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Think of it like a switch only instead of your finger electricity turns it on and off.

You have your device wired on one side of the relay and the other is going to a power source, turn on the power and the relay turns on your device.
They are used to isolate your switch, a relay can handle more power than a switch.
 

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X2 with JD
Also , the "Coil" of the relay is an electromagnet. This is how it works. When a small current passes through the electromagnetic COIL, it pulls the contact lever in the relay down or up, (depending on the application)
and makes the contact for the circuit being controlled.
On bosch relays terminal 85 and 86 are the coil connections.
30 is power source in, usually 30 amps or higher(as Jake said more than just a switch can handle)
87 and 87a are the output contact terminals that hook to the device to be controlled.
Depending on the contact state... normally open or normally closed (contacts) you can turn a circuit on or off by energizing the coil
 

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As a foundation for understanding relays you need to understand electromagnetism and basic electrical circuits.
Google those subjects. There are some very good articles and courses available.
The relay can be described as an amplifier in that a small signal can control a larger electrical load. This small signal can be amplified many times to control any size or type of load.
An example in a vehicle is the temperature sensor that controls a radiator fan.
The sensor is calibrated to switch (close) at a given temperature say 180* but is only capable of handling 1 amp of current. The fan, however, is power hungry say it uses 20 amps or more and has a large draw to start which can be several times the running current.
The sensor does it's job to signal the relay. The relay does the work of starting the fan.

vicrod
 

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x 3 with J.D. For example my 67 standard steering column wiring has two heavy gauge yellow wires in there for the horn that uses about 15 amps of current . On the end of each yellow wire is a brass pin. When you press the horn a small bridge joins the two brass pins and your car horn blows.
This heats up the pins every time its used and eventually fails.
By using a relay ( heavy duty 30 or 40 amp) the two yellow wires now operate the relay only which needs only about 1/2 of an amp to operate its own internal switch.
When the horn is pushed now,(using the same two yellow wires) the power , which is already at the relay flows through the relay and out to the horns. The horn button will never burn out now as it only has 1/2 amp of current at the most going through it.
If you get hold of a relay and a battery, set it up on the bench and use a bulb instead of the horns to make the diagram work for you. ( The battery negative terminal needs a ground, left it off my rough diagram)
Cheers.
Al.
 

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If you have an older car, the headlight current runs up through the dashboard, though the lighting switch, then back out to the headlights. That's a lot of wire to travel through, and the factory tended to use wire that was a gauge (or two) too small. Not only does this dim your headlights, it dims your dash lights as they "fight" for the available current.

I just did a conversion where the headlight switch now activates a relay that sends current to the headlights using only 12 and 14 gauge wire. To be "stealthy" I put the relays in the voltage regulator that is not longer used after switching to a 1-wire alternator.






The results are that the brightness of the low beams literally doubled and the high beams got ~67% brighter as monitored using the light meter in my Nikon.

These same increases in headlight performance are documented elsewhere.
 
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