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Saturday, June 17, 2017

What is Relay,its Symbol and related terminology

·       What is relay? - Relay is an electrical switch which connects or disconnects the electrical circuit. The working principle of relay is based on the electromagnetism principle. As we know that when we pass the current through any coil then magnetic field developed and it attracts the metal nearby.
Relay can be divided into two parts: Input and Output; the input section has a coil which generate magnetic field when small electrical current is supplied from the electrical circuit. The output section has an armature and it is hinged to the yoke and mechanically linked to one or more sets of moving contacts as shown in figure below.

But in the case of solid sate relay (SSR), output section has not any mechanical moving parts that increase its reliability. So in place of armature (in case of electromechanical relay) here we used thyristor, TRIAC or other solid state switching device. An optocoupler (a light- emitting diode coupled with a photo transistor) can be used to isolate control circuits.


Types of Relay:

Relay symbol and related terminology:
  • Relay symbol:
  • Relay Terminology
    • Normally Open(NO): in this type of contacts it connects the circuit when relay is activated and disconnected the circuit when relay is inactive or under normal condition.
    • Normally Closed(NC): in this type of contacts it connects the circuit when relay is inactive or under normal condition and disconnected the circuit when relay is active.
  • Relay types based upon its pole and throw:
    • SPST Relay: SPST relay means single pole single throw. In this type of relay, switching element can be connect or disconnect and it have two terminal and also two terminal for coil. So total terminals for this type of relay has four.
    •  SPDT Relay: SPDT relay means single pole double throw. This type of relay has common terminal which can connects either of two others and also coil has two terminals. So total terminal for this relay has five.
    • DPST Relay: DPST relay means double pole single throw. These types of relay have two pair of terminal equivalent two SPST relays and activated by single coil. Including two terminals for coils so total number of terminals for this type of relay has six.
    • DPDT Relay: DPDT relay means double pole double throw. These types of relay consisting two rows of terminal and it equivalent to two SPDT relay. These relay have eight numbers of terminals- six switching and two for coil.




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