B Tech Notes First Year (Iqbalkalmati.blogspot.com)

8/9/2019 B Tech Notes First Year (Iqbalkalmati.blogspot.com)

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BASIC ELECTRICAL ENGINEERING

Short Answer Questions

1. What is meant by charge?Charge is an electrical property of the atomic particles which matter consists. The charge of anelectron is so small. Charge in motion represents current. The unit of charge is coulomb.

2. What is meant by Current?

The flow of free electrons in a conductor is called current. Unit is ampere (A).I = Q/t

3. What is meant by Voltage?

The poterntial difference between two points is called as voltage. Unit is Vol

ts (V).V=W/Q ,

W=work done in joules &Q = charge in coulombs

4. State Ohm’s Law.

The potential difference across any two ends of a conductor is directly proportional to the currentflowing between the two ends provided the temperature of the conductor remains constant.

5. State Kirchoff’s Voltage Law KVL states that the algebraic sum of voltages in a closed path is zero.

6. State Kirchoff’s current Law.

KCL states that the algebraic sum of currents in a node is zero.

7. Give notes on Nodal Analysis.• KCL is used.

• No: of equations = n-1, n=no: of nodes

8. Give notes on Mesh Analysis.• KVL is used

• Here mesh currents are found.

9. Give short notes on resistor.It is a property of a substance which opposes the flow of electrons. It is denoted by R and its unit

is Ohm ( )

10. Distinguish between a Branch and a node of a circuit.A pair of network which connects the various points of the network is called branch . A point at

which two or more elements are joined together is called node.

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11. Distinguish between a mesh and a loop of a circuit.A mesh is a loop that does not contain other loops. All meshes are loop, but all loops are not

meshes. A loop is any closed path of branches

12. Write down the formula for a star connected network is converted into a delta

network?RA=( R1 R2)/( R1 +R2+ R3)RB=( R1 R3)/( R1 +R2+ R3)

RC=( R2 R3)/( R1 +R2+ R3)

13. Write down the formula for a delta connected network is converted into a star

network?

R1=( RARB+RBRC+RCRA)/RC

R2=( RARB+RBRC+RCRA)/RB R3=( RARB+RBRC+RCRA)/RA

14. Define line currents and phase currents?• The currents flowing in the lines are called as line currents

• The currents flowing through phase are called phase currents

15. Define line voltage and phase voltage?The voltage across one phase and neutral is called line voltage & the voltage between two lines

is called phase voltage

16. Give the phase value & Line value of a star connected system.

17. Give the phase value and line valued of a delta connected system.

18. What is the power equation for a star connected system?

19. What is the power equation for a delta connected system?

20. What is meant by Real power?Real power means the useful power transfer from source to load. Unit is watts.

21. What is meant by apparent power?

Apparent power is the product of voltage and current and it is not true power. Unit is VA



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22. What is reactive power?

If we consider the circuit as purely inductive the output power is reactive power. Its unit is VAR

23. Define Instrument.

Instrument is defined as a device for determining the value or magnitude of a quantity orvariable.

24. Mention the two main differences between an ammeter and a voltmeter.Ammeter Voltmeter

It is a current measuring device It is a voltage measuring deviceAlways connected in series with circuit Always connected in parallel with circuit

The resistance is very small The resistance is very high

25. Give short notes on resistor.It is a property of a substance which opposes the flow of electrons. It is denoted by R and its unit

is Ohm ( )

26. What is control system?A system consists of a number of components connected together to perform a specific function .

In a system when the output quantity is controlled by varying the input quantity then the systemis called control system.

27. What are the two major types of control system?

The two major types of control system are open loop and closed loop

28. .Define open loop control system.The control system in which the output quantity has no effect upon the input quantity are called

open loop control system. This means that the output is not feedback to the input for correction.

29 .Define closed loop control system.The control system in which the output has an effect upon the input quantity so as to maintain the

desired output value are called closed loop control system

30. Mention the errors in Moving iron instruments.• Hysteresis error

• Temperature error • Stray magnetic field error

• Frequency error • Eddy current error

31. Mention any two precautions to be taken while using an Ammeter.

• It should never be connected across any source.

• The polarity must be observed correctly.

• First use the highest range and then decrease the voltage range until the sufficient deflection isobtained.



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32. Give some applications of DC motor.Shunt : driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans,

reciprocating pumps

Series : electric locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyors

Compound : elevators, air compressors, rolling mills, heavy planners

33. Define slip.S = (Ns – Nr)/Ns

Where,Ns = synchronous speed in rpm.

Nr = rotor speed in rpmS = Slip

34. Define synchronous speed.It is given by Ns = 120f / p rpm.Where

Ns = synchronous speed,p = no. of stator poles,

f = supply frequency in Hz

35. Why a single phase induction motor does not self start?When a single phase supply is fed to the single phase induction motor. Its stator winding

produces a flux which only alternates along one space axis. It is not a synchronously revolvingfield, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.

36. Is Induction motor runs with synchronous speed or not.

Induction motor never runs with synchronous speed. It will stop if it tries to achieve synchronousspeed.

37. Define Form factor and Crest factor.

Form factor= RMS valueAverage Value

Crest(peak) factor=Maximum ValueRMS value

38.Which type of instrument is called as universal instrument?

The moving iron instrument are known as universal instruments, because these instruments canbe used for AC and DC.

39. What are the applications of MI instruments?

i) Used as multi-range ammeters and voltmeters.ii) Used as in expensive indicators such as charging and discharging current indicators in

automobiles.iii)Extensively used in industries for measurement of AC voltage and current where errors of the

order of 5% to 10% are acceptable.



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40. What is meant by eddy current damping?

When the conductor moves in a magnetic field an emf is induced in it and if a closed path isprovided ,a current flows known as eddy current. This current intersect with the magnetic field to

produce an electromagnetic torque , which opposes the deflecting torque.

41. .How is electrical power measured?i) Using Voltmeter-ammeter method for DC circuits.

ii)Using Watt meters for AC circuits.

42. .What do you mean by compensation coil in a wattmeter?By connecting a compensating coil in series with a pressure coil ,The error caused by the

pressure coil flowing in the current coil can be neutralized.

43. What are the three types of power used in an a.c circuit?i) Real power or active power P=EI cos

ii) Reactive power Q=EI siniii) Apparent power,S=EI

44. Define average value.

The average value of an alternating current is that value of steady direct current which transfersthe same charge as the alternating current flowing for the same time.

45. Define RMS value.

The effective value of an alternating current is that value of steady ,direct current which producesthe same heat as that produced by the alternating current when passed which produces the same

heat as that produced by the alternating current when passed through the same resistance for thesame interval of time.

46. Define reactive power.

The power consumed by a pure reactance (XL or Xc ) in a a.c circuit is called reactive power. Theunit is VAR. Q=EIsin .

47. What is the basic principle of a dc generator?

Basic principle of a dc generator is Faraday’s law of electromagnetic induction.That is whenevera conductor is moved in a magnetic field dynamically induced emf is produced in that conductor.

48 .What is the purpose of interpoles in modern d.c machine?

In modern d.c machines commutating poles or interpoles are provided to improve commutation.

49. What is the use of commutator and brush in a d.c machine?The commutator converts the alternating emf into unidirectional or direct emf. The brushes are

mainly used to collect current from the commutator.



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50. What is a d.c series motor?In a d.c series motor,the field winding is connected in series with the armature.The field winding

should have less number of turns of thick wire.

51. Why a series motor cannot be started without any load?

Series motor cannot be started without any load because under no load condition the startingtorque is less and motor rotates at dangerous speed and may be damaged.

52. What is meant by transformer?The transformer is a static piece of apparatus by means of which electricalenergy is transformed

from one circuit to another with desired change in voltage and current , without any change inthe frequency.It works on the principle of mutual induction.

53. What are the different types of single phase motor?i)Single phase induction motorii)Single phase synchronous motor.

iii)Single phase series motor

54. What are the two types of rotors of an induction motor?i) Squirrel cage rotor

ii)Slip ring or wound rotor

5 Marks Questions

1. A 3 φ 4 pole 50 hz induction motor runs at 1460 r.p.m. find its % of slip. Solution

N s = 120f/p= 120*50/4

= 1500r.p.m.

Running speed of motorn= 1460r.p.m.Slip S=( N s – N)/ N s*100

=(1500-1460) x 100 / 1500= 2.667%

2. Explain the working principle of Transformer.

A Transformer is a device that transfers electrical energy from one circuit to another by

electromagnetic induction (transformer action).

The electrical energy is always transferred without a change in frequency, but mayinvolve changes in magnitudes of voltage and current. Because a transformer works on

the principle of electromagnetic induction, it must be used with an input source voltagethat varies in amplitude.

There are many types of power that fit this description; for ease of explanation andunderstanding, transformer action will be explained using an ac voltage as the input

source. The amount of power used by the load of an electrical circuit is equal to the current in the

load times the voltage across the load, or P = EI. If, for example, the load in an electrical



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circuit requires an input of 2 amperes at 10 volts (20 watts) and the source is capable ofdelivering only 1 ampere at 20 volts, the circuit could not normally be used with this

particular source.

However, if a transformer is connected between the source and the load, the voltage can

be decreased (stepped down) to 10 volts and the current increased (stepped up) to 2

amperes. Notice in the above case that the power remains the same. That is, 20 volts times 1

ampere equals the same power as 10 volts times 2 amperes.

A Transformer consists of the following parts• A primary coil or winding.

• A secondary coil or winding.• A core that supports the coils or windings.

The primary winding is connected to a 50 hertz ac voltage source. The magnetic field(flux) builds up (expands) and collapses (contracts) about the primary winding.

The expanding and contracting magnetic field around the primary winding cuts thesecondary winding and induces an alternating voltage into the winding.

This voltage causes alternating current to flow through the load.

The voltage may be stepped up or down depending on the design of the primary and

secondary windings.

3. Calculate the amount of resistance (R) in a circuit, given values of voltage (E) and

current (I):

The amount of resistance (R) offered by the lamp


M N t Vi it i b lk l ti bl t


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4. Calculate the amount of voltage supplied by a battery, given values of current (I) and

resistance (R):

the amount of voltage provided by the battery

5. Calculate the electric power in the given circuit and discuss the effect of increasing thebattery voltage.

The formula for determining the power in an electric circuit: by multiplying the voltage in"volts" by the current in "amps" we arrive at an answer in "watts." Let's apply this to the given

circuit.

In the above circuit, we know we have a battery voltage of 18 volts and a lamp resistance of 3 _.Using Ohm's Law to determine current, we get:

Now that we know the current, we can take that value and multiply it by the voltage to determine

power:


M N t Vi it i b lk l ti bl t


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6. What is meant by DEFLECTING TORQUE ?

The deflecting torque is produced by making use of one of the magnetic, chemical,electrostatic and electromagnetic induction effects of current or voltage and causes the

moving system of the instrument to move from its zero position when the instrument is

connected in an electrical circuit to measure the electrical quantity. The method of producing this torque depend upon the type of instrument. In attracting the

type of instrument, this torque to equal toTd = 1/2 I

2 dL/dθ

Whereas in Pmmc instrumentsTd = Bilur

Where B - magnetic densityi - current flowing

l - length of coilu - number of turn

r - radius of coil

7. Find the voltage across each resistors in the following circuit.


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8. The effective resistance of two resistors connected in series is 100 . When connected in

parallel, then effective value in 24 ohm’s. Determine the value of two resistorsSeries R1+R2=100 => R2 =100 - R1

R1R2/R1+R2 = 24

R1R2/100 = 24R1R2 =2400R1 (100-R1) = 2400

100 R1-R12-2400 = 0

R12-100 R1 + 2400 = 0

(R1-60)(R1-40) = 0Therefore R1 = 60; R1 = 40

When R1 = 60 ; R2 = 100 – 60 = 40

When R1 = 40 ; R2 = 100 - 40 = 60

9. Find the Req between two points A & B.

1/Req = ½+1/3+1/3 = 1.17 (Req = 1/1.17= 0.8547)

1/Req = 2+.85+4Req = 7.2

10. Explain about Kirchoffs voltage and current laws. Kirchhoff’s Current Law The sum of current flowing towards a function is equal to the current flowing away from it.

Consider a function formed by 6 conductors. The current in these conductors are i1, i2, .i6.Some

of these currents are flowing towards a 8 other’s away from A

According to Kirchhoff’s Law,

i1+i4+i5+i6 = i2+i3 (Flowing towards) (Flowing away from A)




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Kirchhoff’s Voltage Law (II Law)

In a closed circuit, the sum of the potential drops is equal to thesum of the potential resistance

ABCDA forms a closed circuit.From A B, We have a potential drop of IR1.

From D A, We have a potential drop of V.Sum of potential drops = IR1+IR2+IR3

Potential rise from D A =VIR1+IR2+IR3 = V

11. Explain the principle of operation of DC Motor.

When a current passes through a conductor, lines of magnetic force (flux) are generatedaround the conductor.

The direction of the flux is dependent on the direction of the current flow .

In terms of conventional current flow (positive to negative) then, using your right handpoint your thumb in the direction of the current flow and your fingers will wrap around

the conductor in the same direction of the flux lines.

On the side of the conductor where the lines of flux oppose each other, the magnetic fieldwill be made weaker.




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On the side of the conductor where the lines of flux are not opposing each other, themagnetic field will be made stronger.

Because of the strong field on one side of the conductor and a weak field or, the otherside, the conductor will be pushed into the weaker field.

The armature is connected to the commutator which rides along the brushes which are

connected to a DC power source. The current from the DC power source flows from the positive lead, through the brush

labeled A1 through one commutator section, through the armature coil, through the othercommutator section, through the brush labeled A2 and back to the negative lead .

This current will generate lines of flux around the armature and affect the lines of flux inthe air gap.

On the side of the coil where the lines of flux oppose each other, the magnetic field willbe made weaker.

On the side of the coil where the lines of flux are riot opposing each other, the magneticfield is made stronger.

Because of the strong field on one side of the coil and the weak field on the other side,the coil will be pushed into the weaker field and, because the armature coil is free torotate, it will rotate.

The torque available at the motor shaft (turning effort) is determined by the magneticforce (flux) acting on the armature coil and the distance from the renter of rotation that

force is. The flux is determined by the current flowing through the armature coil and strength of

the field magnets.

12. Explain the working principle of three phase induction motor.

In three phase induction motor, the magnetic field generated by the stator rotates in the accase.

Three electrical phases are introduced through terminals, each phase energizing an

individual field pole.

When each phase reaches its maximum current, the magnetic field at that pole reaches a

maximum value.

As the current decreases, so does the magnetic field. Since each phase reaches itsmaximum at a different time within a cycle of the current, that field pole whose magnetic

field is largest is constantly changing between the three poles, with the effect that themagnetic field seen by the rotor is rotating.

The speed of rotation of the magnetic field, known as the synchronous speed, depends onthe frequency of the power supply and the number of poles produced by the statorwinding.

For a standard 60 Hz supply, as used in the United States, the maximum synchronousspeed is 3,600 rpm.

In the three phase induction motor, the windings on the rotor are not connected to apower supply, but are essentially short circuits.

The most common type of rotor winding, the squirrel cage winding, bears a strong

resemblance to the running wheel used in cages for pet gerbils.




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When the motor is initially switched on and the rotor is stationary, the rotor conductorsexperience a changing magnetic field sweeping by at the synchronous speed.

From Faraday's law, this situation results in the induction of currents round the rotorwindings; the magnitude of this current depends on the impedance of the rotor windings.

Since the conditions for motor action are now fulfilled, that is, current carryingconductors are found in a magnetic field, the rotor experiences a torque and starts to turn.

The rotor can never rotate at the synchronous speed because there would be no relativemotion between the magnetic field and the rotor windings and no current could be

induced.

The induction motor has a high starting torque.

13. Explain the working principle of single phase induction motor.

Single phase induction motor has only one stator winding (main winding) and operateswith a single-phase power supply.

In all single-phase induction motors, the rotor is the squirrel cage type.

The single-phase induction motor is not self-starting.

When the motor is connected to a single-phase power supply, the main winding carries an

alternating current.

This current produces a pulsating magnetic field.

Due to induction, the rotor is energized. As the main magnetic field is pulsating, thetorque necessary for the motor rotation is not generated.

This will cause the rotor to vibrate, but not to rotate.

Hence, the single phase induction motor is required to have a starting mechanism that canprovide the starting kick for the motor to rotate.




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The starting mechanism of the single-phase induction motor is mainly an additional statorwinding (start/ auxiliary winding) as shown in Figure.

The start winding can have a series capacitor and/or a centrifugal switch.

When the supply voltage is applied, current in the main winding lags the supply voltagedue to the main winding impedance.

At the same time, current in the start winding leads/lags the supply voltage depending onthe starting mechanism impedance.

Interaction between magnetic fields generated by the main winding and the startingmechanism generates a resultant magnetic field rotating in one direction.

The motor starts rotating in the direction of the resultant magnetic field.

Once the motor reaches about 75% of its rated speed, a centrifugal switch disconnects thestart winding.

From this point on, the single-phase motor can maintain sufficient torque to operate on itsown.

14. Explain the working principle of single phase Energy Meter.

An electric meter or energy meter is a device that measures the amount of electricalenergy supplied to or produced by a residence, business or machine.

The most common type is a kilowatt hour meter.

When used in electricity retailing, the utilities record the values measured by these metersto generate an invoice for the electricity.

They may also record other variables including the time when the electricity was used.

Modern electricity meters operate by continuously measuring the instantaneous voltage(volts) and current (amperes) and finding the product of these to give instantaneous

electrical power (watts) which is then integrated against time to give energy used (joules,kilowatt-hours etc).

The meters fall into two basic categories, electromechanical and electronic.

The energy meter operates by counting the revolutions of an aluminium disc which ismade to rotate at a speed proportional to the power.

The number of revolutions is thus proportional to the energy usage.

It consumes a small amount of power, typically around 2 watts.

The metallic disc is acted upon by two coils.




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One coil is connected in such a way that it produces a magnetic flux in proportion to thevoltage and the other produces a magnetic flux in proportion to the current.

The field of the voltage coil is delayed by 90 degrees using a lag coil.

This produces eddy currents in the disc and the effect is such that a force is exerted on thedisc in proportion to the product of the instantaneous current and voltage.

A permanent magnet exerts an opposing force proportional to the speed of rotation of thedisc - this act as a brake which causes the disc to stop spinning when power stops being

drawn rather than allowing it to spin faster and faster.

This causes the disc to rotate at a speed proportional to the power being used.

The type of meter described above is used on a single-phase AC supply.

Different phase configurations u se additional voltage and current coils.

The aluminium disc is supported by a spindle which has a worm gear which drives the

register. The register is a series of dials which record the amount of energyused. The dials may be of the cyclometer type, an odometer-like display that is easy to read

where for each dial a single digit is shown through a window in the face of the meter, or

of the pointer type where a pointer indicates each digit.

It should be noted that with the dial pointer type, adjacent pointers generally rotate inopposite directions due to the gearing mechanism.

10 Marks Questions

1. DETERMINE THE EQUIVALENT RESISTANCE BETWEEN TERMINALS A & B

SOLUTION:

50 & 12.5 ARE PARALLEL

50*12.5 / 50+12.5 = 10STEP – I

q g p



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STEP – II

20*30/20+30 = 12

STEP – III

60*20/60+20 = 15

STEP – I V

STEP – V

RAB = 50

q g p



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2. Find the mesh currents in the following network

Solution:

The solution of -1 amp for I2 means that our initially assumed direction of current was incorrect.

In actuality, I2 is flowing in a counter-clockwise direction at a value of (positive) 1 amp:



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3. Explain the working of a moving iron type instruments.

These instruments are widely used in laboratories and switch board at commercialfrequencies because these are cheaper in cost, robust in construction and can be

manufactured with required accuracy.

These are generally of two types:-1. The attraction type.

2. The repulsion type.

The attraction type instrument operate on the principle of attraction of a single piece ofsoft iron into a magnetic field and repulsion type instrument operate on the principle of

repulsion of two adjacent iron pieces magnified by the same magnetic field.

Repulsion type instrument are more sensitive than attraction type instrument as inrepulsion type instrument large separating torque is developed by having two iron

element positional class together inside the field coil where the magnetizing effect ismaximum.

In both type of these instruments, the current under measurement is passed through a coilof wire.

This current carrying coil set up the necessary field depending on the magnitude of thecurrent to be measured.

The coil may be of a few turns of very heavy conductor or of many turns of fine wire.

The instrument to be used as an ammeter is provided with a coil of few turns of thickwire in order to have low resistance and carry large current and that to be used as a

voltammeter is provided with a coil of large number of turns of wire in order to have highresistance and draw as small current as possible.

4. Derive the expression for torque produced in moving iron instrument.

Let L be the self inductance corresponding to a total angular deflection of q radians and changein inductance be dL corresponding to small change in deflection angel dq due to small change in

current. The change in energy of magnetic field,dw = Td dθ

Since change in energy dE = workdone, dwTd dθ = ½ I

2dL

Td = ½ I2dL/dθ

where I is in amperes, L is in Henry and θ is in Radians.

Thus torque is proportional to the square of the instrument current and to the rate of change ofinductance with deflection.

5. An energy meter revolves 10 revolutions of disc for unit of energy. Find the number of

revolutions made by it during an hour when connected across when connected 20A at 210V

and 0.8 power factor leading. If energy meter revolves 350 revolutions, find the % error.

Answer.

Energy consumed in one hour = VI cos φ / 1000 = 210 x 20 x 0.8 / 1000= 3.360 kwh.

The number of revolution the meter should make it is correct :



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=3.360 x registration const in revolution per kwh= 3.360 x 100

= 336

Number of revolution actually made = 350

% error = (350-336) x 100 / 350% error = 0.1466 %

6. Explain how following torque are produced in pmmc instrument and attracted type

moving iron instruments

1. Deflecting torque

2. Control torque

3. Damping torque

1. DEFLECTING TORQUE:- The deflecting torque is produced by making use of one of themagnetic, chemical, electrostatic and electromagnetic induction effects of current or voltage and

causes the moving system of the instrument to move from its zero position when the instrumentis connected in an electrical circuit to measure the electrical quantity. The method of producing

this torque depend upon the type of instrument. In attracting the type of instrument, this torque toequal to

Td = 1/2 I2 dL/dθ

Whereas in Pmmc instrumentsTd = BilurWhere B - magnetic density

i - current flowingl - length of coil

u - number of turnr - radius of coil

2. CONTROLLING TORQUE:- The magnitude of the movement to the moving system would

be somewhat indefinite under the influence of deflecting torque unless some controlling torqueexist. This torque opposes the deflecting torque and increases with increase in deflection of the

moving system without controlling system the irrespective magnitude of current and moreover,once deflected it would not return to its zero position on removing the current. In attraction type

instrument it is produced by spring control and in PMMC too it would be produced by springcontrol.

3. DAMPING TORQUE:- This torque is also necessary to avoid oscillation of the moving

system about it's final deflected position owing to the inertia of the moving parts and to bring themoving system to rest in it's final deflected position quickly.


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8. Discuss the construction and working of an electro-dynamic wattmeter with the help of

diagram?

Answer.

This type of instrument is similar in design and principle to the dynamometer type ammeter andvoltammeter.

WORKING AND CONSTRUCTION:-

When the instrument of this type is used as wattmeter, the fixed coil which is divided intotwo equal portions in order to provide them uniform field , is employed as current coil

and moving coil is used as pressure coil.

The fixed coil which is divided into two equal portion in order to provide them uniformfield, is employed as current coil and the moving coil is used as pressure coil, i.e the fixed

coil carries the current proportional to the voltage across the circuit.

A high non inductive resistance is connected in series with the moving coil in order to

limit current. The magnetic field of the fixed and moving coil react on one another causing the moving

coil to turn about it's axis.

The movement is controlled by hair springs which also leads the current into and out ofthe moving element.

Damping is provided by light aluminium moving in an air dash pot.

The pointer is fixed to the moving coil spindle and moves over a suitable calibrated scale.

THEORY:-Let us be the supply voltage, i the load current and R the load resistance of the moving coil

circuit.Current through fixed coil, if = i

Current through moving coil, im = V/Rdeflecting torque, Td ∝ if im ∝ V/R

For a DC circuit the deflecting torque is thus proportional to the power and for any circuit with

fluctuating torque. The instantaneous to the instantaneous power.



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9. Compare merits and demerits of moving iron type instruments and dynamometer type

instruments. Which one is superior why?

Answer.

1. TORQUE HEIGHT RATIO:- Dynamometer type instruments have equal small torqueheight ratio.

2. FRICTION ERROR:- Dynamometer type instruments have considerable friction error.

3. FRICTION LOSS:- Owing to heavy moving system, dynamometer type instruments havemore friction losses.

4. COST AND SENSITIVITY TO OVERLOAD:- As a result of measures to reduce thefrictional error, the dynamometer type instruments are more sensitive to overloads and

mechanical impacts is in comparison to moving iron type instruments.5. SENSTIVITY:- The sensitivity of dynamometer instrument is typically very poor due to poor

deflecting torque.6. POWER CONSUMPTION:- Dynamometer type instrument have comparatively higher

power consumption.7. EFFECT OF STRAY MAGNETIC FIELD:- There is no effect of stray magnetic field on

moving iron type while dynamometer type are most sensitive towards it.8. HYSTERISIS AND EDDY CURRENT ERRORS:- Dynamometer type instruments are free

from these errors while moving iron have these errors.9. EFFECT OF WAVE FORM:- Dynamometer type instruments are very useful for accurate

measurement of runs voltage while frequency change serious errors in AC measurement inmoving iron type instruments.

10. CALIBRATION:- Dynamometer type instruments have same calibration for AC and DCmeasurements while moving iron type have a difference between AC and DC calibration.

10 Wh h t i ll d lt t d t ? A i il i t t h



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10. Why shunt is usually used voltmeter and ammeter? A moving coil instrument has a

resistance of 5 _ and gives full deflection of 100mv. Show how the instrument may be used

to measure:-

1. voltage upto 50V

2. current upto 10A

Answer.Shunt is usually used in voltmeter and ammeter to extend the range of voltmeter and ammeters.Rm = 5

Vm = 100mvIm = Vm /Rm = 100mv/5 = 20mA

1. For measuring voltage upto 50V.

Series resistance is used with the instrument whose resistance is

R = V/Im - Rm = 50/(20 x 10-3

) - 5R = 2.5 x 10

-3 - 5

R = 2495

2. Such resistance of resistance Rf is used to be connected

Rf = Rm /[I/Im - 1]Rf = 5/[10/20 x 10

-3 -1] = 5 x 2/998

Rf = 0.01002004

11. Explain the principle of operation of attraction type moving iron instruments and

explain how the controlling and damping forces are obtained?

Answer.

The earliest and simplest form of attraction moving iron instruments uses a solenoid andmoving oval shaped soft iron pinoted eccentrically.

To this iron a pointer is attached so that it may deflect along with the moving iron over agraduate scale.

The iron is made of sheet metal specially shaped to give a scale as nearby uniform aspossible.

The moving iron is drawn into field of solenoid when current flows through it.

The movement of the iron always from weaker magnetic field outside the coil into thestronger field inside the coil regardless the direction of flow of current.

When the current to be measured is passed through the solenoid, a magnetic field is setup inside the solenoid, which in turn magnetises the iron.

Thus the iron is attached into the coil causing the spindle and the pointer to rotate.

So much instruments normally have spring control and pneumatic damping forces.


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13 Explain the method of temperature control in open loop and closed loop systems



25/811

13. Explain the method of temperature control in open loop and closed loop systems.

Temperature controllers are needed in any situation requiring a given temperature be kept

stable.

This can be in a situation where an object is required to be heated, cooled or both and toremain at the target temperature (set point), regardless of the changing environment

around it. There are two fundamental types of temperature control; open loop and closed loop

control.

Open loop is the most basic form and applies continuous heating/cooling with no regardfor the actual temperature output.

It is analogous to the internal heating system in a car. On a cold day, you may need toturn the heat on to full to warm the car to 75°.

However, during warmer weather, the same setting would leave the inside of the car

much warmer than the desired 75°.

Closed loop control is far more sophisticated than open loop.

In a closed loop application, the output temperature is constantly measured and adjusted

to maintain a constant output at the desired temperature. Closed loop control is always conscious of the output signal and will feed this back into

the control process.

Closed loop control is analogous to a car with internal climate control. If you set the cartemperature to 75°, the climate control will automatically adjust the heating (during colddays) or cooling (during warm days) as required to maintain the target temperature of 5°.

A temperature controller is a device used to hold a desired temperature at a specified

value.

The simplest example of a temperature controller is a common thermostat found inhomes.

For instance, a hot water heater uses a thermostat to control the temperature of the waterand maintain it at a certain commanded temperature.

Temperature controllers are also used in ovens.

When a temperature is set for an oven, a controller monitors the actual temperature inside

of the oven.

If it falls below the set temperature, it sends a signal to activate the heater to raise thetemperature back to the set point.

Thermostats are also used in refrigerators So if the temperature gets too high a controller



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Thermostats are also used in refrigerators. So if the temperature gets too high, a controllerinitiates an action to bring the temperature down.

14. Explain about open loop and closed loop control system.

Figure 1 shows an open loop system. A computed force is applied to the system which isexpected to respond based on the specifications.

If the system fails to respond correctly (because your estimates were off) or anunanticipated disturbance acted on it, then there is no way to correct the course.

On the other hand, figure 2 shows a feed-back system.

The response C(s) is measured using the sensor H(s) and the resultant is compared withthe input R(s).

The resultant difference (error) is acted upon by the controller which works on theactuator.

The actuator then applies the required force on the system.

The closed loop thus contains the sensor dynamics, the controller dynamics, the actuatordynamics in addition to the system we are interested in.

It should be noted that all measurements have to be done or converted if necessary intoone unit so that comparison with the target signal is possible.

Usually, measurements result in currents and voltages.

Hence, this conversion from a mechanical input to an electrical output is also included inthe sensor, controller and actuator dynamics.

In designing the full control system the dynamics of all the components need to be

accounted for.

If the controller is very slow compared to the system, it will not send the right input at theright time.

In this class, we will assume perfect sensor and actuator dynamics, i.e., what goes into the

sensor (it is commonly denoted by H(s)) and the actuator comes out unmodifiedinstantaneously.So we replace them with unity transfer functions.

15. Explain the relation between voltage, current and resistance



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p g ,

An electric circuit is formed when a conductive path is created to allow free electrons to

continuously move.

This continuous movement of free electrons through the conductors of a circuit is called acurrent , and it is often referred to in terms of "flow," just like the flow of a liquid through

a hollow pipe. The force motivating electrons to "flow" in a circuit is called voltage. Voltage is a specific measure of potential energy that is always relative between two

points.

When we speak of a certain amount of voltage being present in a circuit, we are referringto the measurement of how much potential energy exists to move electrons from oneparticular point in that circuit to another particular point.

Without reference to two particular points, the term "voltage" has no meaning.

Free electrons tend to move through conductors with some degree of friction, or

opposition to motion.

This opposition to motion is more properly called resistance.

The amount of current in a circuit depends on the amount of voltage available to motivatethe electrons, and also the amount of resistance in the circuit to oppose electron flow. Justlike voltage, resistance is a quantity relative between two points.

For this reason, the quantities of voltage and resistance are often stated as being"between" or "across" two points in a circuit.

To be able to make meaningful statements about these quantities in circuits, we need tobe able to describe their quantities in the same way that we might quantify mass,

temperature, volume, length, or any other kind of physical quantity.

For mass we might use the units of "kilogram" or "gram." For temperature we might usedegrees Fahrenheit or degrees Celsius.

Here are the standard units of measurement for electrical current, voltage, and resistance:

16. Explain the construction of DC machine with neat diagram.

A D.C. machine consists mainly of two part the stationary part called stator and therotating part called stator.

The stator consists of main poles used to produce magnetic flux ,commutating poles or

inter-poles in between the main poles to avoid sparking at the commutator but in the caseof small machines sometimes the interpoles are avoided and finally the frame or yoke

which forms the supporting structure of the machine.

The rotor consist of an armature a cylindrical metallic body or core with slots in it toplace armature windings or bars, a commutator and brush gears.

The magnetic flux path in a motor or generator is show below and it is called themagnetic structure of generator or motor.



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Cross sectional view of a DC Machine

Frame

Frame is the stationary part of a machine on which the main poles and commutator poles arebolted and it forms the supporting structure by connecting the frame to the bed plate. The ring

shaped body portion of the frame which makes the magnetic path for the magnetic fluxes fromthe main poles and interpoles is called Yoke.

Yoke

In early days Yoke was made up of cast iron but now it is replaced by cast steel. This is because

cast iron is saturated by a flux density of 0.8 Wb/sq.m where as saturation with cast iron steel isabout 1.5 Wb/sq.m.So for the same magnetic flux density the cross section area needed for caststeel is less than cast iron hence the weight of the machine too. If we use cast iron there may be

chances of blow holes in it while casting. so now rolled steels are developed and these haveconsistent magnetic and mechanical properties.

End Shields or Bearings

If the armature diameter does not exceed 35 to 45 cm then in addition to poles end shields or

frame head with bearing are attached to the frame. If the armature diameter is greater than 1mpedestral type bearings are mounted on the machine bed plate outside the frame. Thesebearings could be ball or roller type but generally plain pedestral bearings are employed. If the

diameter of the armature is large a brush holder yoke is generally fixed to the frame.

17. Explain the Working of dynamometer type wattmeter?



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The principle of operation of the electrodynamometer-type wattmeter is the same as that

for dynamo-electric machines.

The deflection torque is produced by the interaction of two magnetic fluxes.

One of the fluxes is produced by a fixed coil which carries a current proportional to the

load current and therefore called the current coil. The other flux is created by a movable coil which carries a current proportional to the

load voltage and thus called the voltage or potential coil.

A high non-inductive resistance is connected to the potential coil so that its current isalmost in phase with the load voltage.

The control torque is provided by a control spring.

In a dynamometer type wattmeter the fixed coil (current coil) is connected in series withthe load.

This coil is divided in to two parts and they are kept parallel to each other.

The coil is thick in cross section and has less number of turns.

The moving coil (pressure coil) is connected across the load. It is thin in cross - sectionand has hundreds of turns.

It has a non - inductive high resistance in series with it

The wattmeter is an electrodynamic instrument for measuring the electric power or thesupply rate of electrical energy of any given circuit.

The device consists of a pair of fixed coils, known as current coils, and a movable coilknown as the potential coil.

The current coils are connected in series with the circuit, while the potential coil isconnected in parallel.

Also, on analog wattmeters, the potential coil carries a needle that moves over a scale toindicate the measurement.

A current flowing through the current coil generates an electromagnetic field around thecoil.

The strength of this field is proportional to the line current and in phase with it.

The potential coil has, as a general rule, a high-value resistor connected in series with itto reduce the current that flows through it.

The result of this arrangement is that on a dc circuit, the deflection of the needle is



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proportional to both the current and the voltage, thus conforming to the equation

W=VA or P=EI.

On an ac circuit the deflection is proportional to the average instantaneous product ofvoltage and current, thus measuring true power, and possibly (depending on load

characteristics) showing a different reading to that obtained by simply multiplying thereadings showing on a stand-alone voltmeter and a stand-alone ammeter in the same

circuit.

The two circuits of a wattmeter are likely to be damaged by excessive current.

The ammeter and voltmeter are both vulnerable to overheating - in case of an overload,their pointers will be driven off scale - but in the wattmeter, either or even both the

current and potential circuits can overheat without the pointer approaching the end of thescale!

This is because the position of the pointer depends on the power factor, voltage andcurrent.

Thus, a circuit with a low power factor will give a low reading on the wattmeter, evenwhen both of its circuits are loaded to the maximum safety limit.

Therefore, a wattmeter is rated not only in watts, but also in volts and amperes.

18. Explain the construction of transformer with neat diagram.

A transformer is an electrical device used to convert AC power at a certain voltage levelto AC power at a different voltage, but at the same frequency.

The construction of a transformer includes a ferromagnetic core around which multiplecoils, or windings, of wire are wrapped.

The input line connects to the 'primary' coil, while the output lines connect to 'secondary'coils.

The alternating current in the primary coil induces an alternating magnetic flux that'flows' around the ferromagnetic core, changing direction during each electrical cycle.

The alternating flux in the core in turn induces an alternating current in each of thesecondary coils.

The voltage at each of the secondary coils is directly related to the primary voltage by theturns ratio, or the number of turns in the primary coil divided by the number turns in thesecondary coil.

For instance, if the primary coil consists of 100 turns and carries 480 volts and asecondary coil consists of 25 turns, the secondary voltage is then:

secondary voltage = (480 volts) * (25/100) = 120 volts

Two coils of wire (called windings) are wound on some type of core material.

In some cases the coils of wire are wound on a cylindrical or rectangular cardboard form.

In effect, the core material is air and the transformer is called an AIR-CORETRANSFORMER.

Transformers used at low frequencies, such as 50 hertz and 400 hertz, require a core oflow-reluctance magnetic material, usually iron.

This type of transformer is called an IRON-CORE TRANSFORMER.

Most power transformers are of the ironcore type.

The principle parts of a transformer and their functions are:

The CORE, which provides a path for the magnetic lines of flux. Th PRIMARY WINDING hi h i f th



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The PRIMARY WINDING, which receives energy from the ac source.

The SECONDARY WINDING, which receives energy from the primary winding anddelivers it to the load.

The ENCLOSURE, which protects the above components from dirt, moisture, andmechanical damage.

A soft-iron-core transformer is very useful where the transformer must be physicallysmall, yet efficient.

The iron-core transformer provides better power transfer than does the air-coretransformer.

A transformer whose core is constructed of laminated sheets of steel dissipates heatreadily; thus it provides for the efficient transfer of power.

The majority of transformers you will encounter in Navy equipment contain laminated

steel cores.

These steel laminations are insulated with a non conducting material, such as varnish, andthen formed into a core.

It takes about 50 such laminations to make a core an inch thick.

The purpose of the laminations is to reduce certain losses.

BASIC ELECTRONICS



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CONDUCTION IN SEMICONDUCTORS

The branch of engineering which deals with the flow of Electrons through vacuum, gas or semiconductor is called Electronics.

Electronics essentially deals with electronic devices and their utilization.

Atomic Structure

Atom is the basic building block of all the elements. It consists of the centralnucleus of positive charge around which small negatively charged particles called

electrons revolve in different paths or orbits.

An Electrostatic force of attraction between electrons and the nucleus holds upelectrons in different orbits.

Electrostatic force.

+

Centrifugal force.

Figure1.1. Atomic structure

Nucleus is the central part of an atom and contains protons and neutrons. A protonis positively charged particle, while the neutron has the same mass as the proton,

but has no charge. Therefore ,nucleus of an atom is positively charged.

atomic weight = no. of protons + no. of neutrons

An electron is a negatively charged particle having negligible mass. The chargeon an electron is equal but opposite to that on a proton. Also the number ofelectrons is equal to the number of protons in an atom under ordinary conditions.

Therefore an atom is neutral as a whole.

atomic number = no. of protons or electrons in an atom

The number of electrons in any orbit is given by 2n

2

where n is the number of theorbit.

For example I orbit contains 2x12

=2 electrons



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For example, I orbit contains 2x1 =2 electrons

II orbit contains 2x22 = 8 electrons

III orbit contains 2x32 = 18 electrons and so on

The last orbit cannot have more than 8 electrons.

The last but one orbit cannot have more than 18 electrons.

Positive and negative ions

Protons and electrons are equal in number hence if an atom loses an electron ithas lost negative charge therefore it becomes positively charged and is referred aspositive ion.

If an atom gains an electron it becomes negatively charged and is referred to asnegative ion.

Valence electrons

The electrons in the outermost orbit of an atom are known as valence electrons .

The outermost orbit can have a maximum of 8 electrons.

The valence electrons determine the physical and chemical properties of amaterial.

When the number of valence electrons of an atom is less than 4, the material isusually a metal and a conductor. Examples are sodium, magnesium and

aluminium, which have 1,2 and 3 valence electrons respectively.

When the number of valence electrons of an atom is more than 4, the material isusually a non-metal and an insulator. Examples are nitrogen, sulphur and neon,

which have 5,6 and 8 valence electrons respectively.

When the number of valence electrons of an atom is 4 the material has both metaland non-metal properties and is usually a semi-conductor. Examples are carbon,silicon and germanium.

Free electrons

The valence electrons of different material possess different energies. The greaterthe energy of a valence electron, the lesser it is bound to the nucleus.

In certain substances, particularly metals, the valence electrons possess so muchenergy that they are very loosely attached to the nucleus



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Conduction band

Forbidden gap

Valence band

energy that they are very loosely attached to the nucleus.

The loosely attached valence electrons move at random within the material andare called free electrons.

The valence electrons, which are loosely attached to the nucleus, are known as freeelectrons.

Energy bands

In case of a single isolated atom an electron in any orbit has definite energy.

When atoms are brought together as in solids, an atom is influenced by the forcesfrom other atoms. Hence an electron in any orbit can have a range of energiesrather than single energy. These range of energy levels are known as Energy

bands.

Within any material there are two distinct energy bands in which electrons mayexist viz Valence band and conduction band.

Energy level

Figure1.2 Energy level diagram

The range of energies possessed by valence electrons is called valence band .

The range of energies possessed by free electrons is called conduction band.

Valence band and conduction band are separated by an energy gap in which no

electrons normally exist this gap is called forbidden gap.

Electrons in conduction band are either escaped from their atoms (free electrons) or onlyweakly held to the nucleus. Thereby by the electrons in conduction band may be easily

moved around within the material by applying relatively small amount of energy. (either

by increasing the temperature or by focusing light on the material etc. ) This is the reasonwhy the conductivity of the material increases with increase in temperature.



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y y p

But much larger amount of energy must be applied in order to extract an electron from

the valence band because electrons in valence band are usually in the normal orbit arounda nucleus. For any given material, the forbidden gap may be large, small or non-existent.

Classification of materials based on Energy band theory

Based on the width of the forbidden gap, materials are broadly classified as conductors,

Insulators and semiconductors.

(a) Conductor (b) Insulator (c) Semiconductor

Conductors

Conductors are those substances, which allow electric current to pass throughthem.

Example: Copper, Al, salt solutions, etc.

In terms of energy bands, conductors are those substances in which there is noforbidden gap. Valence and conduction band overlap as shown in fig (a).

For this reason, very large number of electrons are available for conduction evenat extremely low temperatures. Thus, conduction is possible even by a very weak

electric field.

Insulators

Insulators are those substances, which do not allow electric current to passthrough them.Example: Rubber, glass, wood etc.

In terms of energy bands, insulators are those substances in which the forbiddengap is very large.

Conduction

band

overlap

Valence

band

Conduction

band

Valence

band

Conduction

band

Valence

band

ForbiddenGa E =1eV

Forbidden

Gap

EG =6eV

Thus valence and conduction band are widely separated as shown in fig (b).Therefore insulators do not conduct electricity even with the application of a large



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y pp g

electric field or by heating or at very high temperatures.

Semiconductors

Semiconductors are those substances whose conductivity lies in between that of aconductor and Insulator.Example: Silicon, germanium, Cealenium, Gallium, arsenide etc.

In terms of energy bands, semiconductors are those substances in which theforbidden gap is narrow.

Thus valence and conduction bands are moderately separated as shown in fig(C).

In semiconductors, the valence band is partially filled, the conduction band is alsopartially filled, and the energy gap between conduction band and valence band isnarrow.

Therefore, comparatively smaller electric field is required to push the electronsfrom valence band to conduction band . At low temperatures the valence band is

completely filled and conduction band is completely empty. Therefore, at verylow temperature a semi-conductor actually behaves as an insulator.

Conduction in solids

Conduction in any given material occurs when a voltage of suitable magnitude isapplied to it, which causes the charge carriers within the material to move in a

desired direction.

This may be due to electron motion or hole transfer or both.

Electron motion

Free electrons in the conduction band are moved under the influence of the appliedelectric field. Since electrons have negative charge they are repelled by the negative

terminal of the applied voltage and attracted towards the positive terminal.

Hole transfer

Hole transfer involves the movement of holes.

Holes may be thought of positive charged particles and as such they movethrough an electric field in a direction opposite to that of electrons.



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I I+ +

V V

(a) Conductor (b) Semiconductor

Flow of electrons Flow of electrons

Flow of current Flow of holes

Flow of current

In a good conductor (metal) as shown in fig (a) the current flow is due to freeelectrons only.

In a semiconductor as shown in fig (b). The current flow is due to both holes andelectrons moving in opposite directions.

The unit of electric current is Ampere (A) and since the flow of electric current isconstituted by the movement of electrons in conduction band and holes in valence

band, electrons and holes are referred as charge carriers.

Classification of semiconductors

Semiconductors are classified into two types.

a) Intrinsic semiconductors.

b)

Extrinsic semiconductors.

a) Intrinsic semiconductors

A semiconductor in an extremely pure form is known as Intrinsic

semiconductor.

Example: Silicon, germanium.

Both silicon and Germanium are tetravalent (having 4 valence electrons).

Each atom forms a covalent bond or electron pair bond with the electrons ofneighboring atom. The structure is shown below.

Silicon or Germanium

Valence electron



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Covalent bond

Figure1.3. Crystalline structure of Silicon (or Germanium)

At low temperature

At low temperature, all the valence electrons are tightly bounded the nucleushence no free electrons are available for conduction.

The semiconductor therefore behaves as an Insulator at absolute zerotemperature.

At room temperature

Free electron

Valence electron

Holes

Figure 1.4. Crystalline structure of Silicon (or Germanium) at room temperature

At room temperature, some of the valence electrons gain enough thermal energyto break up the covalent bonds.

This breaking up of covalent bonds sets the electrons free and are available for

conduction.

When an electron escapes from a covalent bond and becomes free electrons avacancy is created in a covalent bond as shown in figure above. Such a vacancy is

ll d H l It i iti h d d th i fl f l t i



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called Hole. It carries positive charge and moves under the influence of an electricfield in the direction of the electric field applied.

Numbers of holes are equal to the number of electrons since, a hole is nothing butan absence of electrons.

Extrinsic Semiconductor

When an impurity is added to an Intrinsic semiconductor its conductivity changes.

This process of adding impurity to a semiconductor is called Doping and the

impure semiconductor is called extrinsic semiconductor.

Depending on the type of impurity added, extrinsic semiconductors are furtherclassified as n-type and p-type semiconductor.

n-type semiconductor

free e-

Figure 1.5 n-type semiconductor

Fermi level

Si

Si

Si

Si

Si

As

Si Si Si

Conduction band

Forbidden gap

Valence band



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Figure1.6 Energy band diagram for n-type semiconductor

When a small current of Pentavalent impurity is added to a pure

semiconductor it is called as n-type semiconductor.

Addition of Pentavalent impurity provides a large number of free electrons in asemiconductor crystal.

Typical example for pentavalent impurities are Arsenic, Antimony andPhosphorus etc. Such impurities which produce n-type semiconductors are known

as Donor impurities because they donate or provide free electrons to thesemiconductor crystal.

To understand the formation of n-type semiconductor, consider a pure siliconcrystal with an impurity say arsenic added to it as shown in figure 1.5.

We know that a silicon atom has 4 valence electrons and Arsenic has 5 valenceelectrons. When Arsenic is added as impurity to silicon, the 4 valence electrons of

silicon make co-valent bond with 4 valence electrons of Arsenic.

The 5th Valence electrons finds no place in the covalent bond thus, it becomes freeand travels to the conduction band as shown in figure. Therefore, for each arsenic

atom added, one free electron will be available in the silicon crystal. Though eacharsenic atom provides one free electrons yet an extremely small amount of arsenic

impurity provides enough atoms to supply millions of free electrons.

Due to thermal energy, still hole election pairs are generated but the number of freeelectrons are very large in number when compared to holes. So in an n-type

semiconductor electrons are majority charge carriers and holes are minority chargecarriers . Since the current conduction is pre-dominantly by free electrons( -vely charges)

it is called as n-type semiconductor( n- means – ve).

p-type semiconductor

hole

Si

Si

Si

Si

Si

Ga



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Figure 1.7 p-type semiconductor

Fermi level

Figure 1.8 Energy band diagram for p-type semiconductor

When a small amount of trivalent impurity is added to a pure semiconductor

it is called p-type semiconductor.

The addition of trivalent impurity provides large number of holes in the

semiconductor crystals.

Example: Gallium, Indium or Boron etc. Such impurities which produce p-typesemiconductors are known as acceptor impurities because the holes created canaccept the electrons in the semi conductor crystal.

To understand the formation of p-type semiconductor, consider a pure silicon crystal with

an impurity say gallium added to it as shown in figure 1.7.

We know that silicon atom has 4 valence electrons and Gallium has 3 electrons.When Gallium is added as impurity to silicon, the 3 valence electrons of galliummake 3 covalent bonds with 3 valence electrons of silicon.

The 4th

valence electrons of silicon cannot make a covalent bond with that ofGallium because of short of one electron as shown above. This absence of

electron is called a hole. Therefore for each gallium atom added one hole iscreated, a small amount of Gallium provides millions of holes.

Conduction band

Forbidden gap

Valence band

Due to thermal energy, still hole-electron pairs are generated but the number of holes arevery large compared to the number of electrons. Therefore, in a p-type semiconductor

holes are majority carriers and electrons are minority carriers. Since the current



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holes are majority carriers and electrons are minority carriers. Since the currentconduction is predominantly by hole( + charges) it is called as p-type semiconductor( p

means +ve)

Drift and Diffusion current

The flow of current through a semiconductor material is normally referred to as one ofthe two types.

Drift current

If an electron is subjected to an electric field in free space it will accelerate in a

straight line form the – ve terminal to the + ve terminal of the applied voltage. However in the case of conductor or semiconductor at room temperature, a free

electrons under the influence of electric field will move towards the +ve terminal

of the applied voltage but will continuously collide with atoms all the ways asshown in figure 1.9.

Electron drift due to field

conduction

when electricfield is

present

+Conduction

Applied voltage when no electricfield is applied

Figure 1.9

Each time, when the electron strikes an atom, it rebounds in a random directionbut the presence of electric field doesnot stop the collisions and random motion.As a result the electrons drift in a direction of the applied electric field.

The current produced in this way is called as Drift current and it is the usual kindof current flow that occurs in a conductor.

Diffusion current

The directional movement of charge carriers due to their concentration gradient produces a component of current known as Diffusion current.

semiconductor

The mechanism of transport of charges in a semiconductor when no electric fieldis applied called diffusion. It is encountered only in semiconductors and is

normally absent in conductors.



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y

heavy concentration of less concentrationelectrons electrons

Diffusion current

Even distribution

Net diffusion current is zero

With no applied voltage if the number of charge carriers (either holes orelectrons) in one region of a semiconductor is less compared to the rest of the

region then there exist a concentration gradient.

Since the charge carriers are either all electrons or all holes they sine polarity ofcharge and thus there is a force of repulsion between them.

As a result, the carriers tend to move gradually or diffuse from the region ofhigher concentration to the region of lower concentration. This process is called

diffusion and electric current produced due to this process is called diffusion

current.

This process continues until all the carriers are evenly distributed through thematerial. Hence when there is no applied voltage, the net diffusion current will bezero.

Fermi-level

Fermi level indicates the level of energy in the forbidden gap.



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1. Fermi-level for an Intrinsic semiconductor

Energy level

EC

Ef

Forbidden gapEV

EC Conduction band energy level

EV Valence band energy level

Ef fermi level

We know that the Intrinsic semiconductor acts as an insulator at absolute zerotemperature because there are free electrons and holes available but as the

temperature increases electron hole pairs are generated and hence number ofelectrons will be equal to number of holes.

Therefore, the possibility of obtaining an electron in the conduction band will be

equal to the probability of obtaining a hole in the valence band.

If Ec is the lowest energy level of Conduction band and Ev is the highest energylevel of the valence band then the fermi level Ef is exactly at the center of these

two levels as shown above.

2. Fermi-level in a semiconductors having impurities (Extrinsic)

a) Fermi-level for n-type Semiconductor

Conduction band

Fermilevel

Valence band

Let a donar impurity be added to an Intrinsic semiconductor then the donarenergy level (ED) shown by the dotted lines is very close to conduction

band energy level (Ec).



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Therefore the unbonded valence electrons of the impurity atoms can veryeasily jump into the conduction band and become free electros thus, atroom temperature almost all the extra electrons of pentavalent impurity

will jump to the conduction band.

The donar energy level (ED) is just below conduction band level (Ec) asshown in figure1.10(a). Due to a large number of free electrons, theprobability of electrons occupying the energy level towards the conduction

band will be more hence, fermi level shifts towards the conduction band.

EC

EDmoves Ef upward

EV

ED Energy level of donar impurity

Figure 1.10 (a) Energy level diagram for n-type semiconductor

b)

Fermi-level for P-type semiconductor

Let an acceptor impurity be added to an Intrinsic semiconductor then the acceptorenergy level (Ea) shown by dotted lines is very close to the valence band shown

by dotted lines is very close to the valence band energy level (Ev).

Therefore the valence band electrons of the impurity atom can very easily jump

into the valence band thereby creating holes in the valence band.

Conduction band

Fermi level

Valence band

E

Conduction band



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EC

EfEA

EV

EA Energy level of acceptor impurity

Figure 1.11 (b) Energy level diagram for P-type semiconductor

The acceptor energy level (EA) is just above the valence band level as shown infigure 1.11 (b).

Due to large number of holes the probability of holes occupying the energy level

towards the valence band will be more and hence, the fermi level gets shiftedtowards the valence band.

HALL EFFECT

If a piece of metal or semiconductor carrying a current I is placed in a transverse

magnetic field B then an electric field E is induced in the direction perpendicular to both Iand B. This phenomenon is known as Hall effect.

Y(+ve)

Surface-2

+ + + + + + + +d

I VHw

X (+ve)

BSurface -1

Z (+ve)

Fermi level

Valence band

Hall effect is normally used to determine whether a semi-conductor is n-type or p-type.

To find whether the semiconductor is n-type or p-type



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i) In the figure. above, If I is in the +ve X direction and B is in the +ve Zdirection, then a force will be exerted on the charge carriers (holes and

electrons) in the – ve Y direction.

ii) This force is independent of whether the charge carriers are electrons or holes.Due to this force the charge carriers ( holes and electrons) will be forced

downward towards surface – 1 as shown.

iii) If the semiconductor is N-type, then electrons will be the charge carriers and

these electrons will accumulate on surface – 1 making that surface – velycharged with respect to surface – 2. Hence a potential called Hall voltageappears between the surfaces 1 and 2.

iv)

Similarly when surface – 1 is positively charged with respect to surface – 2,then the semiconductor is of P-type. In this way, by seeing the polarity of Hall

voltage we can determine whether the semiconductor is of P-type or N-type.

Applications of Hall effect

Hall effect is used to determine,

carrier concentration, conductivity and mobility.

The sign of the current carrying charge.

Charge density.

It is used as magnetic field meter.

Carrier lifetime (τ)

In a pure semiconductor, we know that number of holes are equal to the number of

electrons. Thermal agitation however, continues to produce new hole electron pairs whileother hole-electron pair disappear as a result of recombination.

On an average, a hole will exist for τp second and an electron will exist for τn secondbefore recombination. This time is called the carrier lifetime or Mean lifetime.

The average time an electron or hole can exist in the free state is called carrier lifetime.

SEMICONDUCTOR DIODE

Wh t i d t t i l i it bl j i d t t i d t th



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When a p-type semiconductor material is suitably joined to n-type semiconductor the

contact surface is called a p-n junction. The p-n junction is also called as semiconductordiode.

p n

(b)

(a) Depletion region

Fig. 2.0 (a) p-n junction Fig 2.0 (b) symbolic representation

The left side material is a p-type semiconductor having – ve acceptor ions and+vely charged holes. The right side material is n-type semiconductor having +ve

donor ions and free electrons.

Suppose the two pieces are suitably treated to form pn junction, then there is atendency for the free electrons from n-type to diffuse over to the p-side and holesfrom p-type to the n-side . This process is called diffusion.

As the free electrons move across the junction from n-type to p-type, +ve donorions are uncovered. Hence a +ve charge is built on the n-side of the junction. Atthe same time, the free electrons cross the junction and uncover the – ve acceptorions by filling in the holes. Therefore a net – ve charge is established on p-side ofthe junction.

When a sufficient number of donor and acceptor ions is uncovered furtherdiffusion is prevented.

Thus a barrier is set up against further movement of charge carriers. This is calledpotential barrier or junction barrier Vo. The potential barrier is of the order of 0.1

to 0.3V.

Note: outside this barrier on each side of the junction, the material is still neutral. Onlyinside the barrier, there is a +ve charge on n-side and – ve charge on p-side. This region iscalled depletion layer.

+ +

2.1 Biasing: Connecting a p-n junction to an external d.c. voltage source is calledbiasing.

1 Forward biasing



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1.

Forward biasing

2. Reverse biasing

1. Forward biasing

When external voltage applied to the junction is in such a direction that it cancelsthe potential barrier, thus permitting current flow is called forward biasing.

To apply forward bias, connect +ve terminal of the battery to p-type and – veterminal to n-type as shown in fig.2.1 below.

The applied forward potential establishes the electric field which acts against thefield due to potential barrier. Therefore the resultant field is weakened and the

barier height is reduced at the junction as shown in fig. 2.1.

Since the potential barrier voltage is very small, a small forward voltage issufficient to completely eliminate the barrier. Once the potential barrier iseliminated by the forward voltage, junction resistance becomes almost zero and a

low resistance path is established for the entire circuit. Therefore current flows inthe circuit. This is called forward current.

p n

no external field

External field

Fig.2.1 forward biasing of p-n junction

2. Reverse biasing

When the external voltage applied to the junction is in such a direction thepotential barrier is increased it is called reverse biasing.



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potential barrier is increased it is called reverse biasing.

To apply reverse bias, connect – ve terminal of the battery to p-type and +veterminal to n-type as shown in figure below.

The applied reverse voltage establishes an electric field which acts in the samedirection as the field due to potential barrier. Therefore the resultant field at the

junction is strengthened and the barrier height is increased as shown in fig.2.2.

The increased potential barrier prevents the flow of charge carriers across the

junction. Thus a high resistance path is established for the entire circuit and hencecurrent does not flow.

p n

external field

no external field

Fig.2.2 Reverse biasing of p-n junction

2.2 Volt- Ampere characteristics(V-I)

R

Adiode

V V

(i)

IF(mA)

Break over



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Voltage

VR Knee voltage VF

IR(μA)

(ii)

Fig. 2.3 V-I characteristics of p-n junction diode.

(i) Circuit diagram

(ii) Characteristics

The V-I characteristics of a semiconductor diode can be obtained with the help ofthe circuit shown in fig. 2.3 (i)

The supply voltage V is a regulated power supply, the diode is forward biased in

the circuit shown. The resistor R is a current limiting resistor. The voltage acrossthe diode is measured with the help of voltmeter and the current is recorded using

an ammeter.

By varying the supply voltage different sets of voltage and currents are obtained.By plotting these values on a graph, the forward characteristics can be obtained.

It can be noted from the graph the current remains zero till the diode voltageattains the barrier potential.

For silicon diode, the barrier potential is 0.7 V and for Germanium diode, it is 0.3V. The barrier potential is also called as knee voltage or cur-in voltage.

The reverse characteristics can be obtained by reverse biasing the diode. It can benoted that at a particular reverse voltage, the reverse current increases rapidly.

This voltage is called breakdown voltage.

2.3 Diode current equation

The current in a diode is given by the diode current equation

I = I0( eV/ηV

T – 1)

Where, I------ diode currentI0------ reverse saturation current

V------ diode voltageη------- semiconductor constant

=1 for Ge, 2 for Si.VT------ Voltage equivalent of temperature= T/11,600 (Temperature T is in Kelvin)



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Note----- If the temperature is given in 0C then it can be converted to Kelvin by the helpof following relation,

0C+273 = K

2.4 Diode equivalent circuit

It is generally profitable to replace a device or system by its equivalent circuit. Once the

device is replaced by its equivalent circuit, the resulting network can be solved bytraditional circuit analysis technique.

switch rfIf

VF Vo VF

(i) (ii )

Fig.2.4 Diode equivalent circuit. (i) symbol (ii) equivalent circuit

The forward current If flowing through the diode causes a voltage drop in its internal

resistance rf . Therefore the forward voltage VF applied across the actual diode has toovercome

1. potential barrier Vo

2.

internal drop If rf

Vf = Vo + If rf

For silicon diode Vo=0.7V whereas for Germanium diode Vo = 0.3 V.For ideal diode rf =0.

2.4.1 Basic Definitions

1.Knee voltage or Cut-in Voltage.

It is the forward voltage at which the diode starts conducting.

2. Breakdown voltageIt is the reverse voltage at which the diode (p-n junction) breaks down with sudden risein reverse current.

3. Peak-inverse voltage (PIV)It is the max. reverse voltage that can be applied to a p-n junction without causing

damage to the junction.



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If the reverse voltage across the junction exceeds its peak-inverse voltage, then the junction ex

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