What is Relay and How it works?

In electrical and electronic circuits we frequently use several basic components, devices, and so on. These components and devices include switching components, protecting devices, sensing elements, and so on. Let us consider the switching and protecting devices such as transistors, diodes, thyristors, etc.,. Here, in this article let us discuss in detail about a special type of switching and protective device termed as a relay. Primarily, we must know what is relay and how does a relay work.

What is Relay?


A relay can be termed as different type of switch which can be operated electrically. Generally, relays are mechanically operated as switch using an electromagnet and these types of relays are termed as solid-state relays. There are various types of relays and are classified based on various criteria such as based on operating voltage, based on operating technology, and so on. Various types of relays can be listed as latching relay, mercury relay, reed relay, Buchholz relay, vacuum relay, solid state relay, and so on. Before discussing in detail about types of relays, let us discuss how relay works.

Relay Working

To discuss about working of relay, we must consider any one type of relay and here in this article, consider solid state relay to easily understand about relay working.  Solid state relay can be defined as relay, which utilizes the solid state semiconductor devices for performing switching operation. If we compare electromagnetic relay and solid state relay, then we can observe that the solid state relay offers high power gain. These solid state relays are again classified into various types such as transformer-coupled, photo-coupled, reed relay-coupled solid state relay.

The solid state relay working is similar to electromechanical relay, but the solid state relay doesn’t contain any moving parts. Hence, offer increased long term reliability compared to relays with moving contacts. The power MOSFET transistors are used as switching devices in solid state relay working. The electrical isolation between the low power input circuit and the high power output circuit can be provided using an Opto-coupling.

Let us consider a practical example of solid state relay as shown in the figure below. If the output switch is opened or MOSFET is off, then it is said to have infinite resistance. Similarly, if the output switch is closed or MOSFET conducts, then it is said to have a very low resistance. We can use these solid state relays for switching both AC and DC currents.

Solid State Relay
Solid State Relay

The above circuit consists of photovoltaic unit with LED which switches on the MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) with 20mA through the LED. The photovoltaic consists of 25 silicon diodes which generates 0.6V output such that making a total of 15V which is large enough to turn on the MOSFETs.


Practical Solid State Relay Working

To understand about relay working in depth, let us consider practical three phase solid state relay with ZVS. Three single phase units with power triac and snubber network are used for zero voltage switching in order to control each phase individually.

Three Phase Solid State Relay with ZVS by Edgefxkits.com
Three Phase Solid State Relay with ZVS by Edgefxkits.com

This project consists of 8051 microcontroller which sends switching signals to each phase through Opto-isolators. The Opto-isolators drives the loads through a set of triacs which are connected in series with loads. For every zero voltage pulse the microcontroller generates output pulses such that to turn on the load for every zero crossing of supply waveform.

Three Phase Solid State Relay with ZVS Project Block Diagram by Edgefxkits.com
Three Phase Solid State Relay with ZVS Project Block Diagram by Edgefxkits.com

The above figure shows the block diagram of practical three phase solid state relay with ZVS which consists of the power supply block, microcontroller block, TRIAC set, and loads. The zero crossing feature of an Opto-isolator (which acts as TRIAC driver) avoids abrupt current inrush on inductive and resistive loads by ensuring low noise generation. Two push buttons are used to generate output pulses from the microcontroller.

To verify load switching at the zero voltage point we can check the waveforms of voltage applied to the load by connecting to a CRO or a DSO. The relay working can be extended for switching heavy loads in the industries by using two back to back thyristors. By incorporating overload protection and short circuit protection, we can achieve high reliability.

Advantages of Solid State Relay

  • The solid state relay working is entirely silent, slimmer, and allows tight packing.
  • Independent of the amount of usage the SSRs have constant output resistance.
  • The relay working is clean and bounceless compared to mechanical relay working.
  • Even in explosive environments also SSRs can be used as they don’t cause sparking even under relay working.
  • As there are no moving parts, these SSRs are long lasting compared to mechanical relays.

Disadvantages of Solid State Relay

  • To the gate charge circuit, the isolated bias supply is essential.
  • The voltage transients may cause spurious switching.
  • Because of the body diode, the SSRs have high transient reverse recovery time.

Do you want to know about the various types of relays in detail? Are you interested in designing electronics projects on your own? Then, post your comments, suggestions, ideas, and queries in the comments section below.

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