Relay Logic System

Relay Logic System | IPCS Global

Relays are an important component in electrical panels. These are the electromagnetic switches which open and close the contact. These are used in electrical circuits. These are used in low power signals in control circuits. Relay control the one circuit by opening and closing of contact in another circuit. Relays have potential free contact which means that it can work on AC as well as DC.  IPCS Global Pune provides industrial automation training on the latest trends. It offers Automation course in Pune, Industrial Automation course in Pune, PLC course in Pune, SCADA course in Pune, PLC training in Pune.

What are the types of Relays? 

Solid state relays, thermal relays, safety relays, mercury relays, latching relays, control relays, machine tool relays, PCB mounted relays, reed relays, hybrid relays,

What is a Solid state relay?

Solid state relays are known as SSRs. These are mostly used in Industries.  They do not have the mechanical moving parts but consist of semiconductor and electronic components.  Switching done through the semiconductors like diodes, triac, thyristors.  Due to the use of these semiconductors these operates very fast as compared to electromechanically relays, there are again subcategories of these relays opto-coupled solid state relay, 

Transformer coupled solid state relays, a very small voltage drop occurs on this relay.  Solid state relays are mostly used in resistive and heavy loads like heaters. These loads need a large amount of current. 

The input part consists of an LED which is triggered by input supply voltage. Again, there is isolation between the input and output circuit. The output part has a photo coupler for capturing the light and converting it into electrical energy.  When input voltage becomes zero load also gets off. Due to opto-coupling technology, switches have high insulation levels and more sensitivities. These relays are highly resistant to shocks and vibrations. 

But the main disadvantage of this type of relay is voltage drop across the semiconductor which is the wastage of power in the form of heat energy.

Electromagnetic Relays – Electromagnetic relays work on the principle of electromagnetic attraction.  When supply is given to relay the magnetic field generated across the magnet and this magnet works as a switch which makes and breaks the contact.

Relays have the contact N0/NC.  These relays are constructed by wounding copper coils on the metal core. 

Working of Electromagnetic Relays –   These relays have the two operating conditions.

1) Normally open condition

2) Normally close condition 

Normally open condition – The electromagnetic coil is not energetic in this condition. When power supply is given it energies and normally open contact becomes close and load connected across it becomes on. 

Normally closed Condition – The contact is in the closed condition and the relay coil is not energized. But when load is connected across this NC contact it is initially ON when supply given to relay coil the load will be off.

Relay contacts are made from low resistance material. Silver, Tungsten, certain alloys are used for making contact. 

Advantages of Electromechanical relays –   These relays are quick to operate and can easily reset. They can operate with both AC and DC supply and can control AC/DC load. The operating time of these relays’ changes with current and also with additional arrangement of copper rings. Simple in size.

Disadvantages of Electromagnetic relays – To operate these types of relays high burden instrument transformers are needed. Dust and other external particles have the impact on the working of relays. These require routine maintenance and testing. Because of electromagnetic components there is a speed limit on operation. Direction feature not given in these relays. Electromagnetic relays can also be controlled remotely and automatic.

Application of Electromagnetic relays – Many DC and AC equipment are protected by the use of electromagnetic relays.  Applications where high temperature and environment is not good for operators at this condition one can use electromagnetic relays to operate from remote.

 Selection Criteria for Relays – contact current and voltage rating.  Number of poles. Mounting type -wall mounting and drill rail mounting. Operating life of contact and mechanical life. Coil voltage and coil current. Relay enclosure and package. Isolation between the coil contact, contact plating, size of relay, snubber circuit for relay contact protection, duty cycle of application, hysteresis effect, Operating temperature range. Contact type of arrangement that is form A , form B, form C, form D. 

Testing of Relays –   We can use a digital multimeter for checking the resistance value of the relay across the coil. The resistance of AC relay –   10KΩ and DC relay it is – 500Ω. For other testing keep DMM on continuity mode, check the continuity between NC and pole.  Check the discontinuity between NO and pole. 

Hybrid Relays – these are made from SSR and Electromagnetic relay.  Solid state relay uses the load current, so it overcomes the arcing problem. In this type there is no power loss in the form of heat.


Relays are electronic switching devices. Video includes pin configuration Allen Bradley relays. Relays are classified as AC relay and DC relay. These are SPST,SPDT,DPDT. In industrial automation relays are for electrical supply switching with PLC controllers. IPCS global provides training on electrical automation which includes the working of relays, contactors and other switchgear. IPCS Global Pune provides Industrial Automation training, Python training, Digital marketing, BMS and CCTV installation training, Data science, and Data analytics training, Machine learning, Web development, etc.


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