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What Is a Relay?

what is a relay - diagram

Many electronics have some variation of a relay inside. Relays can be used to switch a low-current trigger to high current, switch a circuit on or off, reverse polarity, and much more. When adding LED lights, such as off-road light bars, driving/work lights, or other auxiliary lights to a vehicle, you must add a circuit to power the light adequately. Relays are a cost effective, flexible way of doing this. Understanding the basics of relays will help installations go much smoother and allow for customization along the way.

There are many different types of relays, however this post is geared toward explaining the standard 5-pin SPDT (single pole, double throw) automotive relay.

what is a relay - 5 pin spdt

The Anatomy of a Relay

Relays are essentially two parts in one package. The first part is an inductive coil (pins 85 and 86). This coil is the “trigger” function of the relay. By applying both a positive (+) and ground (-) to the contacts, current passes through the coil. When current passes through the coil, a magnetic field is generated and moves the switch (the arrow in the diagram). This is where the second part of the relay comes into play.

what is a relay - switch

The second part of the relay is the “switch.” The switch has three interface contacts—common, normally closed, and normally open. “Normally” refers to the state of the relay not being energized.

  • Normally open (87) indicates when the relay is NOT energized (on); this pin is NOT connected to the common terminal (30) or an open circuit.
  • Normally closed (87a) indicates when the relay is NOT energized (on), this pin IS connected to the common terminal or a closed circuit.
  • Pins 87a and 87 are not connected to one another—only to pin 30—which is why it is referred to as the common terminal.
relay energized and not energized
Left: ground not connected—relay not energized; Right: ground connected—relay energized (switch moves from left to right)
Relay Applications

While relays can be used for virtually any task, there are some very useful, simple applications for adding LED lighting to a vehicle. Please note that this wiring only pertains to the relay—not the wires in the vehicle. Before interfacing with a wire in a vehicle, always ensure you test the wire with a digital multimeter first to confirm circuit function.

Positive Switching

Positive switching is used for applications where the “trigger” for your switch or circuit is positive. This could be an ignition, brake, or reverse light wire for example.  This could be used for applications such as off-road light bars, work lights, additional brake lights, and other exterior lighting.

what is a relay - positive switching

Negative Switching

Negative switching is used for applications where the “trigger” for your switch or circuit is negative. Negative triggers are often found in door, hatch, and trunk triggers as well as many interior switches. This could be used for applications such as footwell lighting, truck bed lighting, under-hood lighting, and other interior lighting.

what is a relay - negative switching

Switch Activated With Ignition Override

This is useful for applications where you want to turn your light on or off with a switch but only want the light to remain on when the car is running or key is in the “ignition” position. The main advantage to this is—even if the switch is left on—the light (or output circuit) will shut off when the key is turned off so that the battery will not be run down.

what is a relay - ignition override switch


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