12volt Relay ,do I Need It

[mr magoo]

not sure if this is the place to post this but here it is anyways

my father bought me a air horn for my boat after nearly getting run over by a 40 footer while dressing a couple of yft on the deck,now in the kit it comes with a relay but the diagram shows the horn conected to a steering wheel of a car,seems to me(im just a simple panel beater) to be complicating something that is simple ,compressor, horn switch and fuse

the compresser has a positive and a negative terminal

what does a relay do

any help would be appreciated

cheers arman

[Holmesie]

Gday arman, in a car you would use a relay to avoid running the compressor off the car-horn wiring (which would not normally handle the high amps a compressor would draw).

On your boat I presume you would use a press button, in which case the relay should not be required!

hope that helps!

Edited June 6, 2008 by Holmesie

[Boban]

The relay is necessary as the weak point in the circuit is the switch. The use of a relay means that the switch(the one you press) is used to switch the relay(which is in itself a high capacity switch).

The simple answer is yes unless you can get a switch to handle the current that will be drawn by the horn. Use the relay.

[Davemmm]

Relays get used for a couple of purposes. Sometimes it allows you to switch a couple of different circuits with one switch but in this case it is used to save the switch from excess current.

If the switch that is being used is a dodgy, low current contact (such as what is used on a steering wheel) then the only current that runs through the switch is the small amount that the relay coil draws. The larger current that the compressor draws runs through the heavier relay contacts.

I would hook it up straight through the switch. Most switches should be able to handle a small compressor but if in the long run it doesnt simply replace the switch with a heavy duty one.

The other issue with a relay is it has a couple of moving parts and the ongoing action of the boat will wear them down and it will fail.

Dave

[mr magoo]

sorry forgot to add that the very ,very small and short instructions say it requires a 20 amp fuse is that to strong for your average switch,thanks

cheers arman

[IWANNABROCK]

Use the relay and the fuse that is recomended. those compressor's draw a lot of current to get going,the relay is a heavy duty switch to prevent burning out the switch you press to activate horn, and drawing to much current through wiring.

[Boban]

sorry forgot to add that the very ,very small and short instructions say it requires a 20 amp fuse is that to strong for your average switch,thanks

cheers arman

That indicates a high current draw. If you are going to purchase a switch just make sure it can handle 20A at 12volts. Most of those supplied with the kit will not handle that.

I think you will find a relay or two in most outboards BTW. My biggest concern with the relay would be corrosion at the contacts.

Like Dave says you can avoid them if you get the appropriate switch. I would not wire it without the relay if using the supplied switch. It will work but not for long.

[BFB]

Be careful how to wire it up. A car horn works oposite to most electrical switches as the horn button makes the earth (neutral) connection where most over switches switch the positive wire.

[Davemmm]

20amp is a bit high for most switches - use the relay

[mr magoo]

thanks guys for taking the time to reply

i got a 25amp switch and did away with the relay

much appreciated

cheers arman