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Relay Function | Types of Relay | Relay Working

Learn what is a relay, how it works, how it is designed and constructed and what are the different type of relay based and their working.

The main use of the relay was seen in the history to broadcast and receive information, called Morse code where the input signal used to be either 1 or 0, the changes in the signals are reported in the context of mechanical ON and OFF a light, bulb or a beep sound done. This means that those pulses of 0 sec to 1 sec are converted into mechanical ON and OFF using relay an electromagnet. Later this was improved and used in various applications. Let's see how this electromagnet works as a switch and why its name called relay.
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What is a Relay?

The relays are classified into several types, a standard and commonly used relay are made of electromagnets which are normally used as a switch. One thing is nothing else, it can be an interpretation of the device, which means that the signal from one side of the device is controlled by the other side switch function. Hence the relay is a switch that controls electromechanically (open and closed or NO and NC) circuits. The main operation of this device is to make or break contacts without any human involvement so that it can be turned on or off. It is mainly used to control high power circuit using less power signal. Normally a DC signal is used to control the circuit, which is operated by high domestic voltage, such as controlling AC home appliances with DCv signal.

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Glass Relays 

Design of a Relay

An electromechanical relay is basically made using some electromagnet, a movable armature, contact, yoke and some mechanical parts like a spring/frame/stand, these parts are visible in the inner photos of the relay below. All these are arranged logically to create a relay.


An electromagnet plays a major role in relay work. It is a metal in which the magnetic property is not, but it can be converted into a magnet with the help of an electrical signal. We know that when the stream passes through the driver, it attains the properties of the magnet. Therefore, when a metal is curved from a copper wire and is powered by sufficient power supply, then the metal can act as a magnet and attract metals within its range.

Construction of Relay and its operation:

The following picture shows how a relay interior looks and how it is made.

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On a cover, a core is placed with the copper winding (makes a horoscope). A movable armature has spring support or stands like an adjoining structure, and a metal contact-connected on the other side, all of these arrangements are placed above the core as if the coil is activated. It attracts armature. The movable armature is generally considered to be a normal terminal, which is to be connected with external circuitry. There are also two pins in the relay, which are normally closed and opened normally (NC and NO), normally connected to the closed pin armature or the normal terminal, while the normally opened pins are free (When the coil is not energized). When the coil is activated, the armature moves and is normally connected

Open the contact until the current flow through the coil. When it is de-energized, it goes into its initial state.

Normal circuit representation of relay is shown in the figures given below;

Pole and Throw combinations of relay:

  • SPST – Single Pole Single Throw
  • SPDT – Single Pole Double Throw
  • DPST – Double Pole Single Throw
  • DPDT – Double Pole Double Throw
Switches classified based on the number of poles and throw combinations. A pole can be considered as an input terminal and a moving part connected to it, while a throw can be considered as an output terminal. Its classification is as follows.

Single Pole, Single Throw (SPST):

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There are only one pole and one throw. Generally, the path is either closed or opened (remains untouched by any terminal). Push Button switch is the best example of this type. When we push the button, the contact is in the closed position and when the release is released, the contact is in an open position, which can be understood from the image above.

Single pole, double throw (SPDT):

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This type of switch has only one pole but it has two throws. So, the contact is always made, a slide switch can be considered as its example. The slider is always linked to any contact. That is a closed path always exists all the time if both the terminals are connected to a circuit.
Double Pole, Single Throw (DPST):

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There are two poles and one throw in it. Its contact either opens or closes which is done simultaneously. Toggle switch works on this property. When the switch is moved from one position to another, both contacts are transferred together.

Double Pole, Single Throw (DPDT):

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There are two poles in this type of switch, but there are two throws in the different pole. Therefore, it has been named as a double throw and the switching action has been uniform and simultaneous for both poles. A switch on a standard timer is of DPDT because when we are charging the timer and when the switch on the timer is turned on. It automatically stops charging, meaning that the charging circuit internally open.

The applications of the relay are infinite, its main function is to control high voltage circuits (230V circuit AC) with low voltage power supply (DC voltage).

The relay is not only used in large electrical circuits. It is also using in the computer circuits to do arithmetic and mathematical operations.

Switches are used to control electric motor switches. To operate an electric motor, we require a supply of 230V AC, but in some cases/applications. There may be a situation of turning the motor with DC supply voltage. In those cases, a relay can be used.

Automatic Stabilizer is one of its applications where a relay is used. When the supply voltage is in addition to the rated voltage, the set of relays feels voltage variation and controls the load circuit with the help of a circuit breaker.

If a circuit is used for selection if there are more than one circuit in a system.
Common usages;

It is being used in the Building management system (BMS) to get signals from the devices. i.e. temperature, ON, OFF, start and stop control etc.…

Relays, however, they are basically electric power switches. It comes in many sizes, shapes and power ratings suitable for all types of applications. The relay can also have single or multiple contacts. The main power relay "contacts" use for the main voltage or high current switching application in the same packages are call relays.

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Professionally prepared message. A detailed description of the purpose, design of the device and the principle of its action have now become more clear.
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