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.
34
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.
35
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.
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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
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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