How to Wire a 12V Relay

by Richard Asmus
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12 volt relais, relay image by Sascha Zlatkov from Fotolia.com

Most 12 volt relays operate accessories in a motor vehicle. Applying a small amount, as low as .2 amps of current to the relay coil closes contacts to feed power to an accessory that requires more current to operate, usually over 20 amps. To wire an automotive relay, make at least four connections: two wires to the coil to operate the relay itself and two wires to the relay contacts to apply power to your accessory. You need sufficient electrical skill to select the tools, wire and connectors for the job.

Step 1

Connect wires to the coil of your relay. This requires two wires from each side of the coil to positive and negative voltage from your vehicle's electrical system. You also need to run either of the wires through a switch to actually operate the relay.

Step 2

Connect the "common" (C) terminal of the relay to a battery supply. This can be the same supply you use to power the coil. See the reference at autoshop101.com for relay wiring details.

Connect the power input lead from the equipment you want to operate with the relay to the "normally open" (NO) terminal of the relay. When the relay operates, the internal contact will put the voltage on this pin.

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