I am adding additional LED ribbon to the lighting in the boot. I have a 5w courtesy light in the boot. I tried wiring up using a relay and a feed from the power supply to the towbar, but it transpires that the switch on the courtesy light is switched on the earth side. As a result, the live and the earth to the courtesy bulb are always 12v. I can tell which is +12v by disconnecting the light unit and using the multimeter.
How do I wire a relay to cope with this?
Put the relay coil in parallel with the bulb.
Then a fused positive feed to common on the relay and the LEDs between the normally open contact and chassis.
How much current do your LED lights take? You may not even need a relay...
Highly unlikely that you need a relay. Just wire them in parallel with the courtesy light. It'll only add a couple of watts to the load if that. If you are worried just change the bulb for an led and the total draw will be under the existing 5w
Just stuff the wires in the existing lighting terminals. Secure and bobs your uncle. No need for a relay.
Thanks for the advice. The LED strips are 4.8w per metre, and I have 2 90cm strips, so just under 9w of additional load.
Given the bulb is 5w, a total of 14w is maybe too much for the wiring, just over 1A of current, that's why I'd rather use a relay.
Won't putting the relay parallel to the bulb essentially cause a short circuit?
The relay coil wired parallel with the bulb will be fine.
Although I'd highly doubt you'll need a relay.
0.35mm2 modern thin wall automotive wire is rated at 7 amps, and I'd doubt they'd use anything that thin for interior lights.
Older thicker wire of that size would still be good for a couple of amps.