Brief Introduction to Relays and Its Types

Introducion of Relays

Relay switch was invented in 1835 by Joseph Henry.A Relay is an Electromagnetic Switch used for control purposes where a relatively small current can operate (Control) a Pilot which then controls a large circuit (Load). It functions as Coupler which couples Input and Output circuitry. Relays earliest applications were of amplifier/repeater in telegraph circuits and Computers, calculators and telephone exchanges where it was used for performing logical operations. Snubber circuit is often part of relays to absorb the energy due to induction in electromagnetic coil and to suppress the voltage spikes.

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Basic Design and Operation of Relays

So How a Relay Works.

When electricity flows through the Coil of wire as shown in above figure, it becomes a magnet and attracts the common terminal to open or close the circuit attached with relay. Single Pole Single Throw (SPST) can control only one circuit as shown in figure below, but Single Pole Double Throw (SPDT) can control two circuits. Double Pole Single Throw (DPST) and Double Pole Double Throw (DPDT) also act accordingly as can be seen in the figure. It depends how many circuits one want to control simultaneously.

As shown in the figure below is IEC Relay terminals labeling and IEC labeling is becoming standard. 'X' here shows the number related to 'contact number' of Relay and second digit will be (3,4) for Normally Open contact and (1,2) for Normally Close contact of relay and 'Coil' of Relay is represented by (A1, A2). Similarly one can verify the Relay Pin labeling of SPDT as well.

So here below is very simple example of SPST electromagnetic relay.

A "Relay" is Coupling a Float Switch to Operate a "Pilot" circuit of Pump.Here Input is "Float Switch" and Output is "Pilot of Pump" being Controlled by Relay. If Float energizes the Coil of Relay then "Common Contact" of relay will get attracted to 'Energized Coil' and Pilot circuit of pump will be activated to start the Pump.

Types of Relays

Co-Axial Relays

Coaxial Relays are used in Transceivers (Transmitter + Receiver) for radio systems having one antenna as receiver and transmitter as well. Purpose of Coaxial relay there is to switch antenna from 'being transceiver' to 'becoming receiver' of radio signals and also provides isolation between the two and to avoid receiver from transmitting the "received signal" its characteristic impedance is kept at 50 ohms.

Latching Relays

Latching Relays are sometimes very important. They maintain their state which could be very important for PLC program.

So in the figure above if we do not use a latching relay then we will have to stand there while continuously pressed "Start" button. So Latching relay here comes to our rescue. When "Stop" button will be pressed then Relay will de-energize and so will relay coil.

Overload Protection Relays

Overload Relays protects from sustained excessive current and thus protects very costly equipment such as motors. Causes for overload could be locked motor rotor, if motor is under-sized for given load, frequent starts of motors, loss of phase, under voltage or over voltage etc.. Most of time it is in built within motor starter circuitry.

So current passes through a coil i.e. electric heater which upon over-heating will attract Bimetal junction and circuit will be broken and can be seen in ladder logic that "OL" is normally close contact and hence motor would stop.

Reed Relay

As the solenoid coil gets energized it becomes an electromagnet and hence attracts the contacts closed in a tube with some inert gas in it and control circuit gets activated. Used in cars, metal detectors, burglar alarms, washing machines and many more places.

Solid State Relay

Solid State Relays perform the same function of Conventional relays but the difference lies how they perform the said tasks. They don't contain moving parts and don't have electromagnetic induction, instead they use opto couplers and triacs to perform the same job. TRIAC or SCR is equivalent to Normally Open contact but when it gets energized (switched) then it closes the circuit similar to electromagnetic relay in energized state.

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in above figure are shown electromagnetic and Solid State Relays side by side.

Time Delay Relay

Time Delay Relays control the output based upon time. For example if transient was very short so we use time relays "that if x state stays continuously for y period of time then only relay should activate, not otherwise" depending upon the specific application. These are "Shock absorb Relays". A complete article could be written on these types of relays and they are very important by industrial point of view.

Application of Relays

Want to work in Process or Power industry or want to become a successful Control Engineer then to understand Relays is a 'MUST'. It can get you jobs easily . Industry is totally dependent upon these itty bitty things called Relays.
It was a brief introduction though.

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About Me

My name is Fahad and I am an Electronics Engineer. I am currently working at a Power Plant in Pakistan as Junior Engineer in Instrumentation and Control Division (I&C). My duties include Maintenance and Debugging of Distributed Control System (DCS) and Programmable Logic Controllers (PLC) of whole Plant. If my posts are relevant to your work or may help someone else, Please do resteem, upvote and follow.

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