ACKNOWLEDGEMENT

At this moment of accomplishment, we are presenting our work with great

pride and pleasure, we would like to express our sincere gratitude to all those who

helped us in the successful completion of our venture. First of all, we would like to

thank our Principal Dr. Mohd Asgar who provided us with all facilities and amenities

for the development of our project. We would like to thank our HOD, Prof S P

Sharma for helping us in the successful accomplishment of our project. We are

exceedingly grateful to our project coordinator Assistant Professor, Mr.Vishal Puri

for his timely and valuable suggestions. We also sincerely thank Mr.Shakeel Lab

Technicians, department of Electronics and Communication for their constant

support and encouragement for our project. We would also like to thank our parents

and friends M.Yasir Thakur, Junaid Khalid, Pz Nayeem for their over whelming and

whole hearted encouragement and support without which this would not have been

successful.Above all we thank God almighty for constantly motivating us with His

love, and giving us courage at each stride to step forward with confidence and self-

belief.

Muhammad Yahaya Shah

Anwar-ul-Haq

Saleem Ramzan

Danish Rashid

1

CONTENTS

Abstract

Biasing arrangement for SCR

SCR Characteristics

Uses of SCR

2P4M SCR data sheet

How to study I.V characteristics of SCR

TRIAC (Triode for alternating currents)

Uses of TRIAC

How to study IV characteristics of TRIAC

The triac used in that and its data sheet

TRIAC Characteristics

DC Motor principle

DC Motor speed control.

Application of DC Motors

Starting of DC machines

DC shunt motor

Speed control of DC shunt motor

Components Used

References

2

Abstract

Power electronic refer to control and conversion of electrical power by power

semiconductor devices where in these devices operate as switches advent of

silicon controlled rectifiers, abbreviated as SCR,s led to the development of new

areas of applications called the power electronics prior to the introduction of

SCRs mercury are rectifiers were used for controlling electrical power, but such

rectifier circuits were part of industrial electronics and the scope for

applications of mercury-arc rectifiers was limited. Once the SCRs were

available, the applications area spread to many fields such as drives, power

supplies, aviation electronics, high frequency inverters and power electronics

originated.

The few applications of power electronics are:

Uninterruptible power supplies and standby power supplies (emergency

power supplies) for critical loads such as computers, medical equipments

etc.

Power conversion for HVAC and HCDC transmission systems.

Speed control of motors which are used in traction drives, textile mills,

rolling mills, cranes, lifts, compressors, pumps etc.

Solid state power compensators, static contractors, transformer tap

changers etc.

High voltage supplies for electrostatic precipitators and X-ray generators

etc.

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Introduction

The word thyristor comes from the Greek and means “door” as in opening a door and

let something pass through it. A thyristor is a semiconductor device that uses internal

feedback to produce switching action. Usually four layers and also five layers

semiconductor devices are called the thyristor. According to their construction they

have at least two terminals to maximum four terminals. Specifically the five layer

members (TRIAC, DIAC) of thyristor family are used their four semiconductor layer

in the state of conduction. The thyristor family members include:

a) SCR (Unidirectional)b) TRIAC (Bidirectional)c) DIAC (Bidirectional)d) SHOCKLY DIODE (Unidirectional)e) SIDAC (Bidirectional)

The two terminal devices DIAC ,SIDAC and SHOCKLY DIODE and the three

terminal device TRIAC and SCR are mostly used in the section of power electronics

where a large power are need to be controlled, regulated and switched.

Silicon Controlled Rectifier (SCR):

One of the common and leading devices of the thyristor family is silicon controlled

rectifier. After diode and transistor the most important device is SCR and invented in

1947. This member of thyristor family is unidirectional that is SCR control current in

only one direction and this is why it is called rectifier. The device is made of silicon

not germanium the semiconductor material because leakage current in silicon is very

small as compared to germanium. As the SCR is used like a switch so in off condition

it should be carry the leakage current as small as possible.

In construction the SCR is a four layer PNPN semiconductor device with three

terminals. Again we can explain it also three diode sandwiched in series in reverse

with each other or two transistor one is PNP and other is NPN connect to base-

collector and collector-base with each other or one diode p-n and a transistor npn

connected to same polarity layer.

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The SCR also have three terminals that mentions earlier are Anode the positive

terminal, Cathode the negative terminal and the gate that control the triggering. The

negative terminal cathode is connected with the most outer n-type layer, the positive

terminal anode is connected with the most outer p-type layer, and the gate is

connected to the next p-type layer or the base of NPN transistor also called gate. For

the arrangement of layers the SCRs have also three junctions J1 , J2 , and J3. The

symbol of SCmR is shown in fig. The constructional view of SCR is shown in fig.

and the simple connection arrangement of SCR is also shown in fig

a) Construction of SCR b) Simple Connection arrangement of SCR

Biasing arrangement of SCR:

If the anode of the SCR is connected to the positive terminal and the cathode is

connected to the negative terminal of external supply voltage is shown in fig4 then we

find the second junction is in reverse biased and the other two are forward biased. In

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this condition until the break of the reverse biased junction no current flow through

the SCR. In practice this type arrangement are used.

Forward biased SCR connection Reverse biased SCR connection

Otherwise if the anode is connected to the negative terminal and cathode is connected

to the positive terminal is shown in fig5 therefore the second junction is forward

biased and the other two junctions are reverse biased so no current can flow through

the SCR until breaking that two junctions which are in reversed biased. This

arrangement is not used because to break those two reverse biased junction we need a

large amount of current that may damage the SCR.

SCR operation mechanism:

In the arrangement of anode positive and cathode negative we need to break only one

reverse biased junction to flow the current through the SCR. To break that junction

earlier an external circuit connected to the gate that is injects current into the p-type

layer this forward biased the second junction of SCR and make the SCR in

conduction early, without gate current a large amount of anode current is required to

break that junction.

6

SCR characteristics:

The V-I characteristics of SCR is shown in fig 8. The first quadrant of the

characteristics curve shows the forward characteristics of SCR where shows the

different break over voltage VBo0 ,VBo1 ,VBo2 for the different gate current IG0

,IG1 ,IG2 where IG2 >IG1>IG0 i.e. large gate current need for early break over at a

low supply voltage. For different gate current there are different holding current.

The third quadrant of characteristics curve shows the reverse characteristics of SCR.

In this case, two reverse junction need to be break to reach the conduction state though

it is possible but it needs a large amount of current, which may damage the SCR.

It is very interesting to off the SCR after its on state that is in conduction state it does

not off if we remove the applied gate current it is because a large amount of anode

current is flowing. In this case, we must reduce the anode current below the holding

current IH.

Uses of SCR:

SCR can be used as follows:

1) SCR as static conductor.

2) SCR for power control.

3) SCRs for speed control dc motor.

4) SCR for over light detector.

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5) Series static switch

6) Variable resistance phase control

7) Battery charging regulator

8) Emergency light system.

9) Relay controls

10) Motor control

11) Inverters

12) Heat control

2P4M SCR DATASHEET

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HOW TO STUDY I-V CHARACTERISTICS OF SCR

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Circuit Diagram

Observation Table: -

VAK IA

Procedure: -1) Study the circuit given on front panel of kit.

2) Connect mill-ammeter & voltmeter in the circuit

3) Connect dc power supply in gate & A to K circuit.

4) Keeping gate current constant increase VAK in steps to note anode- cathode

current IAK , for each step till SCR fires

5) Note the value of IH by gradually decreasing the voltage VAK

6) Plot SCR characteristics between IAK & VAK .

Result: -Characteristics of SCR were studied & found that SCR turned on when IA > IL &

Remained in on state until IA > IH

TRIAC (Triode for Alternating Current):

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TRIAC is one of the bidirectional devices of thyristor family. TRIAC have five layer

and three terminals, the name TRIAC comes from its three electrodes (terminals)

shown in fig. It has no cathode terminal, one of the three is gate and the others are A1

(MT1 i.e. main terminal) and A2 (MT2) as it conducts by terminal. Triac can be

triggered with either positive or negative gate pulses when the anode terminal

potentials are positive or negative respectively. The symbol of TRIAC is shown in fig.

The five layers TRIAC can be divided into two haves one is SCR1 and other is SCR2

connected in parallel of opposite polarity i.e. four transistor as each SCR consist of

two transistor shown in fig. The terminal anode1 and anode2 of TRIAC are not

connected only one layer (like SCR) it connects the outer most two.

TRIAC characteristics: The V-I characteristics of TRIAC is shown in fig12. The first quadrant and the third

quadrant are identical to those of the first quadrant of SCR but in normal operation the

gate voltage is positive in first quadrant and the gate voltage is negative in third

quadrant.

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In the family of V-I characteristics curve for a TRIAC, the magnitude of break over

voltage and holding current become smaller as the values of gate current (IG2 >IG1

>IG0) increases like SCR. To turn the TRIAC off the anode current must be reduced

below the holding current.

Use of TRIAC: TRIAC can be used as follows:

1) As a high power lamp switch. 2) Electronic change-over of transformer taps. 3) Light dimmer 4) Speed controls for electric fans and other electric motors 5) Modern computerized control circuits 6) For minimizing radio interference

HOW TO STUDY I-V CHARACTERISTICS OF TRIAC

Theory: -

TRIAC is three terminal bi-directional high power device. Conduction takes place

in both directions i.e. from MT1 to MT2 or MT2 to MT1.

Gate terminal is towards MT1 in operation TRAIC is equivalent to two SCR’s

connected in anti parallel. The layer diagram symbolic representation of TRAIC is as

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shown V-I characteristic of SCR and TRAIC are similar. The only difference is that

VI characteristics is symmetrical in TRAIC.

TRAIC can be run on in four modes

I+ Mode: -

In this mode MT2 is positive with respect to MT1 and gate is made positive w.r.t.

MT1.

I- Mode: -

In this mode MT2 is positive with respect to MT1 and gate is made negative w.r.t.

MT1.

III+ Mode: -

In this mode MT2 is negative with respect to MT1 and gate is made positive w.r.t.

MT1.

III- Mode: -

In this mode MT2 is negative with respect to MT1 and gate is made negative w.r.t.

MT1.

Circuit diagram: -

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Observation Table: -Forward characteristics – IG ( CONSTANT) = 4 , 8mA

Reverse characteristics – IG ( CONSTANT) = 15 , 16mA .

Procedure :-

1) Connect the circuit as shown in circuit diagram .

2) Make the connections for I + mode .

3) Keep the IG constant and note down the voltmeter and ammeter reading .

4) Now make the connections in III - mode .

5) Keep IG constant and note down the voltmeter and ammeter reading .

6) Plot the graph for both the characteristics.

Results :- Characteristics of TRIAC were studied and plot the graph from the reading.

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TRIAC USED IN CIRCUIT AND ITS DATASHEET

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DC MOTOR SPEED CONTROL

The purpose of a motor speed controller is to take a signal representing the demanded

speed, and to drive a motor at that speed. The controller may or may not actually

measure the speed of the motor. If it does, it is called a Feedback Speed Controller or

Closed Loop Speed Controller, if not it is called an Open Loop Speed Controller.

Feedback speed control is better, but more complicated.

Motors come in a variety of forms, and the speed controller's motor drive output will

be different dependent on these forms. The speed controller presented here is designed

to drive a simple cheap starter motor from a car, which can be purchased from any

scrap yard. These motors are generally series wound, which means to reverse them,

they must be altered slightly.

APPLICATION Of D.C. MOTORSome elementary principles of application alone are dealt with here. The focus is

on the mechanical equation of dynamics which is reproduced here once again.

TM−T L=Jdωdt

Here TM and TL are the motor torque and the load torques respectively which are

expressed as functions of ’ω. Under steady state operation

dωdt will be zero. The

application of motors mainly looks at three aspects of operation.

1. Starting

2. Speed control

3. Braking

The speed of the machine has to be increased from zero and brought to the operating

speed. This is called starting of the motor. The operating speed itself should be varied

as per the requirements of the load. This is called speed control. Finally, the running

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machine has to be brought to rest, by decelerating the same. This is called braking.

The torque speed characteristic of the machine is modified to achieve these as it is

assumed that the variation in the characteristics of the load is either not feasible or

desirable. Hence the methods that are available for modifying the torque speed

characteristics and the actual variations in the performance that these methods bring

about are of great importance. When more than one method is available for achieving

the same objective then other criteria like, initial cost, running cost, efficiency and

ease operation are also applied for the evaluation of the methods. Due to the absence

of equipment like transformer, d.c. machine operation in general is assumed to be off

a constant voltage d.c. supply.

The relevant expressions may be written as

As can be seen, speed is a function of E and T is a function of Ia Using these

equations, the methods for starting , speed control and braking can be discussed.

Starting of D.C. MachinesFor the machine to start, the torque developed by the motor at zero speed must exceed

that demanded by the load. Then TM−T L will be positive so also is

dωdt and the

machine accelerates. The induced emf at starting point is zero as the ω = 0 The

armature current with rated applied voltage is given by V/Ra where Ra is armature

circuit resistance. Normally the armature resistance of a d.c. machine is such as to

cause 1 to 5 percent drop at full load current. Hence the starting current tends to rise to

several times the full load current. The same can be told of the torque if full flux is

already established. The machine instantly picks up the speed. As the speed increases

the induced emf appears across the terminals opposing the applied voltage. The

current drawn from the mains thus decreases, so also the torque. This continues till the

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load torque and the motor torque are equal to each other. Machine tends to run

continuously at this speed as the acceleration is zero at this point of operation.

The starting is now discussed with respect to specific machines

DC shunt motor

If armature and field of d.c. shunt motor are energized together, large current is

drawn at start but the torque builds up gradually as the field flux

increases gradually. To improve the torque per ampere of line current

drawn it is advisable to energize the field first. The starting current is given by

V/Ra and hence to reduce the starting current to a safe value, the voltage V can be

reduced or armature circuit resistance Ra can be increased. Variable voltage V can be

obtained from a motor generator set. This arrangement is called Ward-Leonard

arrangement. A schematic diagram of Ward-Leonard arrangement is shown in Fig. By

controlling the field of the Ward-Leonard generator one can get a variable voltage at

its terminals which is used for starting the motor.

The second method of starting with increased armature circuit resistance can be

obtained by adding additional resistances in series with the armature, at start. The

current and the torque get reduced. The torque speed curve under these conditions is

shown in Fig. It can be readily seen from this graph that the unloaded machine reaches

its final speed but a loaded machine may crawl at a speed much below the normal

speed. Also, the starting resistance wastes large amount of power. Hence the starting

resistance must be reduced to zero at the end of the starting process. This has to be

done progressively, making sure that the current does not jump up to large values.

Starting of series motor and compound motors are similar to the shunt motor. Better

starting torques are obtained for compound motors as the torque per ampere is more.

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.

19

Speed Control of D.C. Motors

In the case of speed control, armature voltage control and flux control methods

are available. The voltage control can be from a variable voltage source like Ward-

Leonard arrangement or by the use of series armature resistance. Unlike the starting

conditions the series resistance has to be in the circuit throughout in the case of speed

control. That means considerable energy is lost in these resistors. Further these

resistors must be adequately cooled for continuous operation. The variable voltage

source on the other hand gives the motor the voltage just needed by it and the losses in

the control gear is a minimum. This method is commonly used when the speed ratio

required is large, as also the power rating. Field control or flux control is also used for

speed control purposes. Normally field weakening is used. This causes operation at

higher speeds than the nominal speed.

Strengthening the field has little scope for speed control as the machines are already in

a state of saturation and large field mmf is needed for small increase in the flux. Even

though flux weakening gives higher speeds of operation it reduces the torque

produced by the machine for a given armature current and hence the power delivered

does not increase at any armature current. The machine is said to be in constant power

mode under field weakening mode of control. Above the nominal speed of operation,

constant flux mode with increased applied voltage can be used; but this is never done

as the stress on the commutator insulation increases.

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Thus operation below nominal speed is done by voltage control. Above the nominal

speed field weakening is adopted. For weakening the field, series resistances are used

for shunt as well as compound motors. In the case of series motors however field

weakening is done by the use of ’diverters’ . Diverters are resistances that are

connected in parallel to the series winding to reduce the field current without affecting

the armature current.

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DATA SHEET OF µA741 Opamp

General purpose operational amplifier A741/A741C/SA741C

DESCRIPTION

The mA741 is a high performance operational amplifier with high open-loop gain,

internal compensation, high common mode range and exceptional temperature

stability. The A741 is short-circuit-protected and allows for nulling of offset voltage.

FEATURES

Internal frequency compensation

Short circuit protection

Excellent temperature stability

High input voltage range

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Components Used

Device Value/No./Size Rating Total required

SCR 2P4M 2 Amp 3

TRIAC BT136 2 Amp 3

Resistances

100Ω 0.5W 5

100 Ω 1 W 5

1 K Ω 0.5 W 5

1 K Ω 1 W 3

4.7 K Ω 1 W 2

10 K Ω pot 1 W 7

47 K Ω pot 1 W 5

Capacitors

100μF 50 V 3

10μF 63 V 2

1000μF 200V 3

Diode 1N4007, 1Amp 8

PCB 4 ' 'by 6' ' 3

References

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1. Integrated Electronics Analog & Digital Circuit and System. Thirteenth reprint 1998, published by Tata McGraw-Hill ----Jacob Milliman, Christos C. Halkias

2. Electronic devices and circuit theory, Eighth edition, published by Prentice Hall ----Robert L. Boylestad, Louis Nashelsky

3. Electronics made simple Published by S. Chand & Company Ltd ----V.K. Mehta

4. 4QD manufacture speed controllers, and publish this basic technical

guide:

http://www.4qd.co.uk/faq/index.html

5. SGS Thomson produced a good document about current limiting in

a full bridge circuit.

http://www.st.com/stonline/books/pdf/docs/1668.pdf

6. DC motor driving including methods of speed regulation.

http://www.st.com/stonline/books/pdf/docs/1656.pdf

7. Driving DC motors

http://www.st.com/stonline/books/pdf/docs/1704.pdf

8. Electronics for You

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