Simple Relay system for vehicle lights            by M Lauterbach

Installing relays in your lighting circuit has a couple of benefits.  The 12V current path is normally battery, fuse-board, light switch,  bright/dim switch, head lights.  In some vehicles the fuse is between the bright/dim switch and the head lights for some inexplicable reason.  Here the wiring loom between the battery and the bright/dim switch is not protected with a fuse, but this is a discussion for another time.

Because of the long path which needs to be taken for the electrons to reach the lights, there are voltage losses at each switch, the fuse box, and from the losses in the cable to the lights.  The cable is normally designed for 50w globes maximum.  It is not uncommon to have a 1.5V volt drop between the battery and the lights!

If you insert relays, which feed the lights directly from the battery (through an extra fuse), using thicker cable (2.5 or even 4 mm squared), you reduce the voltage loss and at the same time increase the allowable power consumption at the lights (more powerful globes without the danger of frying  your electrics).  the relays are switched by the existing feed wires to the lamps.

One relay is used for each circuit, and is fed directly form the battery complete with its own protective fuse, which is as close to the battery as possible.

The wires feeding each globe are cut at the lamps, and one set (left or right hand) is redirected to the relays to switch them on or off.  The cut wires at the globes are then connected with a suitably thick wire to these relays.

When you have wired your headlamps like this, the original current through the switches will be measured in milli amps, ie the current necessary to switch the relays on, rather than amps.  The head lamp fuses in the original fuse box can then be changed to the smallest fuse available, protecting your wiring loom even more.  By the same token, your switches will not wear out due to high currents passing through them.  Because the current path length has been reduced, and the cable size is probably also increased, the voltage losses will be minimal.  You will probably even notice an increase in brightness with the same globes!

Your extra spot lights can be wired up in the same way.  They should be triggered by the main beams.  The trigger circuit should pass through an isolation switch on your dashboard, so that they can be switched off when necessary.