I have a project I am working on, and am a little rusty on my electronics theroy...

I have a set of projector fog lights that I want to wire up directly to a switched power source (in the fuse box)... The directions want me to wire it up, using a relay, tapped into my parking lights.

If I don't want to tap into my parking lights, but rather directly into a switched (key ON = power) , do I still need to wire them through the relay? Also, I am changing out the actual on/off switch they supplied with a lighted switch, if this makes a difference.

 

Yes, you need the relay as it's likely the switch can't handle the extra power anymore than the wiring to your parking lights can.

The relay gets connected to the battery through a fuse, to ground, and then to a +12V source that causes the relays electro-magnet to energize pulling the armature down and closing the circuit from the battery input to the output terminal that will feed the lights. The relay is doing all the "work" and the +12V source is just providing enough power to fire the relay ... which is a bunch easier than running the lights' power requirements.

If the fog lights are whimpy you might find a switch big enough to work. Take the wattage of the lamps together and then divide by 12 volts to get the current that the switch will need. Two 30 watt bulbs need 60 watts together, divided by 12 is 5 amps. You would need a 12 VDC switch rated at 5 amps for the circuit without a relay. In practice the 5 amp switch should work a 5 amp load but if you're getting a "made in China" switch I would err on the side of safety and get something bigger.

You'll want to protect the car with a fuse in either case connecting a fuse to the battery feed to the relay or in the +12V line to the switch.

 

Awesome Paul ... Thanks for the info. Makes sense now.

I'll just take the spade on the Relay that was going to be tapped into the parking lights, and put that on the switched power spade in the fuse box. This way, whenever the key is on, the foglights will work independantly.

Looking at the fog lights, they don't seem to be "whimpy" ... they are projector fog lights, with 55w H3c bulbs in each. The relay used is a 30A 12v, and the little switch it comes with is rated at 15a @ 12v.

 

The lights are drawing a bit more than 9 amps which the relay will have no problem with. The little switch will do that much all by itself but its better to run though the relay to save having to run that much current inside the firewall to the dash and then back out to the lights.

Good luck and double check your work.