What is Relay? - Working, Applications & Types

Have you ever wondered what a relay switch is, and why it's used in technology? It can play an essential role in controlling circuits within many different types of applications. In this blog post, we'll explain what a relay switch is and its working principle – as well as its various types – so that you have all the details needed to determine whether it fits your specific needs

What is a relay?

A variety of common electrical appliances must be operated effectively, which is no simple accomplishment when converting a small electrical input into a high-current output. Relays, which are essential in all kinds of electronic equipment are used in many circuits to accomplish these conversions. 


A relay is an electromechanical device, it can be used to establish an electrical connection. A relay is essentially a mechanical switch with the ability to be controlled electronically through an electromagnet, as opposed to being manually turned on or off. It consists of a flexible movable mechanical component.

How does a relay work?

The switching operation on the other side of the device is controlled by the signal received from one side of the device. A relay is a switch that electromechanically controls (opens and closes) circuits. This device's primary function is to establish or break contact with the help of a signal without the need for human intervention to turn it ON or OFF. It is mostly utilised to employ a low-power signal to operate a high-powered circuit. Typically, a DC signal is utilised to operate a circuit that is powered by high voltage, such as when using microcontrollers to operate AC home appliances.

Construction:construction of relay

Fig.1- Working of Relay

  1. The relay is physically and electrically driven. It consists of electromagnetic fields and contact sets that carry out the switching function. Relay construction is primarily divided into four groups. They are the housing, terminations, electromechanical design, bearings, contacts, and terminations.
  2. The most crucial essential component of the relay that influences dependability is the contacts. Good contacts have minimal contact resistance and little wear. The type of current to be interrupted, its size, frequency, and voltage of operation are only a few of the variables that affect the contact material choice.
  3. The design of the magnetic circuit and the mechanical attachment of the core, yoke, and armature are also parts of the electromechanical design. For the circuit to be more effective, the magnetic path's reluctance is kept to a minimum. The coil current and voltage are typically limited to 5A and 220V for electromagnets composed of soft iron.

Working Principle:

The above fig. 1.  depicts the relay's interior part. It has an iron core around which a control coil has been coiled. Through the connections of the load and the control switch, the coil receives power. The magnetic field that surrounds the coil is created as current travels through it.

It works on the principle of electromagnetism. The electromagnetic field that creates the temporary magnetic field is energised when the relay's circuit detects the fault current. This magnetic field moves the relay armature to open or close connections. The high-power relay has two contacts for opening the switch, compared to the small-power relay's single contact. The lower arm of the magnet is drawn to the upper arm by the magnetic field. Therefore, complete the circuit, causing current to flow through the load. When contact is already closed, it moves in the opposite direction to open the contacts.

Relay Contact Types

It can be classified, depending on the poles and throws

  1. Single Pole Single Throw (SPST)
  2. Single Pole Double Throw (SPDT)
  3. Double Pole Single Throw (DPST)
  4. Double Pole Double Throw (DPDT)

Single Pole Single Throw (SPST): Circuit diagram of SPST

Fig. 2. Circuit diagram of SPST

A switch with only one input and one output is referred to as a Single Pole Single Throw (SPST) switch. This indicates that it has a single output terminal and a single input terminal. An SPST switch functions as an on/off switch in a circuit. The circuit is turned on when the switch is closed. The circuit is shut off when the switch is open.

Check out 5V Relay Module

Single Pole Double Throw (SPDT):

Single Pole Double Throw

Fig. 3. Circuit diagram of SPDT

A switch with only one input and the possibility to connect to the switch between two outputs is known as a Single Pole Double Throw (SPDT) switch. This indicates that it has two output terminals and one input terminal. A circuit can use a single pole double throw switch for a number of purposes. Depending on how the circuit is wired, it can act as an on/off switch. Or it can connect circuits to any two different paths that a circuit may need to follow in order to function. For instance, a printer's Ready Mode and Standby Mode can be created by connecting an SPDT switch.

Check out 12V Relay Module

Double Pole Single Throw (DPST)Circuit diagram of DPST

Fig. 4. Circuit diagram of DPST

A double pole single throw (DPST) switch is used to link two source terminals to the corresponding output terminals. It contains four terminals (but never to each other). In an "ON/OFF" configuration, a DPST switch can be utilised with its terminal pairs either connected ("ON") or disconnected ("OFF").

Double Pole Double Throw (DPDT):

Double Pole Double Throw

Fig. 5. Circuit diagram of DPDT

A Double Pole Double Throw (DPDT) switch is a switch with two inputs and four outputs, each of which can be connected to one of the two corresponding outputs. A double pole, the double switch has two possible orientations for each of its terminals. Due to its versatility, the double pole, double throw switch. It may be connected to 4 distinct outputs with just 2 inputs. A circuit can be rerouted into one of two operating modes.

Check out Solid State Relays

Types of relays:

Relays can be divided into several types based on their features, architecture and uses, etc. Here, we've included a few of the more typical relay kinds.

  1. Electromagnetic
  2. Latching
  3. Electronic
  4. Non-Latching
  5. Reed
  6. High-Voltage
  7. Small Signal
  8. Time Delay
  9. Multi-Dimensional
  10. Thermal
  11. Differential
  12. Distance
  13. Automotive
  14. Frequency
  15. Polarized
  16. Rotary
  17. Sequence
  18. Moving Coil
  19. Buchholz
  20. Safety
  21. Supervision
  22. Ground Fault


  1. When galvanic isolation is desired, relays are employed whenever it is necessary to control a high-power or high-voltage circuit with a low-power circuit. Relays were first used on long telegraph lines, where a contact may be controlled by a weak signal received at an intermediate station, renewing the signal for further transmission. Small, low-voltage wire and pilot switches can be used to control high-voltage or high-current equipment.
  2. Lighting control systems
  3. Industrial process controllers
  4. Traffic control
  5. Motor drives control
  6. Protection systems of electrical power system
  7. Computer interfaces
  8. Automotive
  9. Home appliances


We have seen relay works on the principle of electromagnetic attraction when it gets energized it operates according to the signal given. Nowadays, this signal is given by the micro-controllers or microprocessors. These relays are also used in power system operations to detect and prevent the fault.

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