How to wire a relay for 12v car lighting

Question

I would like to reconfigure the current wiring of my daytime running lights.

The relay is currently switched on via an accessory power wire with inline switch.

I would prefer the relay to turn the lights off when the headlights are switched on.

How can I wire the relay to switch off the daytime lights when power is applied to the headlight wire?

Thanks

Answer 1

Power a relay from the switched headlight power. Then put the NC (normally closed) contact in series with the daytime lights. When the relay is unpowered, the daytime lights work normally. When the relay is energized, the NC contact opens, and turns off the daytime lights.

Any "12 V" relay that can handle whatever the current of the daytime lights are is good enough. Make sure to look at the relay's DC current rating. That is usually lower than the AC current rating.

Since you are driving a relay coil directly with no other electronics, you don't need to worry about the voltage spikes on the car power line. Relay coils aren't going to be damaged by the occasional spike.