What's a relay and is it needed for foglights?

[prynce_qt]

I'm about to install my ebay foglights and the whole wiring harness is pretty confusing and decided to do it my way. I'm going to be using the connector in the fuse box as it only turns on when the headlights are on and provides 14 volts. I ground the lights to the frame and the positive directly to a fuse then to the fuse box connector. It works perfectly, turns on when I put the parking or headlights on.

Now what I'm wondering is, what is the point of the relay and do I really need it? I checked the voltage level with 1 foglight on and with 2 on and the voltage only dips a bit but still provides about 12v to power both foglights. So since I have enough volts and the fuse in between, can I leave it as is or do I need the relay between the power source and the foglights?

[DragonZealot]

Looks like you are taping into the power source for the parking light which in most cars is not relayed.

You should use a relay. The power source you are tapping into is from the light switch which is not designed to carry such a big current (close to 10A). The light will work but this will greatly decrease the life of the light switch.

Only time you can do away with a relay is if you want to light to come on with the headlight. In this case you tap into the "contacts" side of the original headlight relay.

[prynce_qt]

I'm not really tapping into anything. I bought a foglight kit and that's where the harness goes into to provide power for the foglights. If the connector is not made to draw so much amp, what will a relay do to help the problem?

[rc51]

Relays are generally used for switching purposes especially when hi load devices are used. Typically fog lights are high enough load to require their own power and ground feeds with proper gauge wire and a relay that would control their switching.

Make sure you are not drawing more current than the wire/switch is rated for otherwise you risk a fire at worse case...and if you're not drawing enough juice than you're not getting full use out of your fogs.

[DragonZealot]

I'm not really tapping into anything. I bought a foglight kit and that's where the harness goes into to provide power for the foglights. If the connector is not made to draw so much amp, what will a relay do to help the problem?

You have to tap into something to get the electricity to power the light.

You use a relay because the switch can not carry the current (about 10A) to run the lights. With a relay, instead of switching a 10A circuit to power the lights, you switch a 0.5A circuit (coil of the relay) to close the contacts of the relay which can handle a much higher current.

[prynce_qt]

I can understand that. But I'm not using a seperate switch to power the lights on. I guess the switch I'm using is the one already built into the car, there is an output voltage connector on the fuse box made for foglights to use for OEM kits. Usually that fusebox connector would go into a relay and another wire from the switch would go into the relay to turn on the relay and output power. But since I'm not using another switch, the power automatically turns on when the parking lights are on, do I need a relay still because all I would be doing is tapping the wire that would come from the switch to the wire that connects to the fusebox to always provide it power when the parking lights are on, it really defeats the purpose unless it is to protect some circuitry.

You have to tap into something to get the electricity to power the light.

You use a relay because the switch can not carry the current (about 10A) to run the lights. With a relay, instead of switching a 10A circuit to power the lights, you switch a 0.5A circuit (coil of the relay) to close the contacts of the relay which can handle a much higher current.

[DragonZealot]

Simply put if you tap into the "contact" side of existing relay then its OK.

If you tap into the "coil" side of existing relay then you need a relay. Otherwise it will burn the switch.

[dealhunting]

You are talking about 2 different types of circuits.

1) control circuit- very low current. typically used to switch on a load circuit or sends signals to some type of ecu.

2) load circuit- typically a high current. ie: lights, elec motors, actuators, etc.

It sounds as though you are hooking up a load circuit to a control circuit and the end result will be burnt/melted connectors or in the extreem case, fire. If the fuse box tap you are using is normally wired through a relay and then into the load, then you WILL burn out that circuit (best case). If the tap has already been wired through a relay or is a high current output, then you might be ok, but unless it was designed specifically to carry the extra load you may still be SOL! Even though your circuit may be rated at 10A, that is usually a max not nominal value. Nominal value is usally only 1/2 the max rated value. Therefore the extra load you have placed on the existing circuit, which it was not designed for, will overheat over time.

The relay you need only costs a couple of bucks and even a beginner would only need 30 min- 1 hr to properly install and wire a fog light relay, especially when the switch and wiring is already in place. Either you pay a little and spend some time now to do it right or spend a lot later to replace your fuse box and whatever else you may ruin.

In either case, you WILL pay!

[DragonZealot]

One more thing the voltmeter can not tell how much current the circuit it can provide. So the fact that you get 14V and it lights up does not mean that it was installed properly.