relays of ece subject important concepts.pptx.

It is subject of ece which related to relays it is most important topic for electronic students and most of the lab viva questions.we have this topic in 3rd year of BTech and we learned so much from this topicWelcome to Gboard clipboard, any text that you copy will be saved here.

1. Relays

2. What is a Relay? switch is a component that opens (turn off) & close (turn on) an electrical circuit. whereas, a relay is an electrical switch that control (switch on & off) a high voltage circuit using a low voltage source. A relay completely isolates the low voltage circuit from the high voltage circuit.

3. COM or Common Terminal COM refers to the common terminal of the relay. NO Terminal NO or Normally Open terminal is also a load terminal of a relay which remains open when the relay is not active. The NO terminal becomes closed with the COM terminal when the relay activates. NC Terminal NC or Normally Closed terminal is the other load terminal of a relay. This terminal is normally connected with COM terminal of the relay when there is no control input. When relay activates, the NC terminal disconnects from the COM terminal & stays open until the relay is deactivated.

4. Poles & Throw Poles refer to the switches inside a relay. The numbers of Switches inside a relay is called the poles of the relay. The number of circuits being controlled per pole is called the throw of a relay.

5. Types Of Relay: Based on Poles & Throw: These following types of relays are classified by the numbers of poles & throw inside a relay. SPST Relay SPDT Relay DPST Relay DPDT Relay

6. Forms Of Relay Types of relays are also classified based on its configuration known as “Forms”. “Form A” Relay “Form A” is an SPST relay with normally open (NO) default state. It has NO terminal that connects the circuit when the relay is activated & disconnects the circuit when the relay deactivates.

7. “Form B” Relay Form B relay is SPST relay with normally closed (NC) default state. The NC terminal connects the circuit while the relay is inactive & it disconnects the circuit when the relay activates.

8. “Form C” Relay Form C relay is SPDT relay with double throw contact terminals known as NC & NO. It controls two circuit i.e. one circuit remain open while the other remains closed. It is also known as “break-before-make” relay because it breaks open one circuit before closing the other circuit.

9. “Form D” Relay Form D relay is also an SPDT relay and has the same principle as Form C relay but it is “make-before-break” contact relay. It closes the next circuit before breaking (opening) the first circuit. It is used for not disrupting the continuity of the circuit.

10. Based On Operation Principles: EMR (Electromechanical Relay) This type of relay has an electromagnetic coil and a mechanical movable contact.

11. EMR (Electromechanical Relay) The EMR relay is designed for AC or DC source depending on the application it is used for. The structure of AC & DC EMR relay differs from each other by having a slight difference in its coil construction. The DC coil has a freewheeling diode for protection against back EMF & de-energizing the coil. The polarity of the source in EMR relay does not matter, it energizes the coil in either way but if there is a back EMF diode installed then polarity should be considered. The main disadvantage of EMR relay is that its contacts produce arc during breaking which leads to increasing its resistance over time & decreasing the lifespan of the relay.

12. SSR (Solid State Relay) SSR relay is made up of semiconductors instead of mechanical parts and it works on isolating the low voltage circuit from high voltage circuit using an optocoupler.

13. SSR (Solid State Relay) When the control input is applied to a solid state relay, an LED lights up which produce infrared light. This light is received by a photosensitive semiconductor device which converts the light signal into an electrical signal and switches the circuit. SSR operates on relatively high speed & has very low power consumption as compared to EMR relay. It has a longer lifespan because there are no physical contacts to burn out. The main disadvantage of SSR relay is its nominal voltage drop across the semiconductor which wastes power in the form of heat.

14. Hybrid Relay Hybrid relays are made using both SSR & EMR relays. As we know that the SSR wastes power in form of heat and EMR has contact arcing problem. The hybrid relay uses both SSR & EMR to overcome their disadvantages.

15. Hybrid Relay In Hybrid relay, SSR & EMR are used in parallel. A relay control circuit is used for switching the SSR first. The SSR takes the load current. So it eliminates the arching problem. Then the control circuit energizes the EMR coil & its contact closes but there is no arching since the SSR is taking the load in parallel. After some time, when the EMR contact settles down, the control input of SSR is removed. The EMR conducts the entire load current without any loss. Since there is no current flow throw SSR & the EMR takes the entire load, there is no power loss in form of heat. Thus, it eliminates the heat problem too.

16. Electrothermal Relay (Thermal Relay) An electrothermal relay is made up of bimetallic (made up of two metals having different thermal expansion coefficients) strip.

17. Electrothermal Relay (Thermal Relay) When the current flow through the conductor, it produces heat. Due to which the temperature of the bimetallic strip rises and expands. The metal having high thermal expansion coefficient expands more than the other metal. Due which the strip bends & closes the contacts to usually activate the trip circuitry. Thermal relays are usually used for electric motor protection.