don't run a seperate headlight power circuit controlled by a relay.

New & salvaged relays.

Here's another generic style relay found at AutoZone, Advance Auto, etc. The function is identical.



What does a relay do?

A relay is used to indirectly control power on/off to high current draw applications. This allows you to use a low current carrying switch to control high amperage equipment without using much more expensive switches and extensive runs of large gage wire. Relays also are often remote mounted keeping hi current cables/wires away from the user. Think about it... a worker at a power distribution facility doesn't run outside, drive to the dam 25 miles away then throw a giant switch every time he needs to route power somewhere. He merely throws a switch which activates a remote relay controlling the power. No need to be right there, no need to run costly heavier gage wires to the worker's location.

On a much smaller scale, you are using relays everyday you drive your rig for everything from headlights to cooling fans. If ya stop to think about it... your starter solenoid is also a remote heavy duty relay that takes low current switched power (from your ignition switch) to activate and control a high current draw device (your starter) without you having to open the hood every time and turn it on long enough to crank the motor.


How does a relay work?

The simple explanation is that a low power switched circuit feeds a solenoid inside the relay case. When power is applied, this solenoid 'energizes' a coil which magnetically closes another SEPARATE circuit contained in the case.

The separate circuit is fed separately by higher capacity wire and is active when the low power circuit is active; off when the low power circuit is inactive.

*NOTE! There are relays available that are ON until switched OFF or that have two circuits inside them! These are not common in automotive use but are used here and there. Most likely you'll never use one for something like this but when you get a relay from the auto parts store you should still confirm the application is correct..

Terminals found on a generic relay often used to control fog lights, horn, etc:

Bottom view of generic relay.


A VERY basic circuit.

A basic relay circuit you can use for devices such as fog lights, electric compressors, c.b. radio linears, etc. When switch is in "OFF" position, no current flows to relay and device circuit is "OFF". When switch is in "ON" position, current flows to relay which switches "ON" high current to your device.

NOTE! THIS CIRCUIT SHOWS A DIRECT CONNECTION TO BATTERY FOR SWITCH! YOU SHOULD CONNECT YOUR SWITCH POWER TO A FUSE THAT IS CONTROLLED BY YOUR IGNITION SWITCH UNLESS YOUR CIRCUIT MUST BE ACTIVE WITH THE KEY OFF!

VERY Basic circuit.

Switch fuse should be 5 amps (or less) to protect against shorts in switch circuit. (Switch circuit can usually be ran from existing switched fuse in fuse panel.)

Device fuse should be same rating as relay capacity. (Usually 30 amps.)


What are some uses for a relay?

To control hi current circuits that your existing fuse box can't handle.
For example, I use a relay to control power to my c.b. linear which uses a 25 amp fuse. High current headlights often come with a relay bundle to keep from frying your original headlight harness while letting your original headlight circuit act as the switching control. Foglights are a good candidate for the 'Simple Relay Circuit'.


Tech Tip: Install an auxiliary fuse box under the hood to handle any high current accessories you might add along the way. Most auto parts carry 6 circuit fuse boxes.




If you have pics of your own repairs or can suggest other methods - please contribute your ideas (and pictures) to this article!

Revised on: October 11, 2006









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