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Posted by: 914itis Mar 5 2013, 01:56 PM

Lets say I am hooking up a horn or a a light with an on -off switch, why not just connect to power source with the switch? Why use a relay ?.

Posted by: 914GT Mar 5 2013, 02:02 PM

A couple of reasons .... it takes the high current load off your ignition switch or whatever controlling it. Second, reduces voltage drops from wiring resistance.

Posted by: PDXMike Mar 5 2013, 02:05 PM

Usually a relay is used so that the high power needed to run the accessory doesn't need run through the entire switch circuit. Either the total length would be too long (too much resistance and loss) or the switch itself wouldn't handle the current.

The best example is the starter. The starter pulls a LOT of current. The switch in the steering column couldn't handle all that current, and running all that power from the battery to the dash and back would add up to a lot of loss simply because of the length of the wires.

Posted by: SirAndy Mar 5 2013, 02:17 PM

Posted by: Cupomeat Mar 5 2013, 02:19 PM

AND, any voltage drop result in a corresponding increase in current for the same load, which burns things out faster.

Posted by: 914itis Mar 5 2013, 02:45 PM

Thanks guys .

Posted by: kg6dxn Mar 5 2013, 07:25 PM

READ THIS!!!

 Relay_Book.pdf ( 1.08mb ) Number of downloads: 203