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Different Types of Relays In a Circuit

Relays are essential components that protect and control circuits and other electrical apparatuses. Regardless of the type of relay being employed, one thing is unchanging: all relays react to voltage or current in order to close or open the contacts or circuits. To better understand how relays work, this blog will cover the most common types of relays, their varying designs, and their distinguishing features.

In simplistic terms, relays are electrical switches that control a high voltage circuit by using a low voltage source, and they completely isolate the low voltage circuit from the high voltage circuit. Their construction is fairly simple in that all relays follow a similar design, consisting of a bonding strap, hinged armature, Normally Open (NO) contact, insulator, relay casing, coil and iron core, spring, and yoke. Beyond the consistency in design, there are also four primary types of terminals in a relay.

Terminal Types

Control input, or coil terminals, are the first terminal type, and they contain two input terminals for the means of controlling their switching mechanism. In most cases, a low power source, like AC or DC, is connected to these terminals to activate and deactivate the relay. COMs, or common terminals, are the output terminals of the relay wherein one end of the load circuit is connected. This terminal can be either internally connected with the other two terminals depending on the state of the relay. Next, NO terminals, or Normally Open terminals, are load terminals that remain open when the relay is inactive. NC terminals, or Normally Closed terminals, on the other hand, are typically connected with the COM terminal of the relay when there is no control input.

Defined by Poles and Throws

Poles and throws refer to the switches inside a relay, wherein the number of switches are called the poles and the number of circuits being controlled per pole is called the throw. For instance, a single throw relay can control one circuit, while a double throw relay, as the name suggests, can control two circuits.

How Relays Work

To grasp how relays work, we will outline the operation of a single pole double throw (SPDT) relay. To begin, without a power source, the relay is inactive and the position of the pole remains at the NC terminal. This produces an electrically short path between the COM terminal and NC terminal. This configuration allows for an ample flow of current through the circuit connected to the COM and NC terminal. When the relay is powered by a low voltage source, the pole of the relay shifts to the NO terminal, which makes the NC terminal open and the COM terminal close. This allows the flow of current through the circuit connected with the COM and NO terminal.

Types of Relays

There are various types of relays and they are typically classified into different categories as per their unique properties. As previously mentioned, an SPDT relay is just one example of a relay that is defined by the number of poles and throws inside the relay, but there are many others. For example, single pole single throw (SPST) relays can control one circuit and its pole has only one position in which it can conduct. Other types include double pole single throw (DPST) and double pole double throw (DPDT) relays.

Based on Operating Principles

Relays may also be classified based on their different operating principles. For instance, electromechanical relays (EMRs) are a type of relay that consist of an electromagnetic coil and a mechanical movable contact. Solid state relays (SSRs) are another example that are made of semiconductors instead of mechanical parts and work by isolating the low voltage circuit from high voltage using an optocoupler. Other relay types include hybrid, reed, electrothermal, and polarized & non-polarized relays.


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