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Old 02-02-2013, 08:32 PM
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90% of all relays work off the same principal......

For example a bosch type relay.....
There is a coil with two terminals, (85 and 86) it doesn’t matter which terminal gets pos or neg.

If you hook up constant 12v to one terminal (85)....... the other terminal (86) connected to one side of a switch, the other side of the switch connected to a ground, when the switch is thrown it provides and ground (neg) and activates the coil.

If you hook up constant neg (ground) to one terminal (85)....... the other terminal (86) connected to one side of a switch, the other side of the switch connected to a 12v source, when the switch is thrown it provides and pos and activates the coil.

Depending on what you want .... either way works just as well. I like to hook fused, constant 12v to the coil (85 or 86) and use a switched neg to activate the relay. IMO that keeps the hot wire short and reduces the resistance etc.......

On most automotive bosch type relays that you can purchase at most auto parts stores, will also have a common terminal (30) and a normally open or NO terminal (87) and there are some that have a normally closed (NC) terminal (87a).

Depending on what you want to do would determine if you needed a 4 or 5 terminal relay.

For example........ You wanted to hook a air horn up on your vehicle and you wanted to use the steering wheel horn button to activate it, but you still wanted to use the normal car horn ...... that’s where a relay works perfectly. You would find the original wire that connects to your normal horn and disconnect it from the normal horm, make a new wire and connect it to the normal horn and run it to NC (87a) terminal.

Take the original wire and hook it to the common terminal (30), since 87a and 30 are making contact, when you press the steering wheel horn button the normal horn honks. Next you hook your air horn pwr lead to NO (87) terminal, now when the relay is energized, as outlined above, it disconnects terminal 87a NC from the common (30) and connects terminal 87 NO to the common (30) so now when you press the horn button pwr is routed to the air horn.

Basicly it’s an “either”/”or” type circuit
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