How much Current Do Relays Draw?

[TomoHawk]

Most Internet tutorials  show schematics where the relay for fog lamps or headlamps is activated by a switch that's connected to a switched 12V power source.  You could also just bridge the 12V source to Pins 87 ( the relay contacts) and 30 (the coil) and go from pin 30 to ground with a switch in the cabin.  The latter switched-ground method uses one less connection to  the battery.

But for the convenience of turning the lights off when you turn off the keyswitch or engine, you need another hot wire.  My question would be, "where do you get the switched power from?"   You could tap into the radio or heater power wires, but those two things use a good bit of power, and you wouldn't want your light going out on you when you blast the radio or turn on the heater.  You'll need to know how much current as relay needs, so you don't overpower the +12V supply for the relay.

What else is there in the S30 that you can tap into  as a switch 12V power source to activate a lamp relay?  Having 12V on a switch would also be good if you use a lighted switch.

[TomoHawk]

I think I found the answer on this web page:   http://www.bcae1.com/relays.htm

It discusses all kinds of stuff about relay, including the coil resistance and hence the current draw.  For a Bosh relay, the current draw is about 150mA.

After looking at the fox box cover and the wiring diagram, I learned that one of the wires going to the defogger switch is on the ACC section of the fuse box, so I think I'll use that for a switched hot lead to a lighted switch and a relay.

[TomoHawk]

I found some good information here:   http://www.bcae1.com/relays.htm

It discusses many things about relays, including the coil resistance, and thus the current draw by the coil (of a Bosch relay) is ~160 mA (~75 ohm coil.)

After looking at the cover of the fuse box and the wiring diagram, I learned that one terminal of the rear defroster switch connector goes to a switched ACC circuit of the fuse box.  Since I'm not using the defroster, I will use that for a lighted switch and a hot wire to activate the relay.If I didn't have that, I'd use the bridged hot wire on the two relay pins and a switched ground.

Now, the only problem is to get the right gauge wires in the right colors, and to assemble and install the wiring in a way that  looks like  a factory or period-correct way **cough!**  

Edited May 5, 2016 by TomoHawk