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this is what the dictionary says about relays..

5. ELECTRONIC ENGINEERING device that regulates another: an electronic or electromechanical switching device, typically operated by a low voltage, that controls a higher-voltage circuit and turns it on or off..

now is it madatory to have one hooked up to make your lights work??...no...i don't have one and my lights work fine...

allan
 

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I think it also acts like a fuse so if a power surge or whatever cant damage your lights as it will kill th relay first.
 

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Superfly
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The main purpose of a relay is to allow remote, electric control of other switches. I suppose it would provide the protection of a fuse if the relay's contacts were made of thin pieces of metal like a fuse but AFAIK, that's not how they are generally used in automobiles.

Using relays with a set of lights is a common application. The wire gauge used to power the lights is quite heavy while the gauge used to control the relay itself can be very light. The benefit here is that you can run the control wiring to the dash using thin gauge wire and a small switch. As for running power to the light, you can use heavy gauge wire PLUS it only has to be long enough to go from the battery to the lights (the relay can sit anywhere in between them). You don't need to run the power wire to the dash and then back to the lights. The shorter, heavier gauge wire lets more current reach the lights.

Check out this site for a better explanation:
http://www.howstuffworks.com/relay.htm

...lars



 

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a relay is simply an electrically opereted switch. its used in places where the switch alone cannot handle the current needed to run a device such as lights. a solenoid (like the one used for your starter or winch) is essentially a big relay. they are not a fuse, but do carry a maximum current rating. they work by magnetically pulling two contacts together to complete the circut. the primary side, which is connected to your switch acts upon a coil inside the relay. the magnetic field pulls a tab or plunger which connects to two contacts of the secondary side. the primary and secondary circuts are wired independent of each other, and therefore the load of the of the sec. circut can be very high while the load on the switch reamins very small. your average generic relay can handle about 30A with only about a 5mA load on the switch. the average starter solenoid is rated for 300A intermitantly. imagine that going trough your ignition switch.

back to your question. id recomend using a relay for anything over 5A, or 60 watts maximum. using a relay everything is a good idea, but just means more wiring. most aftermarket light kits dont include one, and youll likely find the swich gets quite hot and burns out quickly.
 
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