Why do I want to use a relay?

[ringadingh]

Heres a great explanation of why to use a relay when wiring accessories to your bike. Thanks to Yellow Jacket !!

Suppose that you wanted to wire up your lightbar so that it would come on with the running lights. You could wire the lightbar directly into one of the running light wires. However, since the running lights are on a 10 amp fuse and relatively small wires, as soon as you turned it on with the light bar hooked up the fuse would blow. So, then you could say to your self, "Self, I'll just replace the fuse with a bigger fuse, say a 20 amp fuse." Voila! The light bar works and the fuse doesn't blow! Of course, since you're now drawing a lot more amps through those small wires than they are rated for the wires get hot. Before long the wires get hot enough to melt the insulation. Then they melt the insulation on other wires that they may be touching. The fuse doesn't blow until the mess gets shorted to ground but by then it's too late. You will now be looking for a replacement wiring harness. Not much fun.

Now, if you were to take a 30 amp relay (which is just an electrically activated switch), connect a wire from the battery (or an auxillary fuse panel) sufficient to handle the amp draw of the light bar and connect a wire from the relay to the light bar, again with the proper size wire you could now run the light bar with out melting any insulation or burning any wires.

All you need now is something to turn the relay on and off. For this you can simply connect a wire the same size as that which runs the running lamps to the coil connection on the relay. Connect a ground wire to the other coil connection.

The coil of the relay draws only a tiny amount of current so the running lamp circuit won't be overloaded.

When the running lamps come on, the relay (switch) will close and turn on the light bar.
When the running lamps go off, the relay (switch) will open and turn off the light bar.

All will be well.

P.S. If you connect the light bar to an aux fuse panel, you don't need to wire it to another relay, as long as the total load on the fuse panel doesn't exceed the rating of the relay and the wires feeding the fuse panel.