Relay is electrically operated switches that open and close the circuits by receiving electrical signals from outside sources.
The “relays” embedded in electrical products work in a similar way; they receive an electrical signal and send the signal to other equipment by turning the switch on and off.
Table of Contents
Types of Electrical Relay
Relay technology can be divided into two main categories: Movable contacts (mechanical relay) and no movable contacts (MOS FET relay, solid state relay).
Movable contacts
( Mechanical Relay )
This type of relay has contacts that are mechanically actuated to open/close by a magnetic force to switch signals, currents and voltages ON or OFF.
No movable contacts
( MOS FET relay, Solid State Relay )
Unlike mechanical relays, this type of relay has no moving contacts but instead employs semiconductor and electrical switching elements such as triac and MOS FET. By the operation of these electronic circuits, signals, currents and voltages are switched ON or OFF electronically.
Electrical relay Structure and Operating Principles
- Mechanical Relay
Relay consists of a coil, which receives an electric signal and converts it to a mechanical action and contacts that open and close the electric circuit. - MOS FET Relay
MOS FET relay is a semiconductor relay that uses power MOS FETs in output elements.
Electrical relay Characteristics and Mechanism
Characteristics of Electrical Relay
Mechanical Relay
One of the major characteristics of a mechanical relay is the physical spacing between the coil and the contact component in order to achieve appropriate level of insulation (insulation distance) on both input and output.
MOS FET Relay
One of the major characteristics of a MOS FET relay is that it utilizes semiconductor so the contacts do not mechanically open/close. As a result, benefits include reduction of footprint, quiet operation, longer operating life, and eliminating the need for additional maintenance.
Parts Name
1 ) Relay contacts
An electromechanical relay has these two fixed contacts; “NO” normally open and “NC” normally closed. They share a common mobile contact called “COM” or common. Under normal conditions, a relay can be open or closed.
In the open position, the contacts are made only when energized and return to their normal position, that is, normally closed once de-energized.
You can think of these contacts as a metal conductor made of elastic material such as beryllium or phosphor bronze alloy.
They have a special welded contact strap. These contact belts are made of high conductivity material with resistance to damage due to electrical sparks.
A conductive belt of an electromechanical relay can be made of alloys of silver, copper silver, tungsten silver, nickel silver, platinum, and gold.
2 ) Electromagnet
The magnetic field is produced when current passes through a conductor.
When current passes through a coil wound on a soft iron core, a magnetic field is created perpendicular to the direction of the current. This magnetic field when flowing through this soft iron core makes it an electromagnet.
An electromagnet maintains its magnetic property until the current is turned off, so it can be turned off and again play an important role in the principle of operation of the relay.
Conventionally, relays usually have latency in the magnetic field. This means that they retain some of their magnetic property even when the current is removed.
This is due to the magnetic lag created due to hysteresis, a hysteresis is a magnetic phenomenon where the magnetic induction in a product lags behind its own magnetic field/strength.
3 ) Movable armature
An armature in the relay is that moving conductor that makes or breaks contact based on the magnetic flux of the iron core.
When energized, these armatures pull against spring tension to make or break contact based on the normally closed/normally open relay type.
4) Yoke
A yoke in an electromechanical relay is that piece of metal attached to the soft iron core that does the job of holding and attracting the armature.
They are very small metal pieces attached at the top of the central element. In many designs, the armature is hinged to the yoke connected with a lead wire.
It ensures the connectivity/continuity of the current between the contacts and the mobile armature. The other main function of the yoke is to provide a low reluctance path for magnetic flux to flow.
Ideally when we look at an electromagnetic relay; we find an energizing coil called primary circuit. A primary circuit has two key parts, one is movable and spring loaded.
This part is called the truss while the other part that remains fixed is called the yoke. When power is supplied to the energizing circuit; yoke pull the movable armor towards itself closing the air gap in between. Modern yokes are made of ferromagnetic material with composite material as an intermediate layer.
5) Spring
Not all relays have a spring attached, but those that do have one attached to the armature to facilitate its movement. This spring allows the armature to move freely within the generated magnetic field to make or break contact with the electrical connection.
These springs are usually made of flat sheet metal that is cut and then stretched into a spring shape; while some high output relays have a nickel silver spring
Working and function
An electromechanical relay works on the simple principle of electromagnetism. When a low voltage direct current is supplied to the energizing coil, electrical contacts are made by the effect of the magnetic field.
You can think of this phenomenon as a simple switch where the circuit is completed with the push of a button. A simple relay is a two-way switch that connects to a different circuit on one side, with three NC, COM, and NO contacts.
Relays with one common contact and two main contacts in a single pole arrangement are called single pole two way type. Similarly, a single pole with a single throw has a NO connection and two poles with a double throw have two NO and NC respectively.
Initially, when no current is supplied to the energizing coil; contact is made with NC and COM. If you connect a bulb with COM and NC at that time it will light up; Similarly, when the relay is energized, you can connect your bulb with COM and NO to make it glow.
While watching a relay upside down; you will find five points of contact. Three on one side while two on the opposite; the two contacts on the opposite side are for NO and NC while the others are Coil +, COM and Coil – respectively.
When we supply current to these two points coil + and coil – magnetic flux is produced; and the position of the armor is altered. In the same way, when we disconnect the supply, the armature returns to its position and the NC contact closes.
Applications
A DPDT AC coil relay with “ice cube” packaging
Relays are used wherever it is necessary to control a high power or high voltage circuit with a low power circuit, especially when galvanic isolation is desirable.
Telegraph
Microprocessors
Railway signaling
Automotive
Motor device Control
FAQ
What is the principle of relay?
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.
What are the 5 applications of relay?
Relay Drive by Means of a Transistor, SCR.
Relay Drive from External Contacts.
LED Series and Parallel Connections.
Electronic Circuit Drive by Means of a Relay.
Power Source Circuit.
PC Board Design Considerations.
What are the 3 functions of relay?
Relay is an electrical control device, which has the interaction between the input circuit and the output circuit. It plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.
Are relays AC or DC?
The working power of the AC relay is AC, and the working power of the DC relay is DC. The coil diameter of the AC relay is thicker and the number of turns is less, and the coil diameter of the DC relay has more turns than the thin wire diameter.
Why relay is used in PLC?
It is used to energize the starter, which, in turn, switches the motor voltage while the PLC controls the relay.
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