Relays and Their Types

Relays are important electrical components that, at their most basic, act as an electrically operated switch. A relay is often used within a system to control a circuit with an independent low-power signal or for multiple control signals. Relays consist of a set of input terminals, and they often serve as the main protection and switching device for control processes and equipment. There are multiple different relay types, each providing for various applications or functions such as protective, reclosing, regulating, auxiliary, and monitoring relays. In this blog, we will discuss some of the main relay types, their components, and provided functions.

Electromagnetic relays are often considered the most simple type, and these relays feature a wire that is coiled around a solenoid, movable iron armature, iron yoke for a low reluctance path, and one or more contact sets. When an electric current is conducted through the coil of the relay, a magnetic field is produced that activates the armature. The armature is then moved to either establish or break a connection between contacts. Electromagnetic relays are operated through a supply system that activates the armature and contacts. Depending on the application, movement of the armature may be provided by a spring or gravity.

The supply system that operates relays typically comes in the form of either alternating current (AC) or direct current (DC) depending on the relay application. The AC relay utilizes laminated cores that work to prevent eddy current losses. As the supplied direction of AC current changes for every half cycle, another shaded coil or electronic circuit is installed within the relay to provide magnetism so that the circuit is not constantly broken and re-established each cycle. Meanwhile, DC relays feature a freewheeling diode that de-energizes the coil. Nevertheless, both the AC and DC relay operate with the same principle of electromagnetic induction.

Solid state relays (SSR) are a type of relay that provides similar functions as the electromechanical relay, but are devoid of any moving parts. As a solid state electronic component, solid state relays tout long term reliability. Solid state relays switch controlled loads through either a thyristor, TRIAC, or similar component. Common types of solid state relays include those such as transformer coupled, reed relay coupled, and photo coupled solid state relays.

Reed relays are those that are contained within a glass tube and have magnetic strip pairs. These magnetic strips, commonly referred to as “reeds”, can serve as an armature and contact blade. The reeds of the relay then perform switching when a magnetic field is enacted upon the wrapped coil of the relay as contacts are manufactured from magnetic materials. As contacts are placed within a glass tube, they are protected from atmospheric corrosion. Reed relays are beneficial as they feature faster switching speeds as compared to larger relays and require less power for actuation and operation. Reed relays are most often considered hermetic, sealed, and open type.

Latching relays are a relay type that may hold the position of a contact without power being applied to the relay coil. This is very advantageous, as such relays could use powering for a short time to actuate the switch, and once the contacts are adjusted, they can remain in place even in the occurrence of a power loss. Due to this, such relays also do not have a notable “hum” that continuously powered switches may produce, such as AC energized coils. Latching relays are common for avionics equipment, as well as many industrial applications.

Across electrical circuits and electronics, relays provide highly beneficial operations that extend the capabilities of technology. Whether it is for telegraph lines, automobiles, computers, or other electrical components, connectors and conductors, relays prove to be indispensable components. Picking the correct relay for an operation depends on many properties, and these should be evaluated before making a decision. Common factors in choosing include operating lifetime, assembly, needed switching time, size, expected mechanical loads due to acceleration, number of contacts, and much more. Ensuring that the contact current does not exceed given values is also important to mitigate chance of damage, and make and break ratings are useful specifications. 


March 18, 2021

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