What is a Relay? Types, and How They Work (Guide)

In the year 1809, relays were first developed. Electrolytic relays were invented by Samuel in 1809 as a part of the electrochemical telegraph. A simplified version of the telegraph was subsequently created by Henry in 1835 in order to create a telegraph, which was later developed by him in 1831. Despite Davy's discovery of the relay in 1835, Samuel's patent for the initial invention was granted in 1840. The device is similar to a digital amplifier in that it replicates telegraph signals and allows propagation over greater distances. As well as explaining what a relay is, how it works, and the types of relays available, this article covers a variety of relay concepts.

What is Relay?

Electrical relays control or manage multiple circuits using a single low-power signal at the same time. Electromagnets, armatures, springs, and electrical contacts are included. In electromagnets, currents pass through coils which create magnetic fields that attract armatures. As a result, a circuit is either opened or closed by this movement. Circuit management is made safer and more efficient with the use of relays, which can control high-power devices with low-power signals.

Each relay type serves a specific purpose, including electromechanical relays, solid-state relays, and reed relays. Industrial and automotive applications commonly use electromechanical relays due to their robustness and reliability. When fast switching and high durability are required, like in automated industrial systems, solid-state relays, which use semiconductor components, are preferred. In telecommunications and medical devices, reed relays with glass tubes enclosing reed switches are used to provide low power consumption and compact dimensions. Relays provide versatile control and protection mechanisms in a wide variety of electronic systems.

Types of Relay

Electromechanical Relays (EMRs)

Electric relays (EMRs) are switching devices that open and close electrical contacts with the help of moving armature mechanisms. There are two or more sets of contacts, an armature, and a coil (electromagnet). Coils generate magnetic fields that attract the armature when they are energized and control current flow in circuits by opening and closing contacts. In addition to SPST and SPDT, there are also Double Pole Single Throw (DPST) and Double Pole Double Throw (DPDT), which can be selected depending on the switching requirements. Various appliances, industrial machines, and automotive systems use these relays for switching and controlling operations due to their reliability and robustness.

Solid State Relays (SSRs)

Thyristors, triacs, or transistors are semiconductor components used in solid-state relays to replace mechanical parts. SSRs use an electronic mechanism instead of moving parts, which provides faster and more reliable switching, longer lifespan, and greater vibration resistance as compared with electromechanical relays. DC to AC SSRs, DC to DC SSRs, and AC to AC SSRs are classified according to their input and output configurations. A major advantage of these relays over traditional ones is their durability and rapid response time, which make them ideal for applications such as industrial automation, HVAC systems, and lighting control that require frequent operation.

Reed Relays

By generating a magnetic field with the help of a coil, reed relays are activated by the magnetic field generated in a glass tube enclosed by a reed switch. This magnetic strip energizes the coil when it is energized, making contact with the coil and completing the circuit. Relays with these characteristics are well suited to low-power and signal switching applications due to their fast operation, low power consumption, and compact size. There are many applications where reed relays can be found, including those in telecommunications equipment, medical devices, and test instruments where space is limited and precise, reliable switching is required.

Thermal Relays

The principle of thermal expansion is used in the operation of thermal relays. In either case, an electric current through a heating coil or a current flowing through the load causes a bimetallic strip to bend. The bending action opens or closes the contacts, preventing the circuit from overheating and overloading. When the voltage exceeds a preset value, bimetallic strip relays disconnect the load, which prevents equipment damage. Bimetallic strip relays are commonly used in motor control circuits and heating systems.

Hybrid Relays

To improve reliability and performance, hybrid relays combine electromechanical and solid-state features. Initial switching is typically made with solid-state components to minimize arcing and wear, and current is carried by electromechanical contacts. The combination of SSRs and EMRs offers fast switching and long durability. Industrial control systems and automotive applications, for instance, require a combination of solid-state switching reliability and electromechanical contact durability.

Latching Relays

When the control power is removed from a latching relay, the position of the relay (open or closed) remains the same. The armature is moved magnetically to the desired position using two coils (set and reset). Memory storage, power distribution systems, and lighting control systems rely on this characteristic to keep their last state in the event of a power outage. The latching relay only consumes energy when changing its state, not when maintaining it.

Time-Delay Relays

Activating or deactivating the contacts of time-delay relays is delayed after an input signal is received. There are three types of pneumatics: electronic, pneumatic, and electromechanical. A relay can be either an on-delay or off-delay relay. On-delay relays delay the closing of the contacts after the input has been applied, while off-delay relays delay the opening of the contacts after the input has been removed. Process control, conveyor systems, motor start-stop sequences, and motor start-stop relays require time-delay relays for industrial operations. Coordinated and safe operations are ensured by its precision timing functions.

Automotive Relays

Relays designed for automotive use are rugged and compact. Different functions of a vehicle can be controlled by these electromechanical relays, including its lights, horn, fuel pump, and other accessories. Vibrations, extreme temperatures, moisture, and contaminants must all be overcome by automotive relays. A vehicle's critical systems function safely and efficiently thanks to their robust design and reliability.

Polarized Relays

Permanent magnets are used to maintain the polarity of polarized relays. This feature enables them to respond differently to different input signals based on polarity. In applications such as telecommunications and signal routing, polarized relays are used to provide direction-sensitive control. Circuits requiring precise control and functionality can benefit from their ability to differentiate between positive and negative current directions.

Safety Relays

Industrial machinery and operators are safe through the use of safety relays that are designed with redundancy and fault tolerance. In order to monitor the relay's health and ensure proper operation, they usually include multiple contacts and feedback loops. Control systems for critical operations in industrial automation use safety relays in emergency stop circuits, safety interlocks, and safety interlocks. Safety standards and regulations are ensured by the design of these devices, which ensures that in the event of a failure, the system will default to a safe state.

How Does it Work?

An electromagnet is used to operate a relay, which is an electrically operated switch. An explanation of how relays work can be found here:

Basic Components of a Relay

Electromagnet (Coil): This coil produces a magnetic field when current flows through it.

Armature: The arm is energized by the electromagnet and attracts the electromagnet.

Spring: When the electromagnet is de-energized, this force is used to return the armature to its original position.

Contacts: An electrical circuit is opened or closed by these switch components. There can be more than one set of contacts on a relay.

Operation of a Relay

Energizing the Coil: The coil of a relay generates a magnetic field when an electrical current is applied to it. The coil is pulled towards the armature by this magnetic field.

Movement of the Armature: A magnetic field generated by the coil moves the armature, which is connected to the contacts. It depends on the relay design (normally open or normally closed) whether this movement closes or opens the contacts.

Switching the Contacts:

• Normally Open (NO): When the relay is de-energized, the contacts are open. The coil's armature moves when it is energized, closing the contacts and allowing current to flow.

• Normally Closed (NC): De-energized relays close the contacts when they are de-energized. In order to interrupt current flow through the load, the coil's armature moves and opens the contacts once energized.

De-energizing the Coil: A coil turns off its current, and a magnetic field collapses, returning the armature to its original position. Contacts are reversed in this way, either opened (for NO contacts) or closed (for NC contacts).

Wrapping Up

To summarize, relays provide an essential interface between low-power control signals and high-power devices in electrical and electronic systems. There are many types of relays, such as electromechanical, solid-state, reed, thermal, hybrid, latching, time-delay, automotive, polarized, and safety relays, each offering advantages suited to a particular application. Relays play an essential role in ensuring optimal performance, safety, and reliability across various industries by understanding their working principles and selecting the right type of relay. Modern electrical engineering relies heavily on relays for precise control and robust protection, whether in industrial automation or automotive systems.

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