Relay's, is there really a need?

Sport · **Posted:** Tue Apr 20, 2010 11:50 am

My driving lights came with one but I did without it and I wired in a compressor but never added a relay. I am not sure what they are used for, can someone school me?

Sport · **Posted:** Tue Apr 20, 2010 3:45 pm

This is a pretty good explanation and diagram. But I don't think these are absolutely necessary.

http://www.r1200gs.info/howto/relay.html

JeepinJarhead03 · **Posted:** Tue Apr 20, 2010 3:55 pm

often can be used to provide "ignition on" power using ignition signal to throw the switch and allow power to the device

they have their purpose, especially with hard wired devices that you tend to want to leave in a constant on state for ease of use but at the same time there's other ways of going about that using existing switch on sources

ATXKJ · **Posted:** Tue Apr 20, 2010 9:06 pm

Relays are really effective for allowing a long high resistance wire for the on/off switch - and yet have a short low resistance power wire for whatever's drawing the current.

JeepinJarhead03 · **Posted:** Tue Apr 20, 2010 10:02 pm

long wire - check!
short wire - check!
current and power something or other - check!

still over my head - CHECK!

wecwbe · **Posted:** Wed Apr 21, 2010 12:19 am

By using a relay, you can reduce the votage drop across the wiring and get more of the voltage to the lights resulting on brighter lights. Like ATXKJ said, you can use a long high resistance wire for the switch (this means a thin wire like 18 gauge that is easy to route to where you want the switch). Then you use a much thicker wire (low reistance, low voltage drop) to power the lights which runs through the relay. Basically the shorter the circuit path is and the thicker the wire is, the less resistance and therefore voltage drop there is across the wiring and the more voltage will end up being across the lights themselves. This means your lights will be brighter. A relay also lets you use a switch with a lower current rating (ususally meaning a smaller switch). Lights draw a lot of current and without a relay would require a high current rated switch.

Boiler · **Posted:** Wed Apr 21, 2010 12:31 am

Start with what a relay is. A relay is effectively a switch. It makes or breaks a circuit, for example the main power to your driving lights. It is different than a toggle switch in that the relay is activated by a solenoid and a control circuit, vs a toggle being controlled by your finger. The main reason for using them is that you can use a low amp-rating toggle switch to activate basically any size circuit. Also you can have a series of control functions without having to run your large wire everywhere.

I have my lights wired to an accessory circuit and a toggle switch. If the key is in any position other than off, voltage is available at my toggle switch via the accessory jumper I put in at the fuse panel. If the switch is turned on, that voltage powers a relay that makes my batter / lights circuit. I would not want to draw that much current from an accessory panel. I have 3 relays and 3 toggles, each operating a pair of lights.

Most switches have a relatively low current rating.

e_poirier2004 · **Posted:** Wed Apr 21, 2010 1:22 pm

Boiler is right, it allows you to use a more manageable wiring and switch in your dashboard. Example, to control something with 50 amps, the switch would have to be about 2" x3", taking a lot of space and looking ugly. Also, you would need a gauge 6 wire on that switch which is about 3/8" thick. Instead, you use a relay near where you use that accessory and keep the switch as small as you want because all you need to control is the coil of the relay (usually less than 500mA). There's also a safety factor. If something shorts out, you better have it in your engine compartment if it's big current instead of inside your dashboard. It doesn't look like much but a 20A circuit can make quite a decent spark when it gets shorted. I hope it helps. Feel free to ask if you still don't understand.

Relay's, is there really a need?

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