Before we can talk about a relay, let’s understand how the amount of current in a circuit dictates the size of the wire. Here’s what I mean, let’s look at an implementation most of us might be acquitted with. In your house, the switch on the wall is in series with the light/appliance it is controlling. Therefore, all the amps being consumed by the light/appliance go through that very switch. If you have ever removed the wall switch plate, you will see large wires used to construct the circuit. The following picture, from the DIYNetwork.com, depicts wires used for your home. You can easily see these wires are large since they have to be able to handle up to 20 to 30 amps.

In regards to automobile circuits, you will hardly see wires of this size. Off hand, you will large wires connecting the alternator to the battery, and you will see large wires from your battery to your starter. But, you know that when you turn the ignition key that you are turning on the starter, yet you don’t see those large wires in your dash. How do automotive engineers pull this off? The answer is the relay.

A relay is a switch that is controlled by a circuit requiring less current than the circuit being turned on. The image on the left depicts what an actual automotive relay looks like. The schematic on the right depicts a typical relay pin out. Honestly, when I first saw the schematic of a relay, I said how in the heck does this work? I’ll never understand it. But, I was wrong. It is a very simple device.

Remember, we are using one lower current circuit to turn on another higher current circuit. The lower current circuit is the coil, energized by pins 85 and 86. One of these pins must be power and the other must be ground. Whenever current is applied through pins 85 and 86, it creates a magnetic field that physically pulls that internal switch, the arrow in the diagram, to connect pin 30 to 87. When there is no current through pins 85 and 86, the relay is considered in the normally off state. In this normally off state, pins 30 and 87a are connected. In most applications pin 87a is not connected to anything or isn’t even present on the relay.

Let’s use the Java Applet Circuit Simulator to see how relays work and how they are wired up. Please continue by clicking on the Simple Relay button.

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