What Is a Relay?

What Is a Relay?

A relay is an electromagnetic switch that operates electromechanically or electronically. It opens and closes circuits by a relatively small electric current that triggers a much larger one.

Relays are widely used because they have the potential to handle amperage levels that vastly exceed the capacity of their wiring. They are also useful in situations where a signal needs to be repeated over long distances, as is the case for the telegraph (developed first by William Cooke and Charles Wheatstone and then Samuel F. B. Morse in England and the United States, respectively).

In a relay circuit, you'll find two coils, a control terminal, and one or more sets of normally open and normally closed contacts. Relay contacts are usually arranged in groups of Single Break (SB) and Double Break (DB). SB contacts only break an electrical circuit in one place, while DB contacts break it in two places. Relays can be found in a wide range of electrical equipment.

The simplest type of relay is the general purpose relay, which has an iron core with two spring-like contact arms and a metal control coil around it. When the relay's control coil is energized, it causes the iron core to become an electromagnet by applying an AC voltage to it. The magnetic field generated by the energized core attracts the lower contact arm to it, which then pushes against and closes the spring contacts, connecting the upper and lower portions of the circuit and turning on the device they're attached to.

A second type of relay is the time delay relay. It has the same function as a normal NO relay, but when power is applied to its coil, the contact only closes after the coil is continuously powered for a specified amount of time. The time delay allows you to prevent unwanted contacts from closing prematurely.

There are a few other important types of relays, depending on your application. One is the normally closed, delayed reed (NCDR) relay. Its function is identical to the normal NC relay, except that when the contact is opened by the application of power to its coil, it will remain open for a specified amount of time. This allows you to add a safety feature, such as a timed lock for your door, without needing an additional mechanical moving part.

Using a multimeter, you can test a relay for continuity and resistance by setting the meter to either continuity or resistance mode and checking its coil and contact pins. Make sure the meter's set to its rated coil voltage; if it is not, the resulting current will burn out the coil. Check for continuity between pins labeled "C" and "NO," and for continuity between "C" and "NC." Both should show low resistance, indicating closed contacts. If not, the contacts may be slightly open due to manufacturing tolerances. You may want to repeat this test several times for consistency. If you do this, you'll get a more accurate reading.

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