How to Connect Automotive Relays
Automotive relays use a small amount of current to switch accessories that draw large amounts of current, such as the headlights lights and air conditioner, on and off. The major types of automotive relays are single pole single throws (SPST), which open one connection while closing another, and single pole double throws (SPDT), which make a connection to two separate wires. All have five connecting pins and plug into a standard, pre-wired automotive relay socket. With so many applications for relays, you will need enough electrical expertise to make basic wire connections.
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Things You'll Need
- Basic electrical tools
- 5-pin standard automotive relay socket.
- Automotive electrical wire (optional)
- In-line fuse holder (optional)
- On/off switch (optional)
Instructions
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Connect one side of the relay coil to a battery supply. This could come from the fuse panel in your vehicle or you can connect it directly to any battery source. But in this case, you should run the wire through an in-line fuse.
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Connect the other side of the relay coil to one side of the switch that will operate the relay. This may be a new switch or one already in your vehicle. With a new switch, add a wire from the other side to ground. On an existing switch, make sure the other side of the switch goes to ground.
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Connect the common (C) contact to a battery source. This will provide power to the accessory you want to operate with the relay. This lead should also be protected by a fuse, either from your vehicle's fuse box or from and in-line fuse.
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Connect the normally open contact or contacts to the equipment you want to power with the relay. On an SPST relay, you only connect to the NO contact. An SPDT relay has two NO contacts that will have battery power on them when the relay operates. You can use these two connections to power two different things, such as one wire from each NO contact to each of the two headlights.
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Tips & Warnings
Your standard automotive relay socket comes with wires already connected to the pins. If you need longer wires, splice another wire onto the end by twisting the conductors together and then wrap it with electrical tape.
You can also use an SPST relay to open a circuit when you energize the relay, such as to turn off a burglar alarm. Simply wire the power to the alarm via the C and NC (normally closed) contact. Operating the relay will then open the connection.
Use caution when wiring to battery leads to prevent shorts that could cause sparks and molten copper to fly.
