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could anyone explain as simply as possible why they are used as I think my understanding is missing something. I know they are an electromagnetic switch and understand why they are used for example in a starter motor solenoid ( to avoid having to run a heavy duty cable a long way ) but when they operate very low draw circuits which only need thin wires what advantage do they offer, as to power the relay you need to run a wire to it anyhow. why not just continue the wire all the way to the component and have just the one switch which you use to operate the circuit.sorry ..I'm woffling but if anyone understands what I'm asking please explain !
thanks
bill
It saves installing a seperate switch for every component, not all relays are the same a simple one can be n/o or n/c depends on what you want to control some have times in them or current or voltage sensing as well. as a guide terminal 85 is earth 86 is the signal feed used to activate the coil, 30 is perm feed, 87 norm open 87a is norm closed. Hope that helps a bit. Also using a diode relay helps when back feeds become an issue.
My understanding is that they are fitted simply to reduce the length of cable in the car carrying higher currents ( where higher = anything higher than it absolutely must be ).
As sturmey says the normal open / normals closed depending on design. Also electrical isolation is important. You have big expensive ECU isolated from some maybe clunky mechanics or items in nasty environments where shorts may be more likely to occur.
As already said ... its really to isolate the components and save on power losses.
Even in a car, there are big differences in the power levels of components. Your ECU's, instruments and dash work typically at milli-Amp levels.
The head lights, spark plugs, heater motors, electric windows, heated seats etc work at Amp levels.
You could be talking about 100x difference in power levels between the switch on the dash and the thing its powering ... even at the 12V within a car. Its worth having the relays in there to keep the isolation.
Relays on cars are known to burn out with the current that passes through them. So it would burn through small digital chips in no time.
think I'm starting to get it ..thanks guys ..any more idiot proof explanations welcome!