How Does a DC Relay Work?

Written by dan keen
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How Does a DC Relay Work?

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A Relay Is an Electromagnetic Switch

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 circuit can be used to trip a switch, which will control a separate circuit that requires a higher voltage or amperage, or even an AC (alternating current) or audio signal. A DC (direct current) 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, from intermittent windshield wipers to cruise controllers, flashers, fog lamps, stereo equipment and air-conditioning units.

Auto relays (photo from
Auto relays (photo from

Relay Switch NC/NO Designations

Relays come in various configurations for their switch contacts, as well as different DC voltages for to operate their coil. 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 NO or NC. In an open circuit, no current flows, similar to a wall light switch in the "Off" position. 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. Voltage applied across the coil at points A and B creates an electromagnetic field that 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.

Schematic diagram: Single-pole, single-throw relay
Schematic diagram: Single-pole, single-throw relay

Single-Pole, Double-Throw Relays

Relays come in different switch configurations. The switches may have more than one "pole", or switch contact. The previous diagram shows a "single-pole, single-throw" configuration, referred to as SPST. This is similar to a wall light switch. With a single "throw" of the switch, the circuit is closed.

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. An SPDT can be used to alternate the circuit to which a voltage or signal will be sent.

Single-pole, double-throw relay
Single-pole, double-throw relay

Double-Pole, Double-Throw Relays

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

Double-pole, double-throw relay
Double-pole, double-throw relay

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