Car headlight wiring - Diode info please?

Thread Starter


Joined Dec 21, 2008
Ok, I'm wiring up a setup for new headlights for the car. I've got a challenge, I think I know the answer. I'm after confirmation and help finding the correct diode (unless I'm completely wrong).

When I switch the headlights on, the headlight circuit is live (12v). When I switch the mainbeam on, the headlight circuit cuts, and the mainbeam circuit is live (12v). What I want to do is have the headlight circuit live when the mainbeam circuit is live (not the other way though). Below is my diagram using a diode to acheive this. I'm not sure what diode I need, or where I could source one from in the UK.

Any help really appreciated.



Joined Jul 17, 2007
What do you mean by "main beam"? Is that like "high beams" vs "low beams"?

Be aware that headlamps require quite a bit of power. If you try to run all of your lamps at once, your alternator may not be able to keep up.

A diode (even a Schottky diode) will have a significant voltage drop across itself at that kind of power levels (if your headlight beams are 55 Watts each, that's 110 Watts or about 8 Amperes of current). A heavy-duty Schottky diode would likely drop 0.5v or more across itself, dissipating 4 Watts in heat.

You may be better off using a P-channel power MOSFET with it's gate tied to GND, source to the "main beam" circuit, and drain to the "headlight" circuit.

Thread Starter


Joined Dec 21, 2008
Yes, main beam = high beam.

Would a 12v 30A relay do the trick, with the power and signal coming from the high beam power source?

I'm not too concerned about alternator power as the high beam is only on for short periods of time. The circuit is fused at 20A so should be ok from that point of view I believe?


Joined Dec 20, 2007
On my first car, I changed the wiring with diodes so that the low beams stayed on when the high beams were on. The single lights each had two filaments and they never burned out and the relay and alternator also never burned out. My high beams were really bright!