The American National Standards Institute (ANSI) defines a relay as

“an electrically controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or another electrical circuit.”

 Examples of relays include stepping switches and contactors. Relays are typically classified into three categories: light, medium, and heavy-duty; as well as commercial, industrial, and military applications.  In addition to these classifications, there may be various other factors that affect the type of relay being used for a particular application such as its construction materials, voltage ratings, current rating, etc. For the successful operation of a system, it is important to select a suitable relay based on its characteristics after considering all relevant factors.

Most relays are controlled either electromagnetically or electronically, yet some are activated by temperature changes to create a distinct thermal relay from its electromagnetic counterparts. 

Electromagnetic relays feature varying methods of control, such as transistors, integrated circuits, and other electronic components. The method of control used in any given relay is not particularly significant, so long as it does not interfere with the functionality of the device.

Types Of Relays

Armature relays:

Armature relays are an essential part of the electromechanical family, utilizing a moving element to carry contacts or actuate another mechanical unit. These innovative components enable precision operations in various electrical systems and serve as reliable switch controls for numerous applications.

Bimetallic relays:

Bimetallic relays are an essential piece of technology in many applications. The two metal strips within this relay act as a thermal response switch, bending and changing shape with even the slightest fluctuations in temperature – whether induced by machines or its own internal heat source! To ensure perfect alignment at any level of temp-variance, specially compensated bi-metallic can be used for more precise performance.

Clapper relay:

The clapper relay is an electromagnetic device that stands apart from traditional relays due to its unique operating mechanism. This type of relay has a hinged armature attached at one point, often the frame or heelpiece, enabling it to move in response to commands given by electrical currents – distinguishing it from plunger, bimetallic, and meter models with their own distinct activation processes.

Current-sensing relay:

The current-sensing relay is designed to detect when the current in its coil reaches an upper limit. The difference between the current required for pickup (when the relay must operate) and non-pickup (when it must not operate) is carefully controlled. Similarly, there is a close control on the difference between dropout (must release) and non-dropout (must not release) levels of current. This ensures that the relay operates reliably and accurately every time.

Dual-coil relay:

The dual-coil relay is an efficient and compact solution, consisting of two coils wound on either leg of a U-shaped magnetic core. The two coils create an effective magnetic field within the limited space, making it easier to manufacture than other types of relays. This type of relay is often used in conjunction with polarized relays to ensure the proper orientation of the electromagnetic field relative to the permanent magnet paths.  With its reliable performance and compact size, the dual-coil relay is a great choice for many applications.

Electromagnetic relays:

Electromagnetic relays are commonplace in electrical applications which require remote or automated circuit control. This type of relay works by using an electromagnet to move an armature and subsequently operate the contacts. These components can be found virtually everywhere, from household electronics to industrial power systems.

Impulse relay:

The impulse relay is a special single-coil relay, that features an armature-driven mechanism that alternates between two positions when pulsed by electrical power. This mechanical movement shifts the contacts from one position to another and back as pulses are received. The impulse relay can be powered by both AC or DC current, but consumes more energy than standard relays and is therefore typically used with intermittent duty coils.

Latching relays:

Latching relays are designed to maintain their contacts in the same position, even when power is no longer applied to the coil. 

There are two main types of latching relays: mechanical reset and electrical reset. 

1. Mechanical reset relays: Mechanical reset relays come with a coil, an armature mechanism, and a mechanical latching device that locks the armature into place once the coil has been de-energized. To reset this type of relay, manual tripping of the locking mechanism is required.
2. Electrical-reset relays: Electrical-reset relays have similar operating mechanisms but also have a second coil and armature which allow for remote resetting back to its original setting. This makes it easier to control and manage these types of relays.

Ratchet Relay:

The Ratchet Relay, a variation of the impulse relay, typically comprises an electromagnetic actuator that drives a rotating shaft and utilizes an indexing pawl to fit into a star wheel. This type of mechanical arrangement allows for the movable switch member to come in contact with multiple electrical contacts (more than two). The number of switches and configurations available can vary according to the requirements of specific circuits. when the actuator is energized, it rotates the shaft via an indexing pawl engaging each successive spoke in turn as it passes by. This enables one or more contacts on the switch member to be connected with one or more fixed contacts simultaneously. When the power supply is removed from the electromagnet, the spring-loaded detent holds the rotor in place and thus maintains contact between the switch member and fixed contacts. Thus, this type of relay offers flexibility in terms of its switching and contact arrangements. It also provides reliable operation that is suitable for many applications, such as circuit protection, control, and communication.

solenoid-activated relay:

A solenoid-activated relay is an electromechanical device that is composed of a coil, core, and switch contacts. It is manufactured in a wide variety of shapes, sizes, and characteristics to meet the needs of different applications. Solenoid action is often used when there is a need for a relatively large movement of the contacts or high contact force. The device utilizes a solenoid, an electromagnet composed of coiled wire, to generate a magnetic field when current is applied. This magnetism then interacts with the core and switches contacts to move them into contact or break the connection.

Undercurrent relays:

Undercurrent relays are protective alarms used to detect current levels that drop below a predetermined value. When this threshold is breached, the relay will trip and activate an alarm warning of potential danger. This type of relay is particularly useful in high-voltage applications where sudden changes in current can lead to catastrophic consequences. It also finds use in operations that require precise control over current levels, such as manufacturing and power plants.