LCGB Forums

by **Paulnobodyimportant** » Sat Jun 06, 2015 8:26 am

I'm going to fit two FIAM 12c DC horns (similar to those shown below) to my lambretta, which has been converted to DC and has a battery and inline fuse on the battery earth.

However, do I need to include a 12v DC relay switch?

Thanks

Paul

by **missing lynx** » Sun Jun 07, 2015 12:18 am

got the same one to fit to my lynx I'm just going to run four wires (two from each wire on the loom) I use to run compressor air horn without a relay without any problems

by **coaster** » Sun Jun 07, 2015 8:34 am

missing lynx wrote:got the same one to fit to my lynx I'm just going to run four wires (two from each wire on the loom) I use to run compressor air horn without a relay without any problems

I have similar 'alpine' horns fitted to both of my DC converted scoots and have wired them via a relay. In fact one set actually had a relay included in the package. Sure it will WORK without a relay but it won't for long. The air compressor type are even worse and will draw several amps enough to cause significant arcing and destroy your switch contacts. Wiring in a small relay is very straightforward 8-)

by **johnnyXS** » Sun Jun 07, 2015 9:13 am

using a relay means you can isolate battery power from your horn switch reducing the risk of fire.
Looking at how my horn/light switch cables are getting chewed to bits by the throttle and clutch wheels and gear change housing... I would not want to take a heavy duty cable from the battery up to the switch housing and back to the horns in case it got shorted out somewhere.

by **coaster** » Sun Jun 07, 2015 10:29 am

johnnyXS wrote:using a relay means you can isolate battery power from your horn switch reducing the risk of fire.
Looking at how my horn/light switch cables are getting chewed to bits by the throttle and clutch wheels and gear change housing... I would not want to take a heavy duty cable from the battery up to the switch housing and back to the horns in case it got shorted out somewhere.

Actually, you take the live to one side of the horn and the earth is then made via the horn button so the live wouldn't need to go to the horn switch but if you are running full DC the live feed will be present in the handle bar switch for the lights/indicators anyway. I have my horn relay mounted on a bracket where the horn used to be with the live on one side of the coil and the earth running up to the horn switch.

Colin

by **johnnyXS** » Sun Jun 07, 2015 10:43 am

it doesn't make much difference really except the wire from the horn to switch would only be live when you pressed the horn

if you take the live wire from the battery to the horn you would still have to take a switch live wire from the other side of the horn up to the horn switch through the headset and handlebars .
When you pressed the horn the wire from the battery to horn and from the horn to the handlebar switch would all be live

by **Captain Pugwash** » Sun Jun 07, 2015 2:39 pm

The difference is the amount of current that will pass through the horn switch wire to earth.

Without a relay fitted peak current flow will exceed the rating of the standard earth wire, with the relay fitted there will only be milliamperes flowing through the switch wire.

There shouldn't be any real damage to the switch short term but prolonged use and time will probably reduce the life of the low amperage earth wire and switch.
Most standard light & horn switches that I've seen use 3 amp wire. Some use 5amp.

The relay (if used) will most likely need to be fused with a minimum 5amp fuse, my dual horns drew enough current to blow the 5amp fuse, so now I use a 7.5 amp fuse.

But it will work with or without the relay.

by **Paulnobodyimportant** » Sun Jun 07, 2015 4:00 pm

Thanks for replies. I came to the same conclusion last night after talking to random strangers at Keighley soul do, in between pints of strong larger.