A friend once asked about the block of relays pinned to a board behind my car's dash. To him relays were the mysterious little boxes that some cars inexplicably required to operate a horn. More than one was plain ludicrous in his eyes. After all, how many horns could a car have?
The relay's primary appeal is its ability to do a lesser-switch's bidding but it's capable of so much more. Relays can be wired together to create very useful, simple, and utterly reliable networks. What follows are three of those configurations.
Here's how an SPST relay looks...
Here's how an SPST relay looks at rest from the inside. Terminals 30 and 87 represent the contact points, or the switch. Energy cannot flow between them when the relay is at rest.
One transforms dual-filament taillights into tail/brake/turn signals. Another configuration duplicates the OEM circuit that preserves accessory power until a door opens. The third isn't so much universal but it's so useful that it would be foolish to not include it. It acts as a dimmer that toggles between low and high beams with the flick of a momentary switch. But first, a little bit of theory.
A relay is at its core a heavy-duty switch without a lever or knob. Power applied to an electromagnetic coil forces a pole to advance. A yoke mounted to that pole moves a lever that's called an armature, which either makes or breaks connections among contacts within the relay.
Power applied to terminal...
Power applied to terminal 86 and ground to terminal 85 energizes the coil, which advances the yoke and swings the armature. Once the armature advances, energy can flow between terminals 30 and 87. Easy, huh?
We refer to switches and relays as having poles and throws. Pole in this case refers to the number of electrically independent armatures, or switches, inside a relay. Throw refers to the number of stationary contacts that the armature (switch) can touch.
Most universal automotive relays have one armature, or switch, that corresponds with terminal 30. We refer to them as having a Single Pole (SP). The very simplest automotive relays have only one contact point at the end of that switch. When energized, that contact meets a fixed contact inside the relay body labeled terminal 87. We refer to that as a Single Throw (ST). These Single Pole Single Throw (SPST) relays behave like a simple light switch: off (at rest) or on (activated).
An SPDT relay has another...
An SPDT relay has another contact represented by 87a. When the relay is at rest power can flow between 30 and 87a. Energizing the coil causes the relay's arm to break that contact and reestablish it between terminals 30 and 87.
Some SPST relays can handle upwards of 70 amps, making them perfect for high-current applications, like oversized cooling fans. But they aren't without limitations. For one, they operate one way: on or off.
The Single Pole Double Throw (SPDT) relay is the same as the SPST relay but has a fifth terminal. Labeled 87a, it conducts electricity with terminal 30 when the relay is at rest. Energizing the electromagnet breaks that connection and reestablishes another between 30 and 87. In that way it operates like a three-way hallway switch.
SPDT relays can be configured to work in at least five ways. To replicate an SPST switch, simply connect the input power to 30 and the device to 87 but leave 87a bare. That's the backbone of a simple horn relay: the device will operate (blow the horn) only when the relay is activated (usually by a switch in the steering column that completes the ground circuit for terminal 85). An SPDT's action can be reversed by swapping the device to 87a and leaving 87 bare.
Rather than tell we'd prefer...
Rather than tell we'd prefer to show how to configure this simple turn signal relay bank. The optional running-light wiring transforms single-filament front turn signal bulbs into combination running lights and turn signals.
An SPDT relay can act like a railroad switch in the sense that it can send power in one of two ways. What's more, nothing dictates what direction the power can flow through the contacts. For example, connect the device to terminal 30 and you can feed it from one of two power sources. In fact, that's the basis for the first example.
Four relays can turn a toggle switch into a turn signal system. Painless Performance sells a compact version (PN 30120) that includes micro relays, brackets, a flasher module, and enough really high-quality cross-link jacket wire. We'll show how it works but I recommend buying it rather than trying to build it. I can build it but I'd rather buy it (in fact I've bought kits for two cars).