Have you noticed I've mentioned fires four times now? Use caution.

The average headlight circuit is about 20 amps; your fog lights will probably draw something close to 30 amps. If you connect your new fog lamps to your headlight circuit you're now trying to draw 50 amps from a 20-amp circuit. THIS IS BAD!

The relay fixes this problem as it has a low amperage side (the trigger side) and a high amperage side (usually drawing power from the battery). The trigger side uses the headlight power as a trigger. When you turn your headlights on, the relay only draws 0.05 amps to trigger a small electro-magnetic switch. The switch closes a contact that now sends power to your fog lights from the high amperage side of your relay, in this example, your battery.

Take a look at the diagram in Figure A.

FIGURE A

The wiring diagram of a relay. Click picture for a larger image.

Pins 85 and 86 are the low amperage trigger wires. Sending (+) power to 85 and (-) power to 86 will trigger the relay. This will cause the high amperage contact to move from contact 87a to 87. High amperage power is now flowing through your relay from pin 30 to pin 87. Once you remove power from the trigger, the contact returns to pin 87a. I'll talk more about the importance of pin 87a later.

Wiring your fog lights.
So in our fog light scenario, pin 86 would be grounded. Pin 85 would get connected to one of your (+) headlight low beam wires. Pin 30 would get connected to the battery. Pin 87 would get connected to the (+) side of your new fog lights.

When you turn on your headlights, pin 85 would receive (+) power and trigger the switch, sending power from your battery to your fog lights. Now your fog lights turn on and off automatically with your headlights, and because you connected them to the low beam wire, they'll now turn off automatically when you turn on your high beams!

Being creative
You can take this simple example and apply it to anything. You already know that to trigger the relay you need to apply a ground and positive power to pins 85 and 86. In our fog light example we used a positive trigger (the headlights) to turn on our fog lights. But what if you want to turn on something using a negative trigger?

For example, what if you want add more interior lights to your vehicle? Most vehicles use a negative trigger (-) to turn on the dome light. When you open your door there is a small pin switch that grounds a wire to the chassis of your car and turns on the light. Here's how you'd add lights using a negative trigger.

Find a good source for constant power (power that does not turn off when you turn off your key). The best source is the battery or a secondary fuse box that you've installed. Hook the constant power wire to pins 30 and 85 on your relay. Now hook the (-) side of your new lights to a good ground.

If you're grounding to a painted surface, grind off the paint first so you get a good ground. Run a wire from your factory pin switch to pin 86 on the relay and BLAM! Your new lights will now turn on when you open your door!