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Old 01-29-2012, 02:22 PM   #1
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Lights - switch and relay?

What is the difference between a switch and a relay? Why do I need a relay? Do I need both or just one or the other?

I am wiring up 4 lights on the front bumper. Ideally these would be on switched power (off w/ignition off) and would have a switch inside the cabin to turn them on with. I do not want these wired to my headlights so that they come on with brights. Where is a good place to connect to a switched power source?

Thanks,
John

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Old 01-29-2012, 02:27 PM   #2
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You use a relay because most lights will pull more current than the switch can handle. You can pull 12v source for the switch from about any circuit being as there will be very little current draw through the relay coil. Pull the 12v supply for the lights through the relay from the battery itself.

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Old 01-29-2012, 02:28 PM   #3
Knows a couple things...

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John, a relay is nothing but a heavy-duty switch that is controlled by a smaller switch you can mount anywhere. Think of it as a remote controlled heavy duty switch.

Driving lights draw enough current (power) that they can damage a common size switch, plus you would need to run heavy wiring throughout the circuit so the lights would operate at full brightness and to keep the wiring from getting hot if you didn't use a relay.

A relay allows you to run small gauge wiring between the passenger-compartment switch and the control side of the relay, and only use the heavy wiring between the battery, light-side of the relay, and lights.

You can make the whole relay and switch wiring easy by buying a pre-wired switch and relay kit from any aftermarket light dealer. KC, PIAA, etc. all make pre-wired switch and relay kits that make getting your lights work an easy thing to do.
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Old 01-29-2012, 02:29 PM   #4
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Switches activate relays. Relays can isolate the power if you will from th switch. You don't want the power for four offroad lights going thru the switch.

When the switch activates the relay, if you wire it correctly, power will be drawn from the battery without touching the switch wiring.

It really is a safety thing.

And you can find good switched power lines by your glove box and fuse box to power the switch.

If you need good switches consider the Carling Contura switches from www.otrattw.com.

If you search my profile and look for threads I started I have a writeup for wiring lights that may help.
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Old 01-29-2012, 04:16 PM   #5
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Quote:
Originally Posted by Jerry Bransford View Post
John, a relay is nothing but a heavy-duty switch that is controlled by a smaller switch you can mount anywhere. Think of it as a remote controlled heavy duty switch.

Driving lights draw enough current (power) that they can damage a common size switch, plus you would need to run heavy wiring throughout the circuit so the lights would operate at full brightness and to keep the wiring from getting hot if you didn't use a relay.

A relay allows you to run small gauge wiring between the passenger-compartment switch and the control side of the relay, and only use the heavy wiring between the battery, light-side of the relay, and lights.

You can make the whole relay and switch wiring easy by buying a pre-wired switch and relay kit from any aftermarket light dealer. KC, PIAA, etc. all make pre-wired switch and relay kits that make getting your lights work an easy thing to do.
Wow, thanks for the great explanation. I understand now. I should have explained earlier that these lights were installed by the PO but it looked like it was very poorly done so...I'm redoing it. They were installed on the jeep but not electrically connected as he had recently installed a new front bumper. I found that the two pairs of KC's are 125W and 55W. That's a lot of current isn't it?? If I ran them all it'd be 30 amps! Is that hard on my alternator and battery?

Upon further inspection of my jeep I found that the PO had actually installed a relay for the two big lights and that he wired the two small lights into what appears to be the pre-existing connectors for the fog lights. I can't tell if the pre-existing connectors power is switched yet because I removed the horrible switches he had installed inside the cabin before checking it. I'll get to that probably tomorrow.

If they aren't switched where should I connect them? I found a wire behind the glove box marked "switched power" but it also says max of 3 amps.
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