Elec relays.

James.UK · Post by **James.UK** » 12 Oct 2005, 17:52

I seem to recal, that as a lad, we used to run wires from the battery to a switch, then onto the light-whatever via a fuse.. So why does everything have relays now?? What are they for? what do they do? and why do we have them?

Please keep it simple, as my knowledge of electrics. is limited to knowing NOT to wet my fingers and stick em in a wall socket!!

Post by **martyhopkirk** » 12 Oct 2005, 18:36

Partly due to teh current draw of items like huge 55w lamps, fan motors etc - these days yu would need chunky cable and industrial switchgear to stop things from burning out.

The relays are there to prevent having a shed load of thick wires and chunky (expensive) switches.

Plus they add a bit more grief for things to go wrong!

bxbodger · Post by **bxbodger** » 12 Oct 2005, 19:11

My Vitesse, being from 1968,works in the way you describe-a dynamo and 2 fuses!! Everything is low wattage and, well, dim!!- One speed blower, 45watt lamps, etc. There are only 2 relays- one for the overdrive, and the starter solenoid.

Our Kia, being new, has about a million fuses in 3 seperate fuseboxes and powered everything.Relays are the price you pay for the luxury of electric everything, good lights, aircon, etc!

One of the reasons the BX has such rubbishy headlamps is that there's no relays and consequently thin wiring, which can't cope with much in the way of current. I keep meaning to put relays in, triggered by the existing wires, so the lamps can draw a heavier current- maybe this winter!!

uhn113x · Post by **uhn113x** » 13 Oct 2005, 07:49

Hi James
A relay is to switch a large current with a small one.

Car wiring has a finite resistance, that 'slows the current down'
The thinner and longer the wire, the more resistance.

Due to (the misquoted) Ohms Law, the voltage arriving at the wire becomes less, and the wire gets warm; the energy has to go somewhere.

If we have 2 x 55 watt headlights, that is 110w; at 14 volts, the current is 110/14 = 7.85 amps; the wire would have to be quite thick and short not to lose voltage. The light switch would get hot as well.

So, we stick in a relay in a short wire between the lights and the supply, and a thinner one to the relay from the switch; typically, about 1 amp or less would be enough.

Make sense?

bxbodger · Post by **bxbodger** » 13 Oct 2005, 15:32

The starter motor circuit is the best example- you have a thin wire from your starter switch to the starter solenoid, the low current through this thin wire operates the solenoid, which is really a relay by a different name, and connects the big thick wire from the battery to the other big thick wire to the starter motor.

The starter draws a huge current, which is why cheapo jump leads melt!! Imagine the size of the ignition switch if there was no relay.....it would have to be pretty big!!!

A picture paints a thousand words, or so they say-[/url]http://www.misterfixit.com/electrical_circuit.htm[url]

James.UK · Post by **James.UK** » 14 Oct 2005, 02:18

Interesting... Thanks for all the info...

batwad · Post by **batwad** » 14 Oct 2005, 08:42

Don't forget the safety aspect. I'd much rather put my key in the lock of a low-powered relay switch than something that connects directly with the feed to the central locking solenoid