Why use a relay ?

Mrwurm · Mar 30, 2006

I understand the benefits of using a relay in an electrical circuit. You can use a relay to control a high amperage device, such as motor, with a low amperage switch.
But, why do horn circuits always use relay ? I'm getting ready to start a project, adding a horn to my Yamaha ATV, and all the info I find assumes you are using a relay. Does a 12v automotive type horn really draw alot of amperage ? I can't find any specs.

Soundguy · Mar 30, 2006

The horn on my tractor uses no relay.
I'm using 14ga stranded wired,

Soundguy

KentT · Mar 30, 2006

On cars, horns and headlights are typically controlled by relays. You can tell the difference with the higher amperage that a relay provides...

RobertN · Mar 30, 2006

In some cases, it keeps the high current or high voltage circuits very short, ie the horn is close to the battery in most cars. You want to minimize line length to minimize voltage and current drops in your circuit. There is a very short run of wire from the main circuit to the relay, and on to the horn or headlight, made of large wire that has very little line loss(IR losses).

So, you have a high voltage or current circuit with short supply paths. The relay needs very little current to activate. So, it is easier to control from another location.

You could run a longer circuit, but would need much larger wire, which gets expensive...

midlf · Mar 30, 2006

The rating of the switch is as much reason to use a relay as is wire voltage drop. Switches can be much lighter duty, This is important when a horn switch is in the steering wheel and also when the current path in the steering wheel includes the lightweight clock spring.

PineRidge · Mar 31, 2006

Think it might boil down to the car manufacturers being able to run lighter gauge wiring throughout autos by using relays to control the loads.

Can a horn be wired without a relay? Yes, and it will do a good job if the wire is heavy enough supplying current to the horn. Try and power the horn with 22 gauge wire though and you might just have a problem.

Bully · Apr 1, 2006

Keep in mind that the horn relay will let you complete the circuit with one wire using the steering wheel as the ground through the horn button as the switch. A lighter single wire. You couldn't run it to well with two wires at 12-14 gauge. /forums/images/graemlins/wink.gif

Mrwurm · Apr 1, 2006

Thanks to all for responding. The switch I have is an oem momentary kill switch that uses pretty light gauge wire. Maybe 18 gauge. Looks like the relay is needed in this case. Thanks.

hobbyfarm · Apr 5, 2006

A buddy of mine tried to "outsmart" his motorcycle wiring - he installed a pair of Fiamm's in place of the OEM horns.

Two toots (second toot was about 5 seconds long) on the horn and the switch melted together internally. /forums/images/graemlins/crazy.gif

Some horns draw a bunch of current.