What Is a Relay?

Relays are electromagnetic switches that open and close circuits electromechanically or electronically. They operate on the principle that a relatively small electric current can control a much larger one.

A relay has a coil which, when energized, induces a magnetic field and moves a movable armature to change its position. Normally closed (NC) and normally open (NO) terminals are connected to the armature.


Relays are a type of electrically operated switch that can be activated by a low power signal. They are often used in combination with other circuit elements to control high voltage or current devices that would otherwise be difficult or impossible to directly drive.

The relay was first developed in the late 19th century, primarily by telephone companies who wanted to be able to control their switching equipment remotely without needing a human operator to manually operate each switch. A typical electromechanical relay consists of an electromagnetic coil, iron armature with movable contacts and a fixed pin of insulator.

The armature is triggered by the electric current passing through the coil and its movement either opens or closes (depending on construction) the contacts of the COM and NO terminals. When the current stops flowing through the coil, a force, such as a spring or gravity, returns the armature to its relaxed position. A flyback diode or snubber resistor is typically placed across the coil to dissipate the energy from the magnetic field’s collapse at deactivation, which could otherwise generate a voltage spike dangerous to semiconductor circuit components.


The function of a relay is to switch circuits electronically as well as electromechanically. Relays are also capable of performing basic Boolean combinatorial logic operations. They have a normally open and a normally closed contact, which can be used to perform the AND or OR functions of electronic switches.

The relay coil has a built-in current limiter to prevent the relay from releasing too much current when it is switched on. The current limiter can be adjusted to change the minimum current required to switch on the relay.

Before using a relay in a circuit, it is important to check its working voltage by connecting the multimeter across its entrance and exit pins. If the voltage is high, it means the relay is energized and in the ON position. It is also important to ensure that the relay isn’t leaking current by testing its resistance with a multimeter. The resistance should be low, with a maximum value of 1.5 times the relay’s rated working voltage.


The relay is a heavy-duty electrical switch that can control high voltage applications. It works on the principle of electromagnetic attraction. Once the coil current is energized, it creates a magnetic field that attracts the armature and opens or closes the connections. When the current in the coil is cut off, a spring forces the movable armature back to its original position.

Relays are available in many different configurations, including number of breaks, poles, and throws. The type of relay you need will depend on your circuit’s specifications. For example, you might want a relay with two changeover (make before break) contacts and a common terminal, which is known as a SPST switch.

The internal section of the relay consists of an iron core delimited by the control coin. Power supply connects with the iron core via load and control contacts. Once the coil is energised, the magnetic field intensifies and causes the upper arm to move towards the lower fixed one which closes the contacts.


Relays use a frame with multiple terminals. They generally include four types of terminals – coil, common, normally open (NO), and normally closed (NC). The coil terminal is connected to a low-power source and the common terminal connects to the device that needs to be controlled. The NO and NC terminals are either open or closed by default until the relay is activated. The NO contacts are also known as “early-make” contacts, while the NC contacts are called “late-break” contacts.

Basic relays feature a coil voltage rating and an internal switching circuit. They also have a maximum switch current and voltage rating that cannot be exceeded during use. They are available with one or two poles, and the number of isolated circuits that they can control varies. Single-pole, single-throw (SPST) contacts can only carry current through one circuit at a time, while double-pole, double-throw (DPDT) contacts can carry current through two circuits simultaneously. Depending on your needs, you should choose the appropriate relay for your application. 중계

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