INFO: How a Relay Works

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This is a picture of a double pull single throw type relay. For most applications a single pull single throw relay will work as well. The difference between the two will be that the DPST relay has pin 87a and the SPST relay does not

How to hook it up:
Pins 86 and 85 are the coil inside the relay. The coil acts as the internal switch to the relay. Basically, if you put 12 volts (sometimes less) across these two terminals, you will "throw the switch" inside the relay. These pins can be hooked up in two configurations:

In these configurations, it does not matter what side you hook up to what.

1. Hook up either of 86 or 85 to +12v (switched or always on - depends on if you want the relay to work only when the vehicle is on or off). You would then trigger the relay by putting on or taking away ground on the other pin. This is typically how most alarms are configured. They will have a 500ma ground output that switches on whenever you hit the button on the remote or whatever activates this output. In this configuration, the ground wire is your "control wire."

2. Hook up either of 86 or 85 to ground and control the switching by putting on or taking off +12 v on the other pin. Works the same way as before, but the trigger wire is the opposite. In this situation, the +12v wire is your "control wire."

How the other pins work when triggered:

1. If the relay is just sitting in your hand, pins 30 (common) and 87a (N/C = Normally Closed) are internally "connected". If the relay is hooked up in one of the two configurations listed above and it is not "switched on" via your "control wire" these two pins are connected as well. In this case pins 87 and 30 are NOT connected internally.

This is mostly used for situations where you want to control something being turned off. You can also have it control something being turned on by whenever the "control wire" isn't turning the relay on, whatever is connected to 30 and 87a will receive power/ground.

2. Whenever your "control wires" are turning the relay ON, pins 30 (Common) and pins 87 (N/O - Normally Open) are internally connected, and pins 87a and 30 are no longer internally connected.

Examples:

Say you want to supply +12v to something whenever the relay is turned ON with a +12 input (say you turn on your parking lights and want to supply power to your aftermarket foglights, but you only want your aftermarket fog lights to be able to be turned on whenever your parking lights are on. So whenever you turn off your parking lights, the aftermarket fog lights go off as well).

(Part 1)
1. Hook up either 85 or 86 to ground.

2. Hook the other pin up to a +12v signal coming from your parking lights whenever they are turned on.

3. Hook up pin 30 to a fused(!) +12v power supply (aka your battery).

4. Hook up pin 87 to the positive side of your aftermarket fog lights.

Say you wanted to be able to turn on these foglights only when the vehicle was turned on:

(Part 2)
Same setup as before, but in step 2, hook up this pin to a wire that becomes +12v whenever the vehicle is in anything but off. Make sure this wire is hot on each key setting except the engine cranking setting.

The one important thing to note here is that if you are sending +12v through pin 30, this wire should be fused appropriately.

Another example involving using ground through the relay:

1. Hook up everything as noted in Part 1 steps one and two.

2. Instead of hooking up a fused +12v to pin 30, put pin 30 to ground.

3. Hook the positive side of your aftermarket fogs to a fused supply from your battery.

4. Hook pin 87 to the ground side of your aftermarket fogs.

Whenever the relay is switched on in this case, ground is sent to the fog lights and will turn them on.

Another case:
You can also switch between two outputs by hooking up both 87 and 87a.

Say you want one thing to have power when the relay is activated through the "control wire" but another to have power when the relay is turned off.

1. Hook up either ground or +12v to either 85 or 86.

2. Hook up your "control wire" (opposite of step 1 above) to the remaining of 85 or 86.

3. Hook up +12v or ground to 30.

4. Hook 87a to the device you want to receive the input of 30 whenever the relay is turned OFF.

5. Hook up 87 to the device you want to receive the input of 30 whenever the relay is turned ON.

There are few uses for this setup, and one of the few reasons to even buy a SPDT relay over the SPST variety.

(Courtesy of TundraSolutions.com member networkguy)