Basic anatomy of an electrical relay-automotive

An post written by Jerry Hastings which also appears on www.hastprotrucks.com/forum

People are often confused by a relay and most importantly it's function and intended purpose. A relay is simply a switch like a toggle switch, but instead of manually toggling the switch, a relay uses electrical connections other than the "load" circuit to do the switching.

The 5 prong relay is a simple example. On one prong is the load's supply wire from either a positive electrical source or it could simply be to a good ground (providing a ground or negative connection instead of a positive connection). The corresponding prong is connected to the load (ie electrical motor, solenoid or other loads).

An entirely different circuit is then used to accomplish the switching action. This circuit is electrically separated from the load circuit. It consists of a good ground from the appropriate prong on the relay to the actual grounding location. The opposite prong on the relay is then connected to an electrical supply that is either a key operated positive supply or may be a supply generated by the action of another switching action of a related accessory. For simplicity we will call that positive and negative connection the "trigger" supply.

This "trigger" circuit can be negatively controlled if desired, but most of the time it is positively controlled. For simplicity we will use the positively controlled relay for further explanation.

When electrical current flows to the "trigger" prong on the relay (which necessitates utilization of the ground or negative connection), it causes a switching action to occur inside the relay which turns on the "load" circuit. This "trigger" circuit is a low amperage circuit meaning it is more friendly to actual connecting or activating items like the key switch or switch associated with another accessory than the "load" circuit. The "load" circuit is a relatively high amperage circuit.

The "trigger" circuit can also use smaller wires since it is a low amperage circuit unlike the "load" circuit which must use larger wires and larger amperage requirements. Smaller wires and low amperage are imperative in computer connections, wiper switches, light switches, etc.

If you find two fuses in the fuse panel that have the same label, the one with the smallest amperage rated fuse is on the "trigger" circuit and the high amperage fuse is on the "load" circuit.

Of course, larger vehicles like class 8 trucks often have their left and right headlamp circuits isolated from each other. That is primarily for safety reasons since one circuit can fail but the other stays functional. In that case, the fuses (or breakers) and relays are labeled "left headlamp" and "right headlamp".

More complicated relays are often found in todays cars and trucks which are capable of performing multiple tasks and/or controlling more than one load. These relays still function by the same principles as the simple 5 prong relay but can be configured to work on both the positive "triggering" and negative "triggering" circuits in the same relay.