Saturday, 29 February 2020

How works Electromechanical Relay? Construction, Types, Applications

 Electromechanical Relays

  • These relays are constructed with electrical, magnetic & mechanical components & have an operating coil & various contacts.
  • If a relay has moving parts, it is called electromechanical relay.  
  • These are very robust & reliable. Based on the construction, characteristics, these are classified in three groups.
Electromechanical Relay

Electromechanical Relay

Construction And Work Of Electromagnetic Relay

  • Relay are typically used when it is necessary to switch a small amount of power. Relay contains several electronic parts to make them work.
  • These includes an electromagnet, which controls opening and closing of the relay.
  • Next is the armature, or the moving part, which is the electronic part that opens and closes.
  • A spring is also used in relay. This is the part that forces the relay back to its original position after each revolution.
  • In addition, a set of electric contacts is needed in order to transfer the power.
  • The power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. 
  • Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
  • As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.
Construction, Working of Relay

Construction, Working of Relay

Types Of Electromagnetic relay

There are three types of elctromagnetic relay
  1. Attraction relays
  2. Relays with movable coils
  3. Induction relays

1.Attraction Relays

  • Attraction relays can be AC & DC and operate by the movement of a piece of iron when it is attracted by the magnetic field produced by a coil. 
  • There are two main types of relays:

                        1. The attracted armature type
                        2. Solenoid type relay

1. Attracted armature relays
  • Consists of a bar or plate (made of iron) that pivots when it is attracted towards the coil.
  • The armature carries the moving part of the contact ,which is closed or opened, according to the design, when the armature is attracted to the coil.

2. Solenoid type relays
  • In this a plunger or a piston is attracted axially within the field of the solenoid. In this case, the piston carries the moving contacts.
  • In order to control the value of current at which relay operates, the parameters resultant force and restraining force  may adjusted. Attraction relays effectively have no time delay and are widely used when instantaneous operation is required.

Relays with movable coils
  • This type of relay consists of a rotating movement with a small coil suspended or pivoted with the freedom to rotate between the poles of a permanent magnet. 
  • The coil is restrained by two special springs which also serve as connections to carry the current to the coil.
  • The relay has inverse type characteristic.

Induction relays
  • An induction relay works only with AC.
  • It consists of an electromagnetic system Which operates on a moving conductor, generally in the form of a DISC or CUP.
  • Classification of induction relays
  1. Shaded pole relay
  2. Watthour- meter type relay
  3. Cup type relay

1.Shaded pole relay

  • The air gap flux produced by the current flowing in a single coil is split into two out of phase components by a so called „Shading Ring‟ generally of copper, that encircles part of the pole face of each pole at the air gap.
  • The shading ring may be replaced by coils if control of operation of the shaded pole relay is desired.
  • The inertia of the disc provides the time delay characteristics

2. Watt hour –meter structure

  • This structure gets its name from the fact that it is used in watt hour meters.
  • As shown in the top figure below, it contains two separate coils on two different magnetic circuit, each of which produces one of two necessary fluxes for driving the rotor, which is also a disc.


  • This type of relay has a cylinder similar to a cup which can rotate in the annular air gap between the poles & the fixed central core. 
  • The operation of this relay is similar to that of an induction motor with salient poles for the windings of the stator. 
  • The movement of the cup is limited to a small amount by the contact & the stops. A special spring provides restraining torque.
  • The cup type of relay has a small inertia & is therefore principally used when high speed operation is required, for example in instantaneous units.

Use of Electromechanical Relay
  • Relay typically are used in modern household appliances such as hair dryers, kitchen appliances, and lights that need to be switched on and off. 
  • They are also used in cars where things need to be turned off and on.
  • In fuel pumps. 
  • Industrial Applications- Where the control of high voltage and current is intended.
  • Controlling large Power loads.

Related Terms:

Friday, 28 February 2020

Different Types Of Relay

Types Of  Relays

  • Relay is an electrical switch which is used to open or close a circuit with electromagnetic relay or electronically. It operates manually as well as automatically.
  • Relay is used in circuit for protection of electrical circuit, system and electrical equipment. In any electrical system protection of the system is given utmost priority.
  • The purpose of an Electric Power System is to generate and supply electrical energy to consumers. The power system should be designed and managed to deliver this energy to the utilization points with both reliability and economically.
  • Any abnormal operating state of a power system is known is FAULT. And over come this faults the relay gives the protection to the device or system. This faults are of two types 
  1. Short Circuit Fault - Current 
  2. Open Circuit Fault - Voltage

  Types Of Relays

Types Of Relays

Types Of Relay

Classification Of Relay

1. According to the Construction Principle

Depending upon the principle of construction, relay is classified in the following four categories.

  • Electromechanical 
  • Solid State
  • Microprocessor 
  • Numerical

2. According to the Actuating signals

  • Current
  • Voltage
  • Power
  • Frequency
  • Temperature
  • Pressure

Thursday, 27 February 2020

Directional Over Current Relay- Use of directional and non directional relay in over current relay

Directional Over Current Relay

  • Directional Over Current Relay is used to transmission line where the power flow is associated in a special direction.
  • From the word over current, it means that the relay will be operated for only one case if the current value rises from the set value.
  • And directional word is for the current direction that is if relay is set for only forward direction current, it will sense forward direction current, it will not sense reverse direction over current and vice versa.
  • These are suitable in ring main system and radial system.

How does the directional Relay Works?

Directional Over Current Relay
Directional Over Current Relay 

  • Directional over current relay respond to excessive current flow in a particular direction in the power system. The relay typically consists of two elements. One is directional element, which determines the direction of current flow with the respect to the voltage reference.
  • When this current flow is in the predetermined trip direction, this directional element enables (turn on) the other element, which is the standard over current relay, complete with taps and time dial, as found on a normal non- directional over current relay. 
  • Because of these relay is designed to operated on fault current, the directional unit is made so that it operates best on a highly lagging current, which is typical of faults in power systems. So the zone of fault is identified with the help of relay characteristics and maximum angle setting.
  • Directional over current relay are normally used on over coming line circuit breakers on buses which have two or more sources. 
  • They are connected to trip an incoming line breaker for fault current flow back into the source, so that a fault on one source is not fed by the other sources. 
  • In complex distribution or sub-transmission network, these relay may be used to improve coordination of the system.

What is Non Directional Relay?
  • When there is fault in power  system, power flows through fault. Non directional relay operates irrespective of direction of flow of current. 
  • Ex. Breaker at generator end. If there is fault on generator secondary relay has to operate to open circuit breaker.  
  • If there is fault in a winding generator and its drawing power from grid then also circuit breaker has to operate. So we use a non directional relay. It has to operate in fault conditions irrespective of direction of power flow. 

Use of Directional and Non- Directional Relay in over Current Relay

  • In an over current relay we use both directional as well as non directional over current relay.
  • Directional relay is connected towards load side of the line which will trip if over current flows from load towards the source and 
  • The non directional relay is connected at the source side which will trip if fault occurs irrespective of the direction of current. 

  • Now a days we use multiple conductors or transmission line forming a ring structure so as to avoid interruption of power supply to the costumers in case of fault in any line.  
  • Therefore when any of line undergoes a fault. The non Directional relay at the source will automatically trip and separate the source from fault point but load side is still connected to the fault point. 
  • Due to the other transmission lines present in the system the power will still reach to the costumers. As the fault is still present therefore other transmission line will also feed this fault point and thus system might get damage. 
  • Therefore the directional relay we connected at the load side will detect the reverse flow of current and thus trip and avoids other transmission to feed the fault location.

Tuesday, 25 February 2020

How Distance Relay Works?- Operation, Types, Advantages, Limitations.

Distance relay

  • Relay is an electrical switch which is used to open or close a circuit with electromagnetic relay or electronically. It operates manually as well as automatically.
  • So the distance relay is another relay for the protection of the devices. Distance relay is used for the protection of transmission line &amp, feeder.
  • In a distance relay, instead of comparing the local line current with the current at far end of line, the relay compares the local current with the local voltage in the corresponding phase or suitable components of them.

Principle of operation of distance relay

Distance Relay Protection
Distance Relay Protection 

  • The basic principle of measurement involves the comparison of fault current seen by the relay with the voltage at relaying point, by comparing these two quantities.
  • It is possible to determine whether the impedance of the line up to the point of fault is greater than or less than the predetermined reach point impedance.
  • There are two types of torques
 1. Restraining torque
2. Operating torque

  • The relay trips when operating torque greater than restraining torque.

Types of distance relay

Distance relays are classified depending on their operating characteristic in the R-X plane
  • Impedance Relay 
  • Mho Relay
  • Reactance Relay


  • The torque equation T, for such a relay the current actuates the operating torque and the voltage actuates the restraining torque, with the usual spring constant K4.
  • Considering K2 to be negative (as it produces the restraining torque) and neglecting the torque component due to spring, the equation represents a circle in the R-X plane.

Impedance Relay Protection
Impedance Relay Protection

Disadvantages of Impedance Relay

1. It is not directional.
2. It is affected by the Arc resistance
3. It is highly sensitive to oscillations on the power system, due to large area covered by its circular characteristic.


  • The reactance relay is basically a directional restrained over current relay. 
  • The characteristics resembles a horizontal line parallel to the R-axis with constant X value. The portion below the line gives the operating zone of the relay.

Disadvantages of Reactance Relay

1. The reactance relay is designed to measure only reactive component of the line reactance.
2. The fault resistance has no affect on the reactance relay.

What is Mho Relay- How it Works, and its Applications

Mho Relay

  • Mho relay is voltage controlled directional relay. Mho relay is the type of Distance Relay. This type of relays used for the long distance protection of transmission lines.
  • Its characteristics is straight line, when plotted in an admittance. Because of this it is called the Mho Relay.

Mho Relay Circuit Diagram
Mho Relay Circuit Diagram

Working Principle of Mho Relay

  • Due to the voltage elements controlling elements develops and operating torque is obtained in volt amperes.
  • Mho relay is used for long transmission lines. Because it can protect from power swing better than other simple impedance relay. It has directional property also which is essential in transmission line.
  • The Mho relay combines the properties of impedance and directional relays. Its characteristic is inherently directional and the relay only operates for faults in front of the relay location. 
  • In terms of the torque equation the relay characteristics can be obtained by making the constant K1 equal to zero. It is basically a voltage restrained directional relay and the torque
Characteristics Of Mho Relay
Characteristics Of Mho Relay
  • This relay has a larger coverage of R-X plane and therefore it is least affect ted by condition of power swing. The characteristics is shown below.

Applications Of Mho Relay

  • Mho relay is used for protection of EHV/UHV transmission lines. These are generally used for protection of long transmission line due to its stability in case of power swing. Further no separate directional unit is required in case of Mho relay, since these relay are inherently directional.

Related Terms:

Monday, 24 February 2020

Thermal Over Current Relay- Working Principle, Construction.

Thermal Over Load Relay

  • Thermal over load relay works on the principle of heat and temperature rise. When an electric current flows through a conductor heat is produces which causes the conductor to have an increased temperature rise. 
  • When a temperature rise reaches a predetermined set point, the relay act to open the protected circuit.
Thermal Over Load Relay
Thermal Over Load Relay

Construction And Working Of Thermal Relay

There is two bimetallic strips that is two metal soldered together. One has lower coefficient of expansion and other is higher coefficient of expansion.

When over current flows through the coil then it heats the bimetallic strips. Both the metal expands due to the heat generation in coil. 

But the expansion of higher coefficient strip is high than the lower coefficient strip. So because of this dissimilar expansion of both the strips it starts to bend.

The mechanical bending movement is used to open the protected circuit. After the unit cools it will self reset and the load can be started again. 

Thermal Over Current Relay construction
Thermal Over Current Relay  construction

  • This types of relays relatively slow to respond. So this type of relays used for the overloads and not for the short circuit faults which requires high speed clearance.

Differential Relay- Working, Classification, Advantages.

Differential Relay

  • Differential relay operates when the phase difference in two or more than two identical quantities crosses the predetermined value.
  • Differential relay is another type of relay which is used for protection against faults related to current and voltage in the device. This is the most prevalent and successful method to protect the circuit.
  • This type of relay used for the protection of transformers, generators, bus bars, feeders and large motors for localized faults etc.

Working operation of differential relay

  • It compares the phase angle and magnitude of the same quantities in particular device. Arranged the relay to compare the current entering and leaving the circuit which should be same at normal condition and during external fault.
  • There are two current  comes from two part of  electric power circuit. It means two inputs. These two currents meets at junction point where the relay coil is connected.
  • If the polarity and amplitude of both the currents are to adjusted that the phasor sum of these two currents is zero at normal condition. At that time there is no current flowing through the relay coil. 
  • And if any abnormality occurs in the circuit this balance broken, it means the phasor sum of these two currents no longer zero then non zero current flowing through relay coil relay being  operated.
  • So there is two current inputs one is the reference and actual input to comparing to inputs if any difference in the input then output coil will be energized and relay will be  turn off.

Differential relay at Normal Condition
Differential relay at Normal Condition

Differential relay is classified in four categories

1. Current differential relay
2. Voltage differential relay
3. Biased or percentage differential relay
4. Voltage balance differential relay

1. Current differential relay

Current Differential Relay
Current Differential Relay

  • This relay is operated only for the current. It sense and then operates the phase difference between the current entering in the circuit and current leaving the circuit.
  • At the both end of the protected zone the current transformers are connected.With the help of pilot wire  the secondary of current transformer is connected in series. And the operating coil of relay connected to secondary of CT's.
  • If any fault occurs outside the zone and the two CT's both having same ratios are connected properly, then the no current will flow through the relay coil that why it will not operates.
  • But if the fault occurs in withing the zone and the current entering in zone is not equal to current leaving the zone. This fault current flows through relay coil and it gets operated.
  • This type of relays used in current transformers, generators, motor protection etc

2. Voltage balance differential relay
  • Current differential relay is not suitable for the feeders so the voltage biased differential relay is used for feeder protections.
  • When the fault occurs in the protected zone the phasor difference of both lines will not be zero.
  • So the voltage induced in the secondaries of CT's will different and the calculating current will flow through the operating coil of relay. Because of this relay operates and gives command to trip the circuit and the circuit breaker will be open.

3. Biased or percentage differential relay

Percentage Differential Relay
Percentage Differential Relay

  • There is one additional coil is connecting in the biased percentage differential  relay. That coil is called restrained or biased coil. Other arrangement of this type of relay is same like the current differential relay.
  • At the mid point of biased relay the relay coil is connected. 
  • Due to this the we can overcome the difficulty arising out of difference in transformer ratio for high values of short circuit current.

Advantages of Differential Relay

  •  In percentage differential relays, Individual CT inputs are given to the relays. So the relay can process these CT inputs to calculate the differential and bias. 
  • Percentage 2nd and 5th harmonic restraining can be provided in differential relay.
  • To prevent operation of relay on through fault, slope characteristics can be provided in such relays.

Related Terms:

Saturday, 22 February 2020

Over Current Relay- Working Principle, Types, Drawbacks

Over Current Relay

  • This is another type of relay which is used to protect the transmission lines from faults. In this relay there is a setting that save the current value and protects the line if it goes high.
  • Over current relay is the relay which operates at the value of current is greater than the setting time of relay.
  • In this over current relay there is one current operated element is present. So the protection against the over current or exceeds current in the line is the essential protection for the device.
  • Over current protection is directed entirely to the clearance of the fault although with the setting usually adopted some measure of overload protection is obtained.  

Working principle of Over Current Relay

  • In over current relay the current coil is given to it. At the normal current flowing through this coil, the coil generates magnetic effect but that not sufficient to move the moving element of the relay.
  • At this condition restraining force is greater than the deflecting force.
  • When the current increases then the magnetic field also increases and at some level deflecting force that generated by the magnetic effect is increases and crosses to the restraining force.
  • Because of this the moving elements of relay starts moving to change the position of relay. 

Operation OverCurrent Relay
Operation Over Current Relay

Types of Over Current Relay

There are three types of over current relay.

1. Definite current or instantaneous over current relay

2.Definite time over current relay

3. Inverse time over current relay

1. Definite current or instantaneous over current relay
  • Instantaneous relay sense the over current in the circuit and gives tripping command to operate the circuit breaker.
  • This relay takes no time in giving tripping command once the over current sense. So the operating time delay is zero.
  • In the construction of over current instantaneous relay magnetic core is wound by a current coil. The iron pieces are attached by hinge support and restraining springs in the relay.
  • When the excess current flowing through the coil, NO contact remains open  that attractive force pull and the NO contacts gets closed.
  • The operating time of the instantaneous over current relay is of order few milliseconds. 

2.Definite time over current relay

  • It is made with intentional time delay after crossing the pick up value of the current.
  • It is provided with the time setting adjustments and also with pick up adjustment.

3. Inverse time over current relay

  • In the magnitude of the current the operation of relay depends.
  • The current increases the relay takes minimum time to trip the circuit. Inverse Time Relay also referred to as Inverse Definite Minimum Time(IDMT).
  • Inverse means higher the current value lesser the time taken for the relay to trip the circuit. Current in the line and time taken for the relay to trip the circuit breaker follow an inverse proportionally.

Drawbacks Of Relay

  • The continuity in the supply cannot be maintained at the load end in the event of fault.
  • Time lag is provided which is not suitable in short circuits.
  • Not suitable for long distance transmission.
  • Difficult to co-ordinate and requires changes with the addition of load.

Related Terms:

Friday, 21 February 2020

What is Buchholz Relay? Construction, Working Principle, Advantages, Limitations


  • Buchholz relay is a protection device used to detect transformer faults in oil filled transformers.
  • Short circuit faults such as inter turn faults, incipient faults, and core faults may occurs due to the impulse breakdown of the insulating oil or simply the transformer oil. 
  • Buchholz relay will sense such faults and close the alarm circuit.

Buchholz Relay

Buchholz Relay


  • Buchholz relay is placed in the pipe which connects to the conservator and transformer oil tank.
  • The oil filled chamber is present in it. Two hinged floats, one at the top of chamber and the other at the top of the chamber which is with a mercury switch each is present in the oil filled chamber. 
  • The mercury switch on the upper float is connected to an external alarm circuit and the mercury switch on the lower is connected to an external trip circuit.  
Construction of Buchholz Relay
Construction of Buchholz Relay

Working Principle

  • The buchholz relay works in transformer tank. It is mechanically actuated. Whenever there will be a minor internal fault in the transformer such as an insulation fault between turns, breakdown of core of transformer, core heating then it works.
  • When something fault occurs in  transformer tank then it accompanied with the generation of some type of gas and if the fault occurred is high enough it will be accompanied by a surge of oil from the tank to the conservator.
  • If some fault occurs inside the transformer, the oil in the transformer tank gets overheated and gases are generated.  In the intensity of fault occurs in transformer tank, the production of gases depends on it.
  • Also the heat will generates if the fault occurs high and it decomposes the transformer oil. And that gases produced in the can be used to detect the winding fault.
  •  So this is the basic principle of the working of buchholz relay.  

Buchholz Relay Placed in Transformer at-
  • Buchholz relay located in the pipe connecting the main oil tank and the conservator.
  • So it can sense both the oil level and the pressure of any gas generated by the thermal decomposition of the oil. 
Location of Buchholz Relay
Location of Buchholz Relay

Operating Conditions of Buchholz Relay
Buchholz relay operates during the three conditions

1. Whenever gas bubbles are formed inside the transformer due to severe fault.

2. When the level of transformer oil faulse.

3.When transformer oil flows rapidly from the conservation tank to the main or from main to the  conservation tank.


  • It indicates incipient faults.
  • Buchholz relay gives an alarm when the oil level reduces below a certain level due to leakage of oil from transformer.
  • It gives audible warning which informs the operator that there is some fault in transformer.
  • By testing the gas it also detects the failure of insulation.

Limitation of buchholz relay

  • It can be used only for oil immersed transformers having conservator tanks.
  • Only faults below oil level is detected.
  • Setting of mercury switches cannot be kept too sensitive otherwise the relay can operate due to bubbles, vibrations, earthquake, mechanical shocks etc.
  • The relay is slow to operate having minimum operation time of 0.1 seconds and average time 0.2 seconds.

Related Terms:

Thursday, 20 February 2020

What Is Relay Protection? Construction, Operation, Advantages, Disadvantages, Applications.


  • Relay is an electrical switch which is used to open or close a circuit with electromagnetic relay or electronically. It operates manually as well as automatically.
  • Relay is used in circuit for protection of electrical circuit, system and electrical equipment. In any electrical system protection of the system is given utmost priority.
  • In domestic electrical system for protection of the equipment and also for the protection of the operator relay is provided which immediately turn off automatically or fuse blown in case of any abnormality in the equipment and we frequently face it home.
  • Now when it comes to comparatively large power consumption such as in industry for the protection of system and equipment we use relay it sense the current and voltage through transformer and if it crosses limits then it generates trip command. 
  • Relays protection can also be used as logic device. They can have timers build into them. They can be wired so that they have a set-reset function.

Construction Of Relay

Construction Of Relay
Construction Of Relay

Coil of Wire
  • Originally a relay is a device that has a coil of wire, electromagnet, that when energize a contact is used make or break a connection. That is to open or close a switch or switches. 

  • Their are two poles in it. Normally close and Normally open. One pole of the contact remains stationary. When the magnet energizes it pulls the other pole contact to or away from the stationary contact.
  • Normally Open means if there is no voltage present at the coil of relay the contact will act as open switch with respect to common. 
  • Similarly Normally closed means it will act as short with common while there is no voltage present at the coil of Relay. Contacts NO & NC are firmly mounted while common is mounted in a way it can move. A spring is attached to common for damping that is Getting back normal state once coil is de-energized.
  • There are solid state devices called solid state relay that effectively perform the same function. That is it has a set of contacts that when energized open or close isolated contact, or solid state device that act as contacts.
  • The purpose is have one circuit open or close contacts in another circuit. This isolates the two circuits. 
  • The isolation can be so that a circuit of lesser or greater voltage controls another circuit of greater or lesser voltage. Tt can be used to isolate a DC or AC from each other. 

Operation of Relay

Relay Operation
Operation of Relay 

  • The power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. 
  • Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
  • As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.
  • Relays are mainly made for two basic operations. One is low voltage application and the other is high voltage. For low voltage applications, more preference will be given to reduce the noise of the whole circuit. For high voltage applications, they are mainly designed to reduce a phenomenon called arcing.

Types Of  Relay

Their are main two types of the Relays
  • Electromechanical Relay- Constructed with mechanical moving parts.
  • Solid State Relay- It is integrated electronic device.


  • Fast operation and fast reset
  • They can use both AC and DC system for protection of AC and DC  equipment.
  • They have properties such as simple, robust, compact in size, reliable.
  • Very low contact voltage drop thus no heat sink is required.
  • High resistance to voltage transient.


  • Low speed of operation.
  • Directional feature is not available in electromechanical type relays.
  • Component failure leading to relay failure.
  • Change in characteristics over a period due to aging effect.


  • Protection of varies AC and DC equipment's.
  • The over/under current and voltage protection of various AC and DC equipments.
  • For fast heater switching.
  • To control variable resistance heater.
  • solenoid activation control.

   Related Terms:

Monday, 17 February 2020

Field-Effect Transistor- Symbol, Types, Classification, Advantages, Disadvantages, applications.

Field-Effect Transistor(FET)


  • The basic principle of the field-effect transistor (FET) has been known since J. E. Lilienfeld’s patent of 1925. 
  • The theoretical description of a FET made by Shockley in 1952 proved the way for development of a classic electronic device which provides the designer the means to accomplish nearly every circuit function. 
  • At one time, the field-effect transistor was known as a “unipolar” transistor. The term refers to the fact that current is transported by carriers of one polarity (majority), whereas in the conventional bipolar transistor carriers of both polarities (majority and minority) are involved. 
  • This Application Note provides an insight into the nature of the FET, and touches briefly on its basic characteristics, terminology, parameters, and typical applications.
  • The Field Effect Transistor or FET is a electronic device. 
  • Its output impedance is high which requires for many circuits.

General Structure:
Field-Effect Transistor
Field-Effect Transistor

Typical Field Effect Transistor

  • The Field Effect Transistor is a three terminal unipolar semiconductor device that has very similar characteristics to those of their Bipolar Transistor counterparts ie, high efficiency, instant operation, robust and cheap and can be used in most electronic circuit applications to replace their equivalent bipolar junction transistors (BJT) cousins.
  • Field effect transistors can be made smaller and along with their low power consumption and power dissipation makes them ideal for use in integrated circuits such as the CMOS range of digital logic chips.
  • In FET's  there are also two basic classifications of Field Effect Transistor, called the N-channel FET and the P-channel FET.
  • The field effect transistor is a three terminal device that is constructed with no PN-junctions within the main current carrying path between the Drain and the Source terminals, which correspond in function to the Collector and the Emitter respectively of the bipolar transistor. The current path between these two terminals is called the "channel" which may be made of either a P-type or an N-type semiconductor material. 
  • The control of current flowing in this channel is achieved by varying the voltage applied to the Gate. As their name implies, Bipolar Transistors are "Bipolar" devices because they operate with both types of charge carriers, Holes and Electrons. The Field Effect Transistor on the other hand is a "Unipolar" device that depends only on the conduction of electrons (N-channel) or holes (P-channel).
  • The Field Effect Transistor has one major advantage over its standard bipolar transistor cousins, in that their input impedance, ( Rin ) is very high, (thousands of Ohms), while the BJT is comparatively low. This very high input impedance makes them very sensitive to input voltage signals, but the price of this high sensitivity also means that they can be easily damaged by static electricity. 
  • There are two main types of field effect transistor, the Junction Field Effect Transistor or JFET and the Insulated-gate Field Effect Transistor or IGFET), which is more commonly known as the standard Metal Oxide Semiconductor Field Effect Transistoror MOSFET for short.

Symbol Of FET

Symbol  Of FET
Symbol  Of FET

Types Of  FET-

There are main two types of FET are following.

1.Junction Field Effect Transistor or JFET and
2.Insulated-gate Field Effect Transistor or IGFET)

Classification Of FET

Classification Of FET

Classification Of FET

Advantages of FET

  • Current flow is majority carriers only.
  • Immune to radiation.
  • High input resistance.
  • Less noisy.
  • No offset voltages at zero drain current.
  • High thermal stability.

Disadvantages of FET

  • Smaller gain bandwidth compared to BJT.


  • In Amplifiers                                                                              
  • Switches
  • Voltage-Controlled Resistors
  • Mixers
  • Oscillators
  • Current Limiters

Popular Posts