What is a Car Relay Switch?

Updated February 21, 2017

Relays are used in many different applications in automobiles, from intermittent windshield wipers to cruise controllers, flashers, fog lamps, stereo equipment and air-conditioning units.

Relays In The Ignition Circuit

Amperage (current) is required to run an automobile's starter motor. When turning the ignition key (a simple on/off switch), a circuit carrying a much smaller current is completed, which energises an electromagnet. Attached to the electromagnet is a switch whose terminals are designed to handle high current flow. That circuit is connected to the car's starter motor. So, a low current circuit can control a high current circuit with a device called a "relay." A relay is an electrically operated switch used to isolate one electrical circuit from another. In its simplest form, a relay consists of a coil used as an electromagnet to open and close switch contacts. Since the two circuits are isolated from one another, a lower voltage or current circuit can be used to trip a switch, which will control a separate circuit that requires a higher voltage or amperage. In our example, this "starter relay" is often referred to in the auto industry as the "starter solenoid." A direct current (DC) voltage is applied to the relay's coil. The amount of voltage needed to "energise" the coil is one of the relay's specifications. Typically, 12-volt DC relays are designed for various applications in automobiles.

Relay Contact States (NO, NC)

Relays come in various configurations for their switch contacts. They may be as simple as an on/off switch, or as complex as integrating several switches into one unit. In a "double pole" configuration, one switch terminal toggles between two different output terminals. Regardless of the configuration, each switch on a relay can be "normally open" (NO) or "normally closed" (NC), that is, when the coil is at rest and not energised (no current flowing through it), the switch contacts are given the designation of being NO or NC. In an open circuit, no current flows, such as a wall light switch in your home in a position that the light is off. In a closed circuit, metal switch contacts touch each other to complete a circuit, and current flows, similar to turning a light switch to the "on" position. In the accompanying schematic diagram, points A and B connect to the coil. Points C and D connect to the switch. When a voltage is applied across the coil at points A and B, an electromagnetic field is created which attracts a lever in the switch, causing it to make or break contact in the circuit at points C and D (depending if the design is NO or NC). The switch contacts remain in this state until the voltage to the coil is removed. A spring pulls the metal contact back, breaking the connection.

A SPST Relay Configuration

The diagram shows a "single pole single throw" switch configuration, referred to as SPST. This is similar to a wall light switch in your home. With a single "throw" of the switch, the circuit is closed.

A SPDT Relay Configuration

A single pole double throw (SPDT) relay configuration switches one common pole to two other poles, flipping between them. As shown in the schematic diagram, the common point E completes a circuit with C when the relay coil is at rest, that is, no voltage is applied to it. This circuit is "closed." A gap between the contacts of point E and D creates an "open" circuit. When power is applied to the coil, a metal level is pulled down, closing the circuit between points E and D and opening the circuit between E and C. A single pole double throw relay can be used to alternate which circuit a voltage or signal will be sent to.

A DPDT Relay Configuration

In a double pole double throw configuration (DPDT), two identical switches are ganged together, but control separate circuits. Each switch operates as discussed above with the SPDT relay. This schematic diagram shows that switch contact D flips between connecting with terminal C and E. The second switch, with terminal G as the common point, toggles between terminals F and H.

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About the Author

Dan Keen is the publisher and editor of a county newspaper in New Jersey. For over 30 years he has written books and magazine articles for such publishers as McGraw-Hill. Keen holds a degree in electronics, was chief engineer for two radio stations and taught computer science at Stockton State College.