Relay Fundamentals

Relays are electromechanical components ubiquitous in most all automotive electrical systems. A basic understanding of their operation is necessary for nearly all automotive wiring projects.

A relay has two parts, a coil and an armature. In the schematic of the relay below, the coil, which has two connections, is on the left side of the relay. The armature, which has three connections, is on the right side of the relay.

The upper left armature connection is the normally open (NO) pin, while the upper right armature connection is the normally closed (NC) pin. The lower armature connection is the common (COM) pin.

Relay coils have two contacts, which are not polarized. Either pin can be connected to power or ground; the relay will be activated either way.

The armature usually consists of two, three, or sometimes more, contacts depending on the type of relay. Basic SPDT relays have three contacts; NO, NC, and COM. NO stands for normally open, NC stands for normally closed and COM stands for common. The NC and NO pins are alternately connected to the COM pin depending on the state of the coil.

Many automotive relays have their pins numbered. Pins 85 and 86 are for the coil. Pin 30 is COM, while pin 87 is NO and pin 87A is NC. These numbers are usually embossed near the pins on the underside of the relay.