doc-Design of a Full wave controlled converter using DC drive.pdf

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A Project report on

DESIGN OF A FULL WAVE CONTROLLED

CONVERTER USING DC DRIVE

submitted in partial fulfillment of the requirement for the award of the degree of

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL AND ELECTRONICS ENGINEERING

By

M.Sai Sowjanya (08241A0294)

K.Satya Tejaswi (08241A0297)

A.Sindhu Reddy (08241A02A0)

A.V.Shamili (08241A02B1)

V.Yamuna (08241A02B9)

Under the guidance of

MRs.D.SWATHI

Associate Professor

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND

TECHNOLOGY

(Affiliated to Jawaharlal Nehru Technological University, Hyderabad)

2012

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GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND

TECHNOLOGY

(Affiliated to Jawaharlal Nehru Technological University, Hyderabad)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

CERTIFICATE

This is to certify that the project work titled “DESIGN OF A FULL WAVE CONTROLLED

CONVERTER USING DC DRIVE” has been submitted by M.Sai Sowjanya (08241A0294), K.Satya

Tejaswi (08241A0297), A.Sindhu Reddy (08241A02A0), A.V.Shamili (08241A02B1) ,V.Yamuna

(08241A02B9) in partial fulfillment of the requirements for the award of the degree of bachelor of

technology in “ELECTRICAL AND ELECTRONICS ENGINEERING” from Jawaharlal Nehru

Technological University, Hyderabad.The results embodied in this project have not been submitted to any

other University or Institute for the award of any degree or diploma.

Internal Guide Head Of The Department External Guide

Mrs.D.SWATHI, Mr. P. M. SARMA,

Associate Professor, Professor,

Dept. of Electrical& Electronics Engg. Dept. of Electrical& Electronics Engg.

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ACKNOWLEDGEMENT

This is to place on record my appreciation and deep gratitude to the persons without whose support

this project would never have seen the light of day.

I wish to express my propound sense of gratitude to Mr. P.S.Raju , Director , G.R.I.E.T for his

guidance , encouragement, and for all the facilities to complete this project.

I also express my sincere thanks to Mr. P.M.Sarma , Head Of The Department , G.R.I.E.T for

extending his help .

I have immense pleasure in expressing my thanks and deep sense of gratitude to my guide

Mrs.D.Swathi , Associate Professor , Department Of Electrical And Electronics Engineering,

G.R.I.E.T for her guidance throughout this project.

Finally,I express my sincere gratitude to all the members of faculty and friends who contributed

through their valuable advice and helped in completing the project successfully.

M.Sai Sowjanya (08241A0294)

K.Satya Tejaswi (08241A0297)

A.Sindhu Reddy (08241A02A0)

A.V.Shamili (08241A02B1)

V.Yamuna (08241A02B9)

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CONTENTS

TOPIC PAGE NO

1. INRODUCTION 8-15

1.1 Motor Controller 8

1.2 Triggering Of Thyristor 8

1.3 5HP Motor 12

1.3.1 Types 12

1.4 Applications 12

1.5 Triggering Circuit Diagram Used For Project 14

1.6 Multisim Software 14

1.7 Circuit Simulation In Multisim 15

2. POWER SUPPLY 16-19

2.1 Power Supply Circuit 16

2.1.1 Power Supply Practical Output 17

2.2Cosine Waveform Generation 18

2.2.1 Cosine Wave Generation Practical Output 19

3.INVERTING AMPLIFIER CIRCUIT 20-23

3.1 Inverting Amplifier 20

3.2 Circuit using LM741 IC For Inverting Amplifier 22

3.3 Simulation Results 23

3.4 Practical Output At Inverting Amplifiers 23

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4.COMPARATOR CIRCUIT 24-28

4.1 Comparator circuit 24

4.2 Comparator operation 25

4.3 Comparator circuit using LM339IC 26

4.4 Simulation results 27

4.5 Practical output 28

5. 555 TIMER CIRCUIT 29-32

5.1 About 555 Timer 29

5.1.1 Pin Description 30

5.1.2 555 Timer Operating modes 31

5.2 555 Timer Circuit For Generation Of Pulses 31

5.3 Simulation Results For Circuit 32

5.4 Practical Result For Circuit 32

6. PULSE TRANSFORMER 33-34

6.1 Pulse Transformer Operating Principle 33

6.2 Pulse Transformer Circuit 34

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7. FULLY CONTROLLED THYRISTOR BRIDGE 35-38

7.1 Thyristor 35

7.1.1 Function Of Gate Terminal 36

7.2 Fully Controlled Thyristor Bridge Circuit 37

7.3 Circuit Simulation In PSIM 38

8. PCB DESIGN USING EAGLE SOFTWARE 39-41

8.1 Eagle Software 39

8.2 Triggering Circuit For Thyristor 39

8.3 Eagle Schematic 40

8.4 PCB Design 41

HARDWARE 42

CONCLUSION 43

REFERENCES 44

APPENDIX (A) --- LM741 Data Sheet 46

APPENDIX (B) --- LM339 Data Sheet 49

APPENDIX (C) --- 555 Timer Data Sheet 52

APPENDIX (D) --- 2N2222A Data Sheet 57

APPENDIX (E) --- 1N4007 Data Sheet 61

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ABSTRACT

This project is to designed to run a 5HP DC motor. The motor is controlled by using a Dc drive.

The control scheme used in this drive is Inverse Cosine Control scheme through which we obtain

the firing pulses.Firing pulses are generated by comparing cosine wave form with a DC voltage .

DC voltage is obtained using a power supply circuit. The converter circuit used in this scheme is

full wave controlled converter by using thyristors.

Phase controlled AC-DC converters employing thyristor are extensively used for changing

constant ac input voltage to controlled dc output voltage. In phase controlled rectifiers, a thyristor

is tuned off as AC supply voltage reverse biases it , provided anode current has fallen to level

below the holding current.

Controlled rectifiers have a wide range of applications, from small rectifiers to large high voltage

direct current (HVDC) transmission systems. They are used for electrochemical processes, many

kinds of motor drives, traction equipment, controlled power supplies, and many other applications.

BLOCK DIAGRAM

DC motor

Full wave rectifier circuit

DC drive

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CHAPTER 1

INTRODUCTION

1.1 MOTOR CONTROLLER:

A motor controller is a device or group of devices that serves to govern in some

predetermined manner the performance of an electric motor. A motor controller might include a

manual or automatic means for starting and stopping the motor, selecting forward or reverse

rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting

against overloads and faults.

1.2 TRIGGERING OF THYRISTOR:

Turning on the thyristor by giving a gating pulse to it is known as triggering. With anode positive

with respect to cathode, a thyristor can be turned on by any one of the following techniques :

(a) Forward voltage triggering

(b) gate triggering

(c) dv/dt triggering

(d)Temperature triggering

(e)Light triggering.

These methods of turning-on a thyristor are now discussed one after the other.

(a) Forward Voltage Triggering: When anode to cathode forward voltage is increased with gate

circuit open, the reverse biased junction J2 will break. This is known as avalanche breakdown

and the voltage at which avalanche occurs is called forward breakover voltage VB0. At this

voltage, thyristor changes from off-state (high voltage with low leakage current) to on-state

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characterised by low voltage across thyristor with large forward current. As other junctions J1, J3

are already forward biased, breakdown of junction J2 allows free movement of carriers across

three junctions and as a result, large forward anode-current flows. As stated before, this forward

current is limited by the load impedance. In practice, the transition from off-state to on-state

obtained by exceeding VB0 is never employed as it may destroy the device.

The magnitudes of forward and reverse breakover voltages are nearly the same and both are

temperature dependent. In practice, it is found that VBR is slightly more than VB0. Therefore,

forward breakover voltage is taken as the final voltage rating of the device during the design of

SCR applications.

After the avalanche breakdown, junction J2 looses its reverse blocking capability. Therefore, if

the anode voltage is reduced below VB0 SCR will continue conduction of the current. The SCR

can now be turned off only by reducing the anode current below a certain value called holding

current (defined later).

(b) Gate Triggering : Turning on of thyristors by gate triggering is simple, reliable and efficient,

it is therefore the most usual method of firing the forward biased SCRs. A thyristor with forward

breakover voltage (say 800 V) higher than the normal working voltage (say 400 V) is chosen.

This means that thyristor will remain in forward blocking state with normal working voltage

across anode and cathode and with gate open. However, when turn-on of a thyristor is required, a

positive gate voltage between gate and cathode is applied. With gate current thus established,

charges are injected into the inner p layer and voltage at which forward breakover occurs is

reduced. The forward voltage at which the device switches to on-state depends upon the

magnitude of gate current. Higher the gate current, lower is the forward breakover voltage.

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Figure 1.1- Gate current versus break over voltage

When positive gate current is applied, gate P layer is flooded with electrons from the

cathode. This is because cathode N layer is heavily doped as compared to gate P layer. As the

thyristor is forward biased, some of these electrons reach junction J2. As a result, width of

depletion layer around junction J2 is reduced. This causes the junction J2 to breakdown at an

applied voltage lower than forward breakover voltage VB0. If magnitude of gate current is

increased, more electrons will reach junction J2 ,as a consequence thyristor will get turned on at a

much lower forward applied voltage.

Figure shows that for gate current Ig = 0, forward breakover voltage is VB0. For Igl ,

forward breakover voltage, or turn-on voltage is less than VB0 For Ig2 > Ig1 , forward breakover

voltage is still further reduced. The effect of gate current on the forward breakover voltage of a

thyristor can also be illustrated by means of a curve as shown in Fig. 1.1. For Ig < oa, forward

breakover voltage remains almost constant at VB0. For gate currents Ig1 , Ig2 and Ig3 the values of

forward breakover voltages are ox, oy and oz, respectively as shown. In Figure the curve marked

Ig = 0 is actually for gate current less than oa. In practice, the magnitude of gate current is more

than the minimum gate current required to turn on the SCR. Typical gate current magnitudes are

of the order of 20 to 200 mA.

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Once the SCR is conducting a forward current, reverse biased junction J2 no longer

exists. As such, no gate current is required for the device to remain in on-state. Therefore, if the

gate current is removed, the conduction of current from anode to cathode remains unaffected.

However, if gate current is reduced to zero before the rising anode current attains a value, called

the latching current, the thyristor will turn-off again. The gate pulse width should therefore be

judiciously chosen to ensure that anode current rises above the latching current. Thus latching

current may be defined as the minimum value of anode current which it must attain during turn-

on process to maintain conduction when gate signal is removed.

Once the thyristor is conducting, gate loses control. The thyristor can be turned-off

(or the thyristor can be returned to forward blocking state) only if the forward current falls below

a low-level current called the holding current. Thus holding current may be defined as the

minimum value of anode current below which it must fall for turning-off the thyristor. The

latching current is higher than the holding current. Note that latching current is associated with

turn-on process and holding current with turn-off process. It is usual to take latching current as

two to three times the holding current . In industrial applications, ho lding current (typically 10

mA) is almost taken as zero.

(c) dv/dt Triggering : This method is discussed further in separate post.

(d) Temperature Triggering : During forward blocking, most of the applied voltage appears

across reverse biased junction J2. This voltage across junction J2 associated with leakage current

may raise the temperature of this junction. With increase in temperature, leakage current through

junction J2 further increases. This cumulative process may turn on the SCR at some high

temperature.

(e) Light Triggering: For light-triggered SCRs, a recess (or niche) is made in the inner p-layer

as shown in Fig. 4.5 (a). When this recess is irradiated, free charge carriers (holes and electrons)

are generated just like when gate signal is applied between gate and cathode. The pulse of light

of appropriate wavelength is guided by optical fibres for irradiation. If the intensity of this light

thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor

is known as light-activated SCR (LASCR).

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LASCR may be triggered with a light source or with a gate signal. Sometimes a

combination of both light source and gate signal is used to trigger an SCR. For this, the gate is

biased with voltage or current slightly less than that required to turn it on, now a beam of light

directed at the inner p-layer junction turns on the SCR. The light intensity required to turn-on the

SCR depends upon the voltage bias given to the gate. Higher the voltage (or current) bias, lower

the light intensity required.

Light-triggered thyristors have now been used in high-voltage direct current (HVDC)

transmission systems. In these several SCRs are connected in series-parallel combination and

their light-triggering has the advantage of electrical isolation between power and control circuits.

1.3 5HP MOTOR:

5 HP DC Drive has thyristor controlled full converter output with single phase or two

phase input, up to 5HP motor applications.

1.3.1 Types:

1.Side plate type: These types of drives are simply covered with powder coated plates. It can be

easily mounted on the wall or panel.

2.Box type with voltmeter and Ammeter: These types of drives are covered with powder

coated box having feather touch ON/OFF switch, variable pot, power ON indication, volt meter

and ammeter.

1.4 APPLICATIONS

Every electric motor has to have some sort of controller. The motor controller will

have differing features and complexity depending on the task that the motor will be performing.

The simplest case is a switch to connect a motor to a power source, such as in small

appliances or power tools. The switch may be manually operated or may be a relay or contactor

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connected to some form of sensor to automatically start and stop the motor. The switch may have

several positions to select different connections of the motor. This may allow reduced-voltage

starting of the motor, reversing control or selection of multiple speeds. Overload and overcurrent

protection may be omitted in very small motor controllers, which rely on the supplying circuit to

have overcurrent protection. Small motors may have built-in overload devices to automatically

open the circuit on overload. Larger motors have a protective overload relay or temperature

sensing relay included in the controller and fuses or circuit breakers for overcurrent protection.

An automatic motor controller may also include limit switches or other devices to protect

the driven machinery.

More complex motor controllers may be used to accurately control the speed and torque of the

connected motor (or motors) and may be part of closed loop control systems for precise

positioning of a driven machine. For example, a numerically controlled lathe will accurately

position the cutting tool according to a preprogrammed profile and compensate for varying

load conditions and perturbing forces to maintain tool position.

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1.5 TRIGGERING CIRCUIT DIAGRAM USED FOR THIS

PROJECT:

Figure 1.2- Triggering circuit for thyristors

The figure1.2 shows the circuit used for triggering the thyristors of fully controlled bridge

,used for controlling speed of the motor. The circuit consists of amplifier circuit, comparator

circuit, 555imer circuit for pulse generation and finally the pulse transformer circuit. The circuit is

simulated using MULTISIM software.

1.6 MULTISIM SOFTWARE : Multisim is s an electronic schematic capture and simulation program which is part of

a suite of circuit design programs. Simulating circuits with Multisim catches errors early in the

design flow, saving time and money. Multisim includes all the tools necessary to take a design

from inception to finished project. Multisim has a database of the most commonly used components

that can be placed and wired immediately.

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1.7 CIRCUIT SIMULATION IN MULTISIM:

Figure 1.3- Triggering circuit for thyristors in MULTISIM

The entire triggering circuit is simulated in MULTISIM. The outputs waveforms are

observed at every stage so as to cross check whether the desired outputs are obtained from the

circuit.

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CHAPTER 2

POWER SUPPLY

2.1 POWER SUPPLY CIRCUIT:

The circuit given here is of a regulated dual power supply that provides +12V and -12V

from the AC mains. A power supply like this is a very essential tool on the work bench of an

Electronic hobbyist. The transformer T1 steps down the AC mains voltage and diodes D1, D2,

D3 and D4 does the job of rectification. Capacitors C1 and C2 does the job of filtering.C3, C4,

C7and C8 are decoupling capacitors. IC 7812 and 7912 are used for the purpose of voltage

regulation in which the former is a positive 12V regulator and later is a negative 12V regulator.

The output of 7812 will be +12V and that of 7912 will be -12V.

Assemble the circuit on a good quality PCB.

Transformer T1 can be a 230V primary; 12-0-12 V, 1A secondary step-

down transformer.

Fuse F1 can be a 500mA fuse.

Capacitor C1,C2,C5 and C6 must be rated at least 50V.

Figure 2.1- Dual power supply circuit

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The ICs used for voltage regulation are 7812 and 7912 for +12v and -12v supply respectively.

Figure 2.2- 7812 Figure 2.3- General pin description Figure 2.4- 7912

These ICs have three terminals – input, output,ground. 7812 and 7912 are voltage

regulators integrated circuit. It is a member of 78xx series of fixed linear voltage regulator ICs.

The voltage source in a circuit may have fluctuations and would not give the fixed voltage

output. The voltage regulator IC maintains the output voltage at a constant value. The xx in 78xx

indicates the fixed output voltage it is designed to provide. 7812 provides +12V regulated power

supply. Capacitors of suitable values can be connected at input and output pins depending upon

the respective voltage levels.

2.1.1 power supply practical output:

The output waveform obtained at the positive terminal of dual power supply

circuit ,when connected to CRO is show in following figure.

Figure 2.5- CRO output for +12V with 5V/div

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The output waveform obtained at the negative terminal of dual power supply

circuit ,when connected to CRO is show in following figure.

Figure 2.6- CRO output for -12V with 5V/div

2.2 COSINE WAVEFORM GENERATION:

Figure 2.7- Cosine waveform generation circuit

For inverse cosine control scheme, that is usually adopted in line commutated thyristor control

circuits, a cosine waveform is to be generated from the supply sine wave. The above circuit is

used to convert the input sine wave into cosine wave. The circu its uses a suitable combination of

resistor and three capacitors as shown in the figure 2.6 to phase shift the input sine wave so as to

obtain a cosine wave. The cosine wave is the input given to the triggering circuit.

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2.2.1 COSINE WAVE GENERATION PRACTICAL OUTPUT:

Figure 2.8- Cosine waveform in CRO

By connecting the output of the cosine wave generator to the CRO , the above

wave is obtained which is a cosine wave.

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CHAPTER 3

INVERTING AMPLIFIER CIRCUIT

3.1 INVERTING AMPLIFIER:

Figure 3.1 – Inverting amplifier using Op-Amp

In this Inverting Amplifier circuit the operational amplifier is connected with

feedback to produce a closed loop operation. For ideal op-amps there are two very important

rules to remember about inverting amplifiers, these are: "no current flows into the input terminal"

and that "V1 equals V2", (in real op-amps both these rules are broken). This is because the

junction of the input and feedback signal (X) is at the same potential as the positive (+) input

which is at zero volts or ground then, the junction is a "Virtual Earth". Because of this virtual

earth node the input resistance of the amplifier is equal to the value of the input resistor, Rin and

the closed loop gain of the inverting amplifier can be set by the ratio of the two external resistors.

We said above that there are two very important rules to remember about Inverting Amplifiers or

any operational amplifier for that matter and these are.

1. No Current Flows into the Input Terminals

2. The Differential Input Voltage is Zero as V1 = V2 = 0 (Virtual Earth)

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Then by using these two rules we can derive the equation for calculating the closed-loop gain of

an inverting amplifier, using first principles.

Current ( i ) flows through the resistor network as shown.

Figure 3.2 – current division through resistance network

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3.2 CIRCUIT USING LM741 IC FOR INVERTING AMPLIFIER:

Figure 3.3 – Inverting amplifier using LM741 IC

The IC used her in the circuit as inverting amplifier is LM741. The LM741 series are general

purpose operational amplifiers which feature improved performance

Figure 3.4 – Pin description of LM741 IC

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3.3 SIMULATION RESULTS:

Figure 3.5 – Output of inverting amplifier circuit

3.4 PRACTICAL OUTPUT AT INVERTING AMPLIFIERS:

FIGURE 3.6 Practical output at LM741

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CHAPTER 4

COMPARATOR CIRCUIT

4.1 COMPARATOR CIRCUIT:

Figure 4.1 – Voltage comparator circuit

Comparator circuits find a number of applications in electronics. As the name

implies they are used to compare two voltages. When one is higher than the other the comparator

circuit output is in one state, and when the input conditions are reversed, then the comparator

output switches.

These circuits find many uses as detectors. They are often used to sense voltages.

For example they could have a reference voltage on one input, and a voltage that is being

detected on another. While the detected voltage is above the reference the output of the

comparator will be in one state. If the detected voltage falls below the reference then it will

change the state of the comparator, and this could be used to flag the condition. This is but one

example of many for which comparators can be used.

In operation the op amp goes into positive or negative saturation dependent

upon the input voltages. As the gain of the operational amplifier will generally exceed 100 000

the output will run into saturation when the inputs are only fractions of a millivolt apart.

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A typical comparator circuit will have one of the inputs held at a given voltage.

This may often be a potential divider from a supply or reference source. The other input is taken

to the point to be sensed.

4.2 COMPARATOR OPERATION:

Figure 4.2 – Comparator operation

The above drawings show the two simplest configurations for voltage comparators.

The diagrams below the circuits give the output results in a graphical form.

For these circuits the REFERENCE voltage is fixed at one-half of the supply voltage

while the INPUT voltage is variable from zero to the supply voltage.

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In theory the REFERENCE and INPUT voltages can be anywhere between zero and

the supply voltage but there are practical limitations on the actual range depending on the

particular device used.

4.3 COMPARATOR CIRCUIT USING LM339 IC:

Figure 4.3 – Comparator circuit using LM339

The above figure shows the comparator circuit using LM339 IC for comparing the

voltage levels and generating the required pulses. The circuit generates the pulse width

modulated(PWM) pulses that are given to thyristors for triggering.

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Figure 4.4 – Pin diagram Figure 4.5 – Package diagram

The LM339 series consists of four independent precision voltage comparators

with an offset voltage specification as low as 2 mV max for all four comparators. These were

designed specifically to operate from a single power supply over a wide range of voltages.

Operation from split power supplies is also possible and the low power supply current drain is

independent of the magnitude of the power supply voltage. These comparators also have a

unique characteristic in that the input common-mode voltage range includes ground, even though

operated from a single power supply voltage.

4.4 SIMULATION RESULTS:

Figure 4.6 – Output at the comparator

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4.5 PRACTICAL OUTPUT

FIGURE 4.7(a) Practical output at comparator LM339

FIGURE 4.7(b) Practical output at comparator LM339

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CHAPTER 5

555 TIMER CIRCUIT

5.1 ABOUT 555 TIMER:

The 555 timer is an integrated circuit (chip) implementing a variety of timer

and multivibrator applications. It is one of the most popular and versatile integrated circuits

which can be used to build lots of different circuits. It includes 23 transistors, 2 diodes and 16

resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).

The 555 Timer is a monolithic timing circuit that can produce accurate and

highly stable time delays or oscillations. The timer basically operates in one of the two modes—

monostable (one-shot) multivibrator or as an astable (free-running) multivibrator. In the

monostable mode, it can produce accurate time delays from microseconds to hours. In the astable

mode, it can produce rectangular waves with a variable duty cycle. Frequently, the 555 is used in

astable mode to generate a continuous series of pulses, but you can also use the 555 to make a

one-shot or monostable circuit.

The 555 can source or sink 200 mA of output current, and is capable of driving

wide range of output devices. The output can drive TTL (Transistor-Transistor Logic) and has a

temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or

equivalently 0.005 %/°C.

Applications of 555 timer in monostable mode include timers, missing pulse

detection, bounce free switches, touch switches, frequency divider, capacitance measurement,

pulse width modulation (PWM) etc.

In astable or free running mode, the 555 can operate as an oscillator. The uses

include LED and lamp flashers, logic clocks, security alarms, pulse generation, tone generation,

pulse position modulation, etc. In the bistable mode, the 555 can operate as a flip-flop and is

used to make bounce-free latched switches, etc. The 555 can be used with a supply voltage

(VCC) in the range 4.5 to 15V (18V absolute maximum).

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5.1.1 Pin description

Figure 5.1: Pin out diagram of 555 Timer Figure 5.2 555 Timer package

Figure 5.3 Functional Block Diagram of 555 Timer

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5.1.2 555 TIMER OPERATING MODES The 555 has three operating modes:

Monostable mode: in this mode, the 555 functions as a "one-shot".

Astable - free running mode: the 555 can operate as an oscillator.

Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop.

5.2 555 TIMER CIRCUIT FOR GENERATION OF PULSES:

Figure 5.4 555 TIMER CIRCUIT

The output of the transistor is given as input of the 555 timer and output pulses are obtained.the

width of the pulses are varied by varying the values of Capacitance and Resistance connected to

the timer.

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5.3 SIMULATION RESULT FOR THE CIRCUIT:

Figure 5.5 Simulation result at 555 timer output

5.4 PRACTICAL RESULT FOR THE CIRCUIT:

Figure 5.6 Practical output at 555 Timer output

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CHAPTER 6

PULSE TRANSFORMER

6.1 PULSE TRANSFORMER OPERATING PRINCIPLE:

The magnetic flux in a typical A.C. transformer core alternates between positive and

negative values. The magnetic flux in the typical pulse transformer does no. The typical pulse

transformer operates in an ―unipolar‖ mode ( flux density may meet but does not cross zero.)

Figure 6.1(a) Pulse transformer

A fixed D.C. current could be used to create a biasing D.C. magnetic field in the

transformer core, thereby forcing the field to cross over the zero line. Pulse transformers usually

(not always) operate at high frequency necessitating use of low loss cores (usually ferrites).

Figure shows the electrical schematic for a pulse transformer and equivalent high frequency

circuit representation for a transformer which is applicable to pulse transformers. The circuit

treats parasitic elements, leakage inductances and winding capacitance, as lumped circuit

elements, but they are actually distributed elements. Pulse transformers can be divided into two

major types, power and signal.

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An example of a power pulse transformer application would be precise control of a heating

element from a fixed D.C. voltage source. The voltage may be stepped up or down as needed

by the pulse transformer’s turns ratio. The power to the pulse transformer is turned on and off

using a switch (or switching device) at an operating frequency and a pulse duration that delivers

the required amount of power. Consequently, the temperature is also controlled. The

transformer provides electrical isolation between the input and output. The transformers used in

forward converter power supplies are essentially power type pulse transformers. There exists high-

power pulse transformer designs that have exceeded 500 kilowatts of power capacity.

Pulse transformer designers usually seek to minimize voltage droop, rise time,

and pulse distortion. Droop is the decline of the output pulse voltage over the duration of

one pulse. It is cause by the magnetizing current increasing during the time duration of

the pulse. To understand how voltage droop and pulse distortion occurs, one needs

to understand the magnetizing ( exciting, or no-load ) current effects, load current

effects, and the effects of leakage inductance and winding capacitance. The designer

also needs to avoid core saturation and therefore needs to understand the voltage-time

product.

6.2 PULSE TRANSFORMER CIRCUIT:

Figure 6.5 Pulse transformer used in triggering circuit

The single pulse produced by the 555 timer is converted into two pulses to trigger a pair of

thyristors in the fully controlled bridge rectifier.

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CHAPTER 7

FULLY CONTROLLED THYRISTOR BRIDGE

7.1 THYRISTOR

A thyristor is a solid-state semiconductor device with four layers of alternating N and

P-type material. They act as bistable switches, conducting when their gate receives a current

pulse, and continue to conduct while they are forward biased (that is, while the voltage across the

device is not reversed).

The thyristor is a four-layer, three terminal semiconducting device, with each layer

consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals,

labelled anode and cathode, are across the full four layers, and the control terminal, called the

gate, is attached to p-type material near to the cathode. (A variant called an SCS—Silicon

Controlled Switch—brings all four layers out to terminals.) The operation of a thyristor can be

understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the

self-latching action:

Figure 7.1 : Thyristor representation

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Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a

diode

2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to

conduct, but the thyristor has not yet been triggered into conduction

3. Forward conducting mode — The thyristor has been triggered into conduction and will

remain conducting until the forward current drops below a threshold value known as the

"holding current"

7.1.1 Function of the gate terminal

The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).

Figure 7.2 Layer diagram of thyristor.

When the anode is at a positive potential VAK with respect to the cathode with no

voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse

biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased

beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and

the thyristor starts conducting (On state).

If a positive potential VG is applied at the gate terminal with respect to the

cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an

appropriate value of VG, the thyristor can be switched into the on state suddenly.

37

Once avalanche breakdown has occurred, the thyristor continues to conduct,

irrespective of the gate voltage, until: (a) the potential VAK is removed or (b) the current through

the device (anode−cathode) is less than the holding current specified by the manufacturer. Hence

VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.

These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate

trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way

that it is evident that there is a minimum gate charge required to trigger the thyristor.

7.2 FULLY CONTROLLED THYRISTOR BRIDGE CIRCUIT:

Figure 7.3 Thyristor bridge circuit used to control speed of dc motor

The circuit of a single-phase fully-controlled bridge rectifier circuit is shown in

the figure above. The circuit has four SCRs. It is preferable to state that the circuit has two pairs

of SCRs, with THY1 and THY2 forming one pair and, THY3 and THY4 the other pair. The

firing pulses obtained from pulse transformers are given to the gates of the thyristors to trigger

them. Pulses from the first pulse transformer are given to the thyristors THY1 and THY2 to

make them operate in the positive cycle of the input wave and pulses from second pulse

transformer are given to thyristors THY3 The main purpose of this circuit is to provide a variable

dc output voltage, which is brought about by varying the firing angle.and THY4 to make them

operate in the negative cycle of the input wave and dc is obtained at the output of the bridge.

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7.3 CIRCUIT SIMULATION IN PSIM:

Figure 7.4 Simulation of fully controlled thyristor rectifier circuit(firing angle α )

39

CHAPTER 8

PCB DESIGN USING EAGLE SOFTWARE

8.1 EAGLE SOFTWARE: EAGLE (Easily Applicable Graphical Layout Editor) is a schematic capture and

PCB layout tool for hobbyists and DIY enthusiasts. EAGLE contains a schematic editor, for

designing circuit diagrams. Parts can be placed on many sheets and connected together through

ports. The PCB(Printed Circuit Board) layout editor allows back annotation to the schematic and

auto-routing to automatically connect traces based on the connections defined in the schematic.

EAGLE saves Gerber and PostScript layout files and Excellon and Sieb & Meyer drill files.

These standard files are accepted by many PCB fabrication companies.

8.2 TRIGGERING CIRCUIT FOR THYRISTORS:

Figure 8.2 - Triggering circuit

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8.3 EAGLE SCHEMATIC OF THE CIRCUIT:

Figure 8.3- EAGLE Schematic

41

8.4 PCB DESIGN:

Figure 8.4- Triggering circuit

Thus, the final PCB design of the circuit is made using EAGLE software.

42

HARDWARE

DC drive circuit used for the design of full wave controlled converter

43

CONCLUSION

Thus by using the trigerring circuit (which takes cosine wave as input ),the pwm pulses have

been generated and the pulse width being modulated by the 555 timers ( by RC combination) and

the resultant pulses after allowing through pulse transformer (that converts single pulse to two

pulses for firing of two thyristors at a time in a single cycle of input signal) are given to thyristor

bridge which controls the input voltage to the motor (by varying the 10 K potentiometer

connected at the reference voltage of the co mparator IC IN 339) there by controlling the speed of

the motor thus finally using this project the speed control of dc motor is also achieved.and here

the firing pulses generated will trigger the thyristors and hence helps in the design of a full wave

controlled converter

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REFERENCES:

1. G. Moltgen, “Line Commutated Thyristor Converters,” Siemens

Aktiengesellschaft, Berlin-Munich, Pitman Publishing, London,

1972.

2. K. Thorborg, “Power Electronics,” Prentice-Hall International (UK)

Ltd., London, 1988.

3. M. H. Rashid, “Power Electronics, Circuits Devices and Applications,”

Prentice-Hall International Editions, London, 1992.

4. N. Mohan, T. M. Undeland, and W. P. Robbins, “Power Electronics:

Converters, Applications, and Design,” John Wiley and Sons,

New York 1989.

5. J. Arrillaga, D. A. Bradley, and P. S. Bodger, “Power System

Harmonics,” John Wiley and Sons, New York, 1989.

6. Bimbhra , Dr. P.S., Power Electronics ,Khanna Publishers, 2

7. J. M. D. Murphy and F. G. Turnbull, “Power Electronic Control of AC Motors”, Pergamon

Press, 1988.

8. www.google.com

9. www.wikipedia.org

45

APPENDIX

The ICs(Integrated Circuits) and components used in this project are as follows:

LM 741 : Inverter amplifier

LM 339 : Comparator

555 TIMER : Pulse Generator

2222A : Transistor

IN 4007 : Diode

The data sheets of the above ICs are as follows.

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APPENDIX-(A)

LM 741 DATASHEET

47

48

59

49

APPENDIX-(B)

LM 339 DATASHEET

44

51

52

APPENDIX-(C)

555 TIMER DATASHEET

53

54

55

56

57

APPENDIX-D

58

59

60

APPENDIX(E)

IN 4007 DIODE DATASHEET