relay wiring

I know that there is an expert out there who can advise,I would like to include relays in the ignition & lighting circuits on my '74 850, I thought that I had a handle on things until it occured to me that a relay needs a low voltage control circuit to operate the coil, so if the relay carries full battery voltage through it, how is the coil operated?,am I totally wrong and should not get into this, or is there something I have overlooked.
I would appreciate an example of using a relay in the ignition circuit so that full power gets to the Boyer unit.Thanks in anticipation.Ride safely. James.

james said:

I thought that I had a handle on things until it occured to me that a relay needs a low voltage control circuit to operate the coil, so if the relay carries full battery voltage through it, how is the coil operated?,

Click to expand...

It isn't low voltage that is required to operate an automotive type relay but low current.

Basically a relay is just a heavy duty switch that is operated by an electrical circuit instead of manually (by your thumb etc.) and the amount of electrical power needed to operate the relay is generally quite small in comparison to what the relay is able to carry, and the voltage losses from a relay is generally less than from the tiny switch contacts normally found in handlebar switch units, so a relay is just a more efficient switch.

The original handlebar switches can then be arranged to operate relays instead of directly handling the heavy current for lighting, horn and other circuits so they tend to have an easier time and can last longer.

James,

I installed sealed relays from Eastern Beaver for th eignition, horn and hi/low beam power to the H4 QI headlight when rebuilding my mk III.

http://www.easternbeaver.com/

Nice product, responsive vendor. The sealed relays are more compact than Radio Shack units (tried that). I located the headlight relays in the headlight housing and mounted the horn/ign relays near the battery. I also installed their in-line fuses so each circuit is independently protected.

Basically, the wire that now goes from the switch to the "unit" (ign, horn, etc), powers the relay coil (the other coil lead goes to ground).

New wires are run (properly sized to carry requisite current - I used 16 ga) directly from the battery/alt through the relay contacts to the "unit" so resistance in the handlebar swiches, etc doesn't reduce the voltage delivered.

Of course, they won't help if the system voltage is low.

Hope this helps

Dave Winship

Thanks for your quick responses,all is now clear,I can proceed with confidence. Ride safely. James