1

DESIGN AND FABRICATION OF FIRING

CIRCUIT FOR SINGLE PHASE AC

POWER CONTROLLER

A REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF

Bachelor of Technology In

Electrical Engineering

Supervisor : Submitted by :

Dr. A.K Kapoor Sharad Sharada

Professor (08404EN010)

Department of Electrical Engineering Ankur Gupta

(08404EN010)

DEPARTMENT OF ELECTRICAL ENGINEERING

INSTITUTE OF TECHNOLOGY

BANARAS HINDU UNIVERSITY

INDIA

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DEPARTMENT OF ELECTRICAL ENGINEERING

INSTITUTE OF TECHNOLOGY

BANARAS HINDU UNIVERSITY

VARANASI - 221005

CERTIFICATE

This to certify that the report entitled “Design and fabrication of firing circuit for single phase AC

power controller” , being submitted by Sharad Sharada(Roll No. 08404EN010) and Ankur

Gupta(Roll No. 08404EN014) , in partial fulfillment of the requirement for the award of

BACHELOR OF TECHNOLOGY in ELECTRICAL ENGINEERING has been carried out

under my supervision within the prescribed period under the ordinances governing the Bachelor’s

degree in Electrical Engineering in the faculty of Institute Of Technology, Banaras Hindu

University,Varanasi applicable for 2012 examination.

SUPERVISOR APPROVED FOR SUBMISSION

____________________ ______________________

Dr. A.K Kapoor Prof. J.P Tiwari

Professor Head of the department

Department of Electrical Engineering Department of Electrical Engineering

IT-BHU, VARANASI IT-BHU, VARANASI

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ACKNOWLEDGEMENT

We are grateful to The Department of Electrical Engineering, for providing us with the

opportunity to execute this project, which is a part of the curriculum in M.Tech(IDD) programme

at the Institute of Technology, Banaras Hindu University, Varanasi.

We would also like to take this opportunity to heartily express our gratitude to our project

supervisor Dr. A.K Kapoor, Professor, Department of Electrical Engineering for his immense

support, invaluable guidance and constant encouragement all through this project work.

We would like to thank Prof. J.P Tiwari, Head of Department of Electrical Engineering, for

providing us all possible facilities needed for this project. We would also like to acknowledge the

support of

staff of Power Electronics Laboratory, Electrical Engineering Department for cooperating with

us during the course of our project.

We are very thankful to our parents for their constant encouragement and unwavering belief in

us.

Institute Of Technology Sharad Sharada

Banaras Hindu University Ankur Gupta

Varanasi - 221005

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Contents

ABSTRACT ....................................................................................................... 5

CHAPTER 1 ...................................................................................................... 6

INTRODUCTION .................................................................................................................................... 6

1.1 Thyristor and its conduction ............................................................................................................... 6

1.2 Necessity of getting synchronizing pulses .......................................................................................... 8

1.3 Basic Building blocks of the firing circuit .......................................................................................... 9

CHAPTER 2 .................................................................................................... 10

Phase Control IC TCA785 ...................................................................................................................... 10

2.1 Description of Various PINS in TCA785 - ...................................................................................... 11

2.2 Working of TCA785 ........................................................................................................................ 12

2.2.1 Output Pulse and Ramp generation ............................................................................................ 13

CHAPTER 3 .................................................................................................... 15

CALCULATION OF VARIOUS PARAMETERS AND SPECIFICATIONS ...................................... 15

3.1 Firing Circuit specification : ......................................................................................................... 15

3.2 Snubber Circuit Calculations : .......................................................................................................... 16

CHAPTER 4 .................................................................................................... 17

HARDWARE CIRCUIT ........................................................................................................................ 17

CHAPTER 5 .................................................................................................... 22

Summary ................................................................................................................................................. 22

APPENDIX A .................................................................................................. 23

1. Datasheet of TCA785...................................................................................................................... 23

2. Datasheet of BD875 ........................................................................................................................ 28

REFERENCES ................................................................................................ 30

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ABSTRACT

The theme of the report is to design and fabricate the firing circuit for a AC power controller.

The necessity of getting synchronized firing pulses for the gate of the thyristor is discussed. Out

of many variety of firing circuits available our methodology is to use a phase control IC

TCA785. Detail description and functioning of the firing circuit using TCA785 is discussed

along with the waveforms. Experimental results obtained from oscillographic displays at

important points of the circuits are included. In this project, we fabricate a hardware circuit

which implements firing of thyristor using TCA785, test the circuit and also check the desired

power output at the load.

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CHAPTER 1

INTRODUCTION

Thyristors or Silicon Controlled Rectifiers (SCRs) are widely used as a switching device in the

medium and large power levels starting from few kilowatts to several megawatts at voltage

levels of few hundred to several kilovolt levels. Bipolar Junction Transistors (BJTs) and Metal

Oxide Semiconductor Field Effect Transistors (MOSFETs) although have very fast switching

characteristics compared to SCRs, their uses are limited to medium power levels at few hundred

volts. Insulated Gate Bipolar Transistors (IGBTs) are switching devices which have positive

points over the MOSFETs and thyristors . However, their higher cost and inability to work at

very high voltages makes SCR a better choice even today.

1.1 Thyristor and its conduction

A thyristor or SCR is a four layer device having three junctions J1, J2 and J3. Essentially three

terminals named anode, cathode and gate are available as shown below in the figure 1.2 for

external connections. Under the conditions a thyristor either conduct or not conduct, i.e. it

allows current either to flow or not, thyristor will be in reverse blocking mode if VAK < 0,

irrespective of the fact that a gate pulse is present or not. On the other hand the thyristor is said to

be in the forward blocking mode, when VAK > 0 in absence of any gate pulse, some current will

flow through the thyristor. In case of the thyristor is turning on either by exceeding the forward

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break-over voltage or by applying a gate pulse between gate and cathode, called forward

conduction mode.

Therefore if we want to use a SCR as a switching device we must ensure that appropriate gate

pulse is supplied between gate and cathode at desired instant of time.

The number of degrees from the beginning of the cycle when the SCR is gated or switched on is

referred to as the firing angle, symbol as α, and the number of degrees that the SCR remains

conducting is known as the conduction angle.

Fig 1.1 I-V Characteristic of SCR Fig 1.2 Thyristor circuit symbol

Fig 1.3 One Dimensional SCR Model

iA

vAK

IH

IBO

VH

VBO

forward

on-state

forward blocking

state

i > 0G

i = 0G

-VRW

M anode

cathode

gate

VA

K

+ -iA

iG

N2

P2

P1

N1

G

K

A

J1

J2

J3

(N )-

(N )+

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1.2 Necessity of getting synchronizing pulses

In order to control the output power we need to control AC input power which is controlled by

controlling the firing angle of the thyristor by supplying synchronized pulses to the gate of both

the thyristor at specified interval of time. Typical waveform of the supply voltage and necessary

gate pulses are shown in figure 1.4. Thus we see that for successful operation of the power

controller, the gate pulses have to be properly synchronized with the a.c power supply.

Fig 1.4 Typical waveforms of a single phase AC to AC converter

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1.3 Basic Building blocks of the firing circuit

Basic blocks which will be necessary to implement any firing control scheme are shown in figure

1.4. The figure demonstrates with the help of a single line diagram, the major blocks necessary to

generate firing pulses for any scheme. Since the firing pulses must be synchronized with the a.c.

supply, a.c power also goes to the isolation and synchronizing blocks. Isolation is essential as

because the control circuit uses very low power devices such as various chips, logic gates etc and

in our case TCA785. The logic circuit block uses IC TCA785 to implement a particular firing

scheme. The strength of the pulse obtained from IC may not be sufficient to drive the gate of a

thyristor, so amplification of the pulse with the help of darlington transistor along with

isolation(using pulse transformer) is used at final stage.

Fig 1.4 basic block of firing control circuit

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CHAPTER 2

Phase Control IC TCA785

The TCA785 is used for triggering thyristors, triacs and transistors in phase control applications.

The phase angle can be varied from 0 to 180 degrees. This IC can be used for applications such

as converter circuit, ac controller, and three phase current controller.

Main features of TCA785 are given below as –

i. Three phase operation can be achieved using 3 ICs.

ii. It may be used as zero point switch.

iii. Recognition of zero passage is reliable.

iv. Output Current is 250 mA.

v. Large ramp current range.

vi. Operation is possible for wide range of temperature.

vii. Linden Scripting Language (LSL) compatible.

Fig 2.1 Pin Configuration(Top View)

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2.1 Description of Various PINS in TCA785 -

It is a 16 pin IC. It requires synchronisation input at pin 5. Ramp resistance and ramp capacitance

at pins 9 and 10. The trigerring pulses are generated at pin 14 and pin 15. Pin 1 is grounded and

supply voltage is provided at pin 16.

Pin No. Symbol Function

1 GND GROUND

2 Q2’ Inverted Output 2

3 QU Output U

4 Q1’ Inverted Output 1

5 Vsync Synchronising Voltage

6 I Inhibit

7 QZ Output Z

8 Vref Stabilized Voltage

9 R9 Ramp Resistance

10 C10 Ramp Capacitance

11 V11 Control Voltage

12 C12 Pulse Extension

13 L Long Pulse

14 Q1 Output 1

15 Q2 Output 2

16 Vs Supply Voltage

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2.2 Working of TCA785

Fig. 2.2 Internal Block Diagram

The synchronizing signal from the line voltage is given to the Vsync input of the IC. The internal

Zero Crossing Detector passes ZCD (zero crossing detection) pulses to the synchronization

register. The synchronization register controls the charging of external ramp capacitor C10. The

ramp capacitor is charged by the constant current determined by the external ramp resistor R9.

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2.2.1 Output Pulse and Ramp generation

Fig 2.3 Ramp generation and output pulse waveform

A ramp signal is generated in synchronism with the a.c. supply Vs or V5 by using two

comparators and an approximate ramp generator circuit using a transistor and capacitors as

described in Fig. 2.3. The first comparator translates the input sinusoidal voltage into a square

wave voltage. When the square wave voltage is high, the transistor (P-N-P type) collector-base

junction is forward biased; the transistor is non- conducting stage (off) and the capacitor charges

exponentially giving ramp rise of the voltage at the output. However, as soon as the square

voltage is negative, transistor becomes on due to collector-base junction is reverse biased and the

capacitor discharges sharply giving a saw tooth like waveform as shown in Fig. 2.3. This

triangular voltage can now be compared by the second comparator with a variable reference d.c.

voltage (Vref) to get the firing pulse signal at Y. The value of α can be varied in the range

00≤ α ≤ 180

0 by changing the value of the reference voltage (V11 or Vref).

For every half wave, a positive pulse of approx. 30ms duration appears at the outputs Q1 and

Q2. The pulse duration can be prolonged up to 180° via a capacitor C12.

If pin 12 is connected to ground, pulses with a duration between ɸ and 180° will result.

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Outputs Q’1 and Q’2 supply the inverse signals of Q1 and Q2.

A signal of ɸ+180° which can be used for controlling an external logic is available at pin 3.

A signal which corresponds to the NOR link of Q1 and Q2 is available at output QZ (pin 7).

The inhibit input can be used to disable outputs Q1, Q2 and Q’1, Q’2 .

Pin 13 can be used to extend the outputs to full pulse length (180° –ɸ).

Fig. 2.4 Output pulse Waveform

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CHAPTER 3

CALCULATION OF VARIOUS PARAMETERS AND SPECIFICATIONS

3.1 Firing Circuit specification :

1. Synchronizing Voltage = 9V a.c

2. Supply Voltage = 15V d.c

3. Supply frequency = 50Hz

4. Output Current = 250mA

5. Pulse Transformer = 1:1 turns ratio

3.1.1Synchronising resistance calculation :

Vs = 9V (peak)

I = 100µA

R = (9/√2)/(100*10-6

)

= 63KΩ

Thyristor Specification :

V(DRN) Max(V) = 800

V(RRN) Max(V) = 800

I(TSM) = 850

I(holding current) = 250mA

ID(max leakage current) = 15mA

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3.2 Snubber Circuit Calculations :

ωLs = 0.5Ω (Source Impedance)

2πf*Ls = 0.5

Ls = 0.157mH

di/dtmax = Edc/Ls

= 220/0.157*10-3

= 1400*10

3 A/s

dv/dtmax = 70V/µs

Rs = (dv/dtmax)/ (di/dtmax)

= 50Ω

Cs = 4 Ls/ Rs2

= 0.25µF

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CHAPTER 4

HARDWARE CIRCUIT

The hardware circuit setup is shown in the figure 3.1 and figure 3.2 to trigger two antiparallel

thyristors with one IC TCA785. A supply voltage of 15V dc is applied to TCA785. A

synchronising signal is supplied to pin no.5 through a centre tapped transformer(9-0-9). The

trigger pulse can be shifted continuously within a phase angle between 0° and 180° by means of

a 10kΩ potentiometer at pin 11. Output of the IC cannot however, be connected directly to

between the gate and cathode of a thyristor because the output from it will not be able to supply

the necessary current required by the gate circuit of a thyristor. Apart from this, we require

isolation between the control circuit and power circuit. Therefore with the help of a transistor and

a pulse transformer these two objectives of strengthening the pulse and providing the isolation

are met.

During the negative line half-wave the trigger pulse of pin 14 is fed to the relevant thyristor via a

trigger pulse transformer. During the positive line half-wave, the gate of the second thyristor is

triggered by a trigger pulse transformer at pin 15.

Since the darlington transistor BD875 was not available we employed SN100 transistor to make

the darlington transistor pair.

Thyristor were mounted on the heat sink for proper heat dissipation.

Snubber circuit was connected across the two thyristor for their protection .

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Fig 4.1 Circuit Diagram

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Fig. 4.2 Experimental Setup

Fig 4.3 Power Circuit

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Fig 4.4 Firing Circuit

Fig 4.5 Synchronising Output for pin no. 5

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Fig. 4.6 Ramp Voltage waveform

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CHAPTER 5

Summary

The AC power controller and its thyristor firing circuit was designed using IC TCA785. During

testing, ramp voltage, control voltage and synchronizing voltage were observed in the

oscilloscope. However the gate pulses were not satisfactorily obtained. However on applying 3V

of reference voltage on pin no. 8 the IC got damaged. Due to limitation in time the project had to

be restricted to this level. A more careful approach towards the operation of the designed circuit

may give the desired results.

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APPENDIX A

1. Datasheet of TCA785

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25

26

27

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2. Datasheet of BD875

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REFERENCES

1. Muhammad H. Rashid, Power Electronics, Prentice Hall of India Publishers Ltd, 2009.

2. Power devices and machines by Dr. J.S Chitode & U.A Bakshi.

3. P.C. Sen, Modern Power Electronics, Wheeler Publishing, 1998.

4. Design and implementation of Firing circuit for single phase converter by Tirtharaj Sen,

Pijush Kanti Bhattacharjee, Member, IACSIT, Manjima Bhattacharya.

5. O.P Arora, Power Electronics Laboratory: Experiments & Organization, 1st edition,

Wheeler Publishing, 1993.

6. Snubber circuits: Theory,design and application by Philip C.Todd.

7. http://www.alldatasheet.com/datasheet-pdf/pdf/45801/SIEMENS/TCA785.html

8. http://forum.allaboutcircuits.com/showthread.php?t=45224