Relay 101

Here is a short tutorial on common types of automotive relays.

Single pole, single throw (SPST)

Here is a picture of the internals of a typical Bosch-style automotive relay. When +12V is applied between pins 85 and 86 the coil becomes a magnet which pulls the lever down making a connection between pins 30 and 87.

The relay above is a SPST relay with dual outputs. These used to be easy to find in Walmarts, Kmarts, etc., but they're getting harder to find.





Single pole, double throw (SPDT)

This relay is similar to a SPST, but pin 30 is switched to either output pin 87A or pin 87. Pin 87A is connected in the unpowered state.

Here is a schematic showing uses for some SPDT relays.

Why use a relay?

Here are a couple different circuits that perform the same function...turning on a pair of driving lights. Let's compare the two circuits.

Circuit 1 - A simple circuit with a switch and lights. Some concerns:

- The high current going to the lights will pass through the switch. Unless you buy a very heavy duty switch the switch will eventually fail.
- There will be noticeable (measurable) voltage loss by the time the circuit reaches the lights.
- When hooking up a circuit like this most people will simply tap into the fuse panel or a wire under the dash. This will usually overload the factory circuit. It might blow a fuse or melt wire, but more likely will degrade the wire and cause intermittant failures of whatever the circuit is hooked to.

Circuit 2 - A proper circuit with a relay and fuse.

- The switch is only switching the relay. It draws a very small current which is good for the switch and whatever power source has been used.
- There will be little or no voltage loss using a relay. A light burns brighter and lasts longer when supplied with proper voltage.
- All the high current is kept between the battery and lights and is properly protected by a fuse.

Building a circuit properly takes more components and time, but will work better, last longer, and be safer.


Circuit 3 shows turning the relay on/off with the ground wire instead of +12V. This might be an easier way to wire a relay depending on the situation.


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