Setting up a relay...

[giant_mtb]

So, I put some auxiliary "driving" lights in the grill of the Bravada.  Just some cheap-o, 4" rounds.  I got 'em all wired up and everything, but I'd really like to set them up with a relay so that they trip on and off with the high beams instead of using a separate switch all the time.  They're just too bright to use with low beams, and I don't want to be a dick to other drivers.

Can someone explain to me how to use a relay to make them go in sync with the high beams?  AFAIK, I basically have to find the positive power-send that goes to the headlights, tap that wire, put the relay in-line, and hook it up to the lights. 

[Rupert]

Yep. You probably want to tap the headlight power wire before it gets to the headlight relay. Also, you want to make sure to use the high beam circuit.

[giant_mtb]

Why tap it before the high-beam relay?  Wouldn't it make sense to have it tapped after the relay? 

[VTEC_Inside]

As long as its a wire that goes hot when the high beams are turned on you are good.

The one that triggers the existing relay would be fine, but you could just as well tap into one of the powered wires as well.

[giant_mtb]

Alright, cool.  Is there any specific kind of relay I need to get?  A certain wattage/voltage/amperage or anything? 

[r0tor]

Ok, the general purpose of a relay is to keep high voltage or amperage out of the car interior.... A relay has 2 circuits - the power circuit and the tripping circuit.

The power circuit will be wired in the power feeding your fog light.  The relay will have to be sized to handle the wattage of the the driving lights or it will overheat and die or start on fire.

The tripping circuit on a car is generally a 12v circuit but low low amps like whats in your headlight switch.  You close the switch, it completes the 12v circuit, but there is very little current in the circuit so its safe for the operator.


attached is the relay for my foglights... its not on the page, but the relay tripping circuit only gets +12volts when the low beams are on.  When the low beams are on and then when the foglight switch is turned on it completes the 12v circuit and makes the relay energize the power side going to the foglights