This document describes an automatic phase changer circuit that can shift the load to an alternate power phase if the voltage drops below a certain level in one of the phases. The circuit uses three identical sets that each correspond to one of the three phases (R, Y, B). Each set includes a transformer, comparator, transistor and relay. The transformer steps down the voltage which is then rectified and used as input for the comparator. The comparator compares this voltage to a reference voltage and triggers the transistor and relay if the phase voltage is low, shifting the load to another phase with sufficient voltage. This automatic switching prevents equipment downtime if one phase loses power.

1. AUTOMATIC PHASE CHANGER
A Minor Project Report
Submitted in partial fulfillment of the requirement for the award of Degree of Bachelor of
Engineering in Electrical
Submitted to
RAJIV GANDHI PROUDYOGIKI VISHWAVIDHYALAYA
BHOPAL (M.P.)
MINOR PROJECT REPORT
Submitted By
Bhanu Bhawesh (0103EE111009)
Guided by
DEPARTMENT OF ELECTRICAL ENGINEERING
LAKSHMI NARAIN COLLEGE OF TECHNOLOGY BHOPAL
Session: June- 2014

2. LAKSHMI NARAIN COLLEGE OF TECHNOLOGY, BHOPAL
Department of Electrical Engineering
CERTIFICATE
This is to certify that the work embodied in this Major Project title “Automatic
Phase Changer” has been satisfactorily completed by Bhanu Bhawesh,
students of Final year. It is a bonafied piece of work, carried out under my
supervision and guidance in the Department of Electrical & Electronics
Engineering, Lakshmi Narain College of Technology, Bhopal, for partial
fulfillment of the Bachelor of Engineering during the academic year 2013-14.
Under the Supervision of
Project Guide
___________________
Prof.
Approved By
________________
Dr.
Head of the Department
Department Of Electrical Engineering
Forwarded by
________________
Dr.
Principal
LAKSHMI NARAIN COLLEGE OF TECHNOLOGY,BHOPAL
I
LAKSHMI NARAIN COLLEGE OF TECHNOLOGY, BHOPAL

3. Department of Electrical Engineering
DECLARATION
We, Bhanu Bhawesh, students of Bachelor of Engineering, Electrical &
Electronics , Engineering, Lakshmi Narain College of Technology, Bhopal, here
by declare that the work presented in this Minor/Major Project is outcome of
our own work, is bonafide, correct to the best of my knowledge and this work
has been carried out taking care of Engineering Ethics. The work presented does
not infringe any patented work and has not been submitted to any University for
the award of any degree or any professional use.
_______________
Bhanu Bhawesh
Enrollment No. 0103EE111009
Date: --/--/----
II

4. ACKNOWLEDGEMENT
Reasons can be given & they can be many but nothing can replace the efforts of numerous
people behind the work, put in by the creator, giving us constant support all the way. On the
very outset I would like to thank, Principal, LNCT, Bhopal for providing me the glorious
opportunity to work on this project. I express my hearty gratitude to Dr. Head of Department
Electrical Engineering. Her invaluable guidance, advice & time from her extremely busy
schedule made this work an enthusiastic experience. I wish to express my most sincere
gratitude for her whole hearted cooperation.
I am grateful, beyond my sense of gratitude, to Prof. , Project Coordinator,
Department of Electrical Engineering, for his kind cooperation throughout the planning &
preparation of this project. Working on this project was quite a wonderful experience & that
too an unforgettable one. Without his unhindered support, sustained interest, unlimited
patience, sound counsel & tremendous knowledge in the field of electronics, this work would
have not been possible.
Nothing can be taken away from my project guides Prof. Department of Electrical
& Electronics Engineering, who has been instrumental in guiding us through the various
aspects of designing of our project & implementing the idea of the project “Automatic Phase
Changer’’.
III

5. TABLE OF CONTENTS
TOPIC NAME PAGE NUMBER
Certificate I
Declaration II
Acknowledgement III
Table of Content IV
List of figures VI
List of tables VI
Abstract VII
Chapter 1: Introduction 1
Chapter 2: Literature Survey 2
Chapter 3: Block Diagram 4
Chapter 4: Block Diagram Description 5
4.1 Phase Input 5
4.2 Power Supply 5
4.3 Voltage Sensing Circuit 5
4.4 Relay Box 5
Chapter 5: Circuit Diagram 6
Chapter 6: Circuit Diagram Description 7
6.1 Power Supply 7
6.2 LED 7
6.3 Transistor 7
6.4 Relay 8
Chapter 7: Working 9
Chapter 8: List of Components 11
Chapter 9: Component Description 12
IV
9.1 Comparator 12

6. 9.2 Zener Diode 16
9.3 Diode 17
9.3.1 Diode Construction 18
9.4 Rectifiers 19
8.4.1 Standard Rectifiers 20
9.4.2 Fast Rectifiers 20
9.4.3 Ultra Fast Rectifiers 21
9.5 Relay 22
9.5.1 Operation 23
9.5.2 Application 24
9.6 Transformer 25
9.6.1 Step Down Transformer 26
9.7 Capacitor 2 9
9.7.1 Operation 30
9.8 Transistor 32
9.9 Voltage Regulator 36
9.9.1 Electronic Regulator Circuit 37
9.9.2 IC7812 37
9.9.3 Advantages 38
9.9.4 Disadvantages 38
9.10 Optocoupler 39
9.10.1 Operation
Chapter 10: PCB Design 41
10.1 PCB Design Procedure 42
10.2 PCB Preparation 43
10.3 Eching 44
10.4 Drilling 45
10.5 Soldering 46
Chapter 11: Cost Estimation 47
Chapter 12: Advantages And Application 49
Chapter 13: Future scope And Conclusion 50
References
V
LIST OF FIGURES

7. 3. Block Diagram of Project 4
5. Circuit Diagram of Project 6
9.1 Comparator pin diagram 12
9.2 Zener Diode Circuit 16
9.2 VI Characteristic of Zener Diode 16
9.3 VI Characteristics of Diode 18
9.5 Relay 22
9.8 PNP Transistor 32
9.9 Voltage regulator 36
9.10 Optocouplar 39
LIST OF TABLES
8. Cost of Components 11
11. Cost Estimation 47
VI
ABSTRACT
In three-phase applications, if low voltage is available in any one or two phases, and you
want your equipment to work on normal voltage, this circuit will solve your problem.
However, a proper-rating fuse needs to be used in the input lines (R, Y and B) of each phase.

8. The circuit provides correct voltage in the same power supply lines through relays from the
other phase where correct voltage is available. Using it you can operate all your equipment
even when correct voltage is available on a single phase in the building.
The circuit is built around a transformer, comparator, transistor and relay. Three identical sets
of this circuit, one each for three phases, are used.
The mains power supply phase R is stepped down by transformer X1 to deliver 12V, 300
mA, which is rectified by diode D1 and filtered by capacitor C1 to produce the operating
voltage for the operational amplifier (IC1). The voltage at inverting pin 2 of operational
amplifier IC1 is taken from the voltage divider circuit of resistor R1 and preset resistor VR1.
VR1 is used to set the reference voltage according to the requirement. The reference voltage
at non-inverting pin 3 is fixed to 5.1V through zener diode ZD1. The phase voltage is
compared against the reference voltage and if the phase voltage is low the relay trips and
shifts the load to other phase.
VII
1. INTRODUCTION
Power instability in developing countries creates a need for automation of electrical power
generation or alternative sources of power to back up the utility supply. This automation is
required as the rate of power outage becomes predominantly high. Most industrial and

9. commercial processes are dependent on power supply and if the processes of change-over are
manual, serious time is not only wasted but also creates device or machine damage from
human error during the change-over connections, which could bring massive losses. The
starting of the generator is done by a relay which switches the battery voltage to ignition coil
of the generator while the main power relay switches the load to either public supply or
generator. Fig 1 shows the generalized block diagram of the system. The approach used in
this work is the modular approach where the overall design was first broken into functional
block diagrams, where each block in the diagram represents a section of the circuit that
carries out a specific function. The functional block diagram of Fig. 1 also shows the
interconnection between these blocks. Each section of the block is analyzed below. A manual
change-over switch consists of a manual change-over switch box, switch gear box and cut-out
fuse or the connector fuse as described by Rocks and Mazur (1993). This change-over switch
box separate the source between the generator and public supply, when there is power supply
outage from public supply, someone has to go and change the line to generator. Thus when
power supply is restored, someone has to put OFF the generator and then change the source
line from generator to public supply.
In view of the above manual change-over switch system that involves manpower by using
ones energy in starting the generator and switching over from public supply to generator and
vice-versa when the supply is restored. The importance attached to cases of operation in
hospitals and air ports in order to save life from generator as fast as possible makes it
important for the design and construction of an automatic change-over switch which would
solve the problem of manpower and the danger likely to be encountered changeover. The
electronic control monitors the incoming public supply voltage and detects when the voltage
drops below a level that electrical or electronics gadgets can function depending on the
utility.
1
2. LITERATURE SURVEY
The aim behind the mini project is to improve the professional competency by selecting those
areas which otherwise are not covered in the normal course. This is to enhance our
knowledge into various fields, and thus to gain work experience, confidence, and logical
thinking. Our aim was to select a topic which is simple enough to be done within the

10. specified time. So we are planned to do a simple project using basic electrical and electronic
concept that we have studied yet. We interested to apply and modify the basic concept than a
new topic to be selected.While selecting a topic for our mini project, the first thing which
came to our mind was that it should be a product that has got considerable importance in the
modern era.
2.1. Selection:
Our concentration was to develop a system which can reduce the problems or
difficulties in our life. Also one more thing was in mind that to develop a system which can
be applied for several applications associated with modern science and developments in
technology. So the concept of automatic phase changer was selected which can be used in 3-
phase applications. In 3 phase applications, if low voltage is available in any one of two
phases and want equipment to work in normal voltage this circuit will solve your problem. It
is a simple circuit. The circuit consists a comporator,transistor,transformer and relays. We use
741 Op-Amp in ‘comparator’mode. This allows it to compare two input voltages.
2.2 Design of the circuit:
The circuit is built around a transformer, comparator, transistor and relay. Three identical
sets of this circuit, one each for three phases, are used. Here we used a step down
transformer. Here the IC LM358 working as the comparator is used here is surrounded by all
other components. Transistor BC557 acting as a switch. Relay is electromagnetic type. In
automatic phase changer the main processes can be divided into four.
 Step down the main supply
 Rectification
 Comparing
 Switching
Main supply R, Y, B is stepped down to desired voltage and current. Each transformer is
individually connected to the phases R, Y, B respectively. In this case, only one phase work
at a time. The diodes (IN4007) are used to rectify the ac to dc. The capacitors for removing
the noises in the dc. The resistors and potentiometers of the circuit is gives the specified
voltage input to the comparator. Based on the comparator output, the transistor (BC557) goes
to on and off positions. Thus we can say that transistor work as a switch.
Transformer - 12 V, 300mA; Transistor – BC557 (PNP); Diode - IN4007; Zener
Diode 5.1 V; Capacitor - 1000microF, 12 V; 470microF, 35 V; Resistor - R1 & R2 –
3.3k, R3 – 10k; Potentiometer - 10k.
2
2.3. Assembling the Project:
Main components needed for the project are resistors, capacitors, diodes, transformer,
comparator and relays. The components were mounted on the bread board and were wired up.
A 12V dc supply was generated. The main circuit consist comparator, transformer, transistor
and relay. Three identical sets of this circuit connected on the breadboard. Each one

11. corresponds three phases. Then the output is verified by connecting a load (bulb) at the output
and got the desired output.
3
3.BLOCK DIAGRAM

13. 4
4. BLOCK DIAGRAM DESCRIPTION
4.1 PHASE INPUT
It is the input source of the device. This section contains three identical phase lines that
represented by R,Y,B respectively. It carries 250 v each between two phase lines. Phase input
can be provided by the use of step down transformer.
4.2 POWER SUPPLY
It provides necessary power supply 5v to the circuit components. The power supply circuit
uses rectifiers, filters and voltage regulators to ensure the input voltage between safe
operative range
4.3 VOLTAGE SENSING CIRCUIT
This section provides appropriate control for the switching action of relay box. in this device
LM358 IC is used. It also protects the controller from further damage due to large current.
4.4 RELAY BOX;
Depending up on the output of IC the relay circuit get energized or de energized. If` low
voltage is available in any one of the phase, the relay circuit get shifted from current phase to
another phase where correct voltage is available.

14. 5

15. 5. CIRCUIT DIAGRAM
Automatic phase changer
6
6. CIRCUIT DIAGRAM DESCRIPTIONS
The circuit is built around a transformer,comparator,transistor,and relay.Three identical sets
of this circuit one each for three phase are used.The main power supply phase R is stepped
down by transformer XI to deliver I2v,500ma, which is rectified by diode DI and filtered by
capacitor CI to produce operating voltage for the operational amplifier(ICI).The voltage at
the non-inverting pin 3 of operational amplifier ICI is taken from the voltage divider circuit
of resistor RI and the variable resistor VRI.The variable resistor is used to set the reference
voltage according to the requirement.The reference voltage at the inverting pin 2 of ICI is

16. fixed to 5. Iv by using a zener diode ZDI. The supply voltage available in phase R is in range
of 200v-230v, the voltage at the non-inverting pin 3 of ICI remains high. As a result,transistor
Tl does not conduct , relay RLI remains de-energized and Phase R supplies power to the load
LI via normally closed contact of relay RLI.
6.1POWER SUPPLY
The present chapter power supply circuit built using filter and SCR Starting with a dc voltage
cell of 9 v and then regulated to obtain a desired voltage.
6.2 LED (Light emitting diode)
It is a (semiconductor) light source LED are aced as indicator lamps in many device and are
increasingly used for other lightly. Modern version are available ,ultraviolet and infrared
wavelength with high brightness. W’hen LED is forward biased electrons are able to
recombine with electron holes with the device, releasing in the form of photon.This effect is
called electroluminescence. The color of light is determined by the energy gap of the
semiconductor. A LED is often small in area and integrated optical components may be used
to shape as radiation pattern. LED are used in application as diverse as replacement for
(aviation lightly), (automatic lightly) the compact size,possibility of narrow bandwidth,
switching speed at extreme reliability of LEDS has allowed new text at video display at
sensor to be developed.
6.3 TRANSISTOR
Transistor is a semiconductor device used to amplify and switch electronic signals it is made
of solid piece of semiconductor material, with at least three terminals for connection to an
external A Voltage or current applied to one pair of the transistor terminals changes the
7
current {lowing through another pair of terminals because the controlled (output) power can
be much more than the controlling(input)power, transistor provides of a signal.
6.6 RELAY
A relay is an electrical switch that opens and closes under control of another electric circuit.
In the original form,the switch is operated by an electromagnet to open or close one on many
sets of contacts.Because a relay is Able to control an output circuit having
higher power,than the input circuit, it can be considered,in a broad sense,to be a form of

17. electrical amplifier. The contacts can be Normally open (NO), Normally closed (NC),Or
Change-over contacts.Normally open contacts connect the circuit when the relay is energized
The circuit is d is connected when the Relay is inactive.It is also called Form A contact
or”make” contact. Form A contact is ideal for application that require to switch high current
power source from a remote device JS formally closed contacts disconnect the circuit when
the relay is activated . The circuit is connected when relay is inactive. It also called Form B
contactor "break"contact.Form B contact is ideal applications that require the circuit remain
closed until the relay is activated Change over contacts control Two circuits one normally
open contact and one normally dosed contact.It is also called Form C contact.
8

18. 7. WORKING
In three-phase applications, if low voltage is available in any one or two phases, and you
want your equipment to work on normal voltage, this circuit will solve your problem.
However, a proper-rating fuse needs to be used in the input lines (R, Y and B) of each phase.
The circuit provides correct voltage in the same power supply lines through relays from the
other phase where correct voltage is available. Using it you can operate all your equipment
even when correct voltage is available on a single phase in the building. The circuit is built
around a transformer, comparator, transistor and relay. Three identical sets of this circuit, one
each for three phases, are used. Let us now consider the working of the circuit connecting red
cable (call it ‘R’ phase). The mains power supply phase R is stepped down by transformer X1
to deliver 12V, 300 mA, which is rectified by diode D1 and filtered by capacitor C1 to
produce the operating voltage for the operational amplifier(IC1). The voltage at inverting pin
2 of operational amplifier IC1 is taken from the voltage divider circuit of resistor R1 and
preset resistor VR1. VR1 is used to set the reference voltage according to the requirement.
The reference voltage at non- inverting pin 3 is fixed to 5.1V through zener diode ZD1. Till
the supply voltage available in phase R is in the range of 200V-230V, the voltage at inverting
pin 2 of IC1 remains high, i.e., more than reference voltage of 5.1V, and its output pin 6 also
remains high. As a result, transistor T1 does not conduct, relay RL1 remains de-energized
and phase ‘R’ supplies power to load L1 via normally closed (N/C) contact of relay RL1.
As soon as phase-R voltage goes below 200V, the voltage at inverting pin 2 of IC1 goes
below reference voltage of 5.1V, and its output goes low. As a result, transistor T1 conducts
and relay RL1 energizes and load L1 is disconnected from phase ‘R’ and connected to phase
‘Y’ through relay RL2. Similarly, the auto phase-change of the remaining two phases, viz,
phase ‘Y’ and phase ‘B,’ can be explained. Switch S1 is mains power ‘on’/’off’ switch.
Use relay contacts of proper rating and fuses should be able to take-on the load when
transferred from other phases. While wiring, assembly and installation of the circuit, make
sure that you: 1. Use good-quality, multi-strand insulated copper wire suitable for your
current requirement. 2. Use good-quality relays with proper contact and current rating. 3.
Mount the transformer(s) and relays on a suitable cabinet. Use a Tag Block (TB) for
incoming/outgoing connections from mains.
9

19. 8. LIST OF COMPONENTS
10
Sr.
no.
Components and Specifications Specifications Quantity
1 Comparator IC LM358 1
2 Zener Diode 1N4007 12
3 Relay 50/60 Hz 1
4 Step down transformer 220V-12V
300 mA
2
5 Transistor bc557 1
Capacitor 100 and 1000 uf 4
7 Voltage regulator IC 7812 1
8 Resistance 470 & 10K ohm 7
9 Octocouplar MCT2E 2
10 Bridge rectifier
-
3
11 Connecting wires
-
As per
requirements

20. 9.COMPONENT DESCRIPTION
9.1 COMPARATOR
The IC LM358 i.e. the operational amplifier is used as a comparator in the circuit given
above. As shown in the figure the IC LM358P is a 8 pin IC in which the pin no. 2 is known as
the inverting terminal of the IC LM358P because it is connected to the negative potential.
The pin no. 3 is known as the non inverting terminal of the IC LM358P . The pin no. 2 is
connected to the reference voltage. The reference voltage is the voltage which we set as a
standard voltage in the circuit. The pin no. 3 is connected to the input voltage. Now if we
applied the input voltage to the IC LM358 then it will compare the input voltage to the
reference voltage and if the input voltage goes low then the output of the comparator is goes
low. And if the input voltage is equal to the reference voltage then the output of the
comparator is high.
Pin Diagram Of
Comparator
11

21. 9.2 ZENER DIODE
A Zener diode is a type of diode that permits current not only in the forward direction like a
normal diode, but also in the reverse direction if the voltage is larger than the breakdown
voltage Breakdown voltage The breakdown voltage of an Insulator is the minimum voltage
that causes a portion of an insulator to become electrically conductive. The breakdown
voltage of a diode is the minimum reverse voltage to make the diode known as "Zener knee
voltage" or "Zener voltage". The device was named after Clarence Zener. Clarence Melvin
Zener was the American physicist who first described the electrical property exploited by the
Zener diode, which Bell Labs then named after him, who discovered this electrical property.
A conventional solid-state diode
Diode In electronics, a diode is a two-terminal electronic component that conducts electric
current in only one direction. The term usually refers to a semiconductor diode, the most
common type today. This is a crystalline block of semiconductor material connected to two
electrical terminals... will not allow significant current if it is reverse-biased below its reverse
breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional
diode is subject to high current due to avalanche breakdown. Unless this current is limited by
circuitry, the diode will be permanently damaged. In case of large forward bias (current in the
direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage
and internal resistance. The amount of the voltage drop depends on the semiconductor
material and the doping concentrations.
12

22. A Zener diode exhibits almost the same properties, except the device is specially designed so
as to have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with
the conventional device, a reverse- biased Zener diode will exhibit a controlled breakdown
and allow the current to keep the voltage across the Zener diode at the Zener voltage. For
example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2
V if reverse bias voltage applied across it is more than its Zener voltage. The Zener diode is
therefore ideal for applications such as the generation of a reference voltage (e.g. for an
amplifier Amplifier Generally, an amplifier or simply amp, is any device that changes,
usually increases, the amplitude of a signal. The relationship of the input to the output of an
amplifier—usually expressed as a function of the input frequency—is called the transfer
function of the amplifier, and the magnitude of... stage, or as a voltage stabilizer for low-
current applications.
The Zener diode's operation depends on the heavy doping Doping (semiconductor) In
semiconductor production, doping is the process of intentionally introducing impurities into
an extremely pure semiconductor to change its electrical properties. The impurities are
dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are
referred to as extrinsic...of its p-n junction
P-n junction A p–n junction is formed by joining p-type and n-type semiconductors together
in very close contact. The term junction refers to the boundary interface where the two
regions of the semiconductor meet...allowing electron
Electron The electron is a subatomic particle carrying a negative electric charge. It has no
known components or substructure, and therefore is believed to be an elementary particle. An
electron has a mass that is approximately 1/1836 that of the proton.
13

23. The intrinsic angular momentum of the electron is as to tunnel from the valence band of the
p-type material to the conduction band of the n-type material. In the atomic scale, this
tunneling corresponds to the transport of valence band electrons into the empty conduction
band states; as a result of the reduced barrier between these bands and high electric fields that
are induced due to the relatively high levels of dopings on both sides. The breakdown voltage
can be controlled quite accurately in the doping process. While tolerances within 0.05% are
available, the most widely used tolerances are 5% and 10%. Breakdown voltage for
commonly available zener diodes can vary widely from 1.2 volts to 200 volts.
Another mechanism that produces a similar effect is the avalanche effect as in the avalanche
diode Avalanche diode An avalanche diode is a diode that is designed to go through
avalanche breakdown at a specified reverse bias voltage and conduct as a type of voltage
reference..... The two types of diode are in fact constructed the same way and both effects are
present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the
predominant effect and shows a marked negative temperature coefficient . Above 5.6 volts,
the avalanche effect Avalanche breakdown Avalanche breakdown - is a phenomenon that can
occur in both insulating and semiconducting materials. It is a form of electric current
multiplication that can allow very large currents to flow within materials which are otherwise
good insulators. It is a type of electron avalanche.- Explanation... becomes predominant and
exhibits a positive temperature coefficient. In a 5.6 V diode, the two effects occur together
and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the
component of choice in temperature-critical applications. Modern manufacturing techniques
have produced devices with voltages lower than 5.6 V with negligible temperature
coefficients, but as higher voltage devices are encountered, the temperature coefficient rises
dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.
All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella
term of "Zener diode".
14

24. V-I Characteristics of zener diode:-
15

25. 9.3 DIODE
Symbol
Diode Function Diodes allow electricity to flow in only one direction. The arrow of the
circuit symbol shows the direction in which the current can flow. Diodes are the electrical
version of a valve and early diodes were actually called valves.
Forward Voltage Drop Electricity uses up a little energy pushing its way through the diode,
rather like a person pushing through a door with a spring. This means that there is a small
voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for
all normal diodes which are made from silicon. The forward voltage drop of a diode is almost
constant whatever the current passing through the diode so they have a very steep
characteristic (current- voltage graph).
Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all
real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits
because it will be very much smaller than the current flowing in the forward direction.
However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is
exceeded the diode will fail and pass a large current in the reverse direction, this is called
breakdown.
16

26. 9.3.1 Diode Construction
The physical construction of a diode with a diffusion junction is shown in the figure below.
When a diode is reverse biased ie. a positive voltage is applied to the cathode with respect to
the anode, an electric field is formed between the cathode and anode specifically across the
depletion region. The diode is 'reverse biased' and cannot conduct except for small leakage
currents. However, if the electric field becomes too strong 'avalanche breakdown' occurs and
the diode will become a short circuit and often be damaged. To counteract this the physical
distance between the anode and cathode is increased by increasing the size of the bulk region
and changing impurity atom doping levels.
In the construction process, N type silicon substrate heated to ~1000oC in presence of vapour
containing positive charged impurity atoms. P region diffused into N region. The resultant
effect is to cause more charge carriers to be present within the diode when it is conducting.
For the diode to switch OFF, the charge carriers must either recombine (minority) or be
removed, the latter mechanism appearing as a reverse current (reverse recovery) flowing in
the diode as it turns OFF. Put simply, diodes with higher voltage ratings have larger bulk
regions, require more time to remove internal charges at turn OFF and are thus slower
switching.
17

28. In the construction process, N type silicon substrate heated to ~1000oC in presence of vapour
containing positive charged impurity atoms. P region diffused into N region. The resultant
effect is to cause more charge carriers to be present within the diode when it is conducting.
For the diode to switch OFF, the charge carriers must either recombine (minority) or be
removed, the latter mechanism appearing as a reverse current (reverse recovery) flowing in
the diode as it turns OFF. Put simply, diodes with higher voltage ratings have larger bulk
regions, require more time to remove internal charges at turn OFF and are thus slower
switching.
18

29. 9.4 RECTIFIERS
9.4.1 Standard Rectifiers
Rectifiers are electronic high voltage diodes, which allow current to flow in only one
direction. Essentially, they act as one-way valves, and are used to convert AC current to DC
current.
The performance of high voltage diodes is determined by a number of voltage, current and
time coefficients:
VRRM: Maximum Reverse Voltage, which is the maximum reverse voltage of the diode.
VF: Forward Voltage, which is the voltage across the diode terminals resulting from the flow
of current in the forward direction.
IR: Reverse Current flows when reverse bias is applied to a semiconductor junction.
trr: Reverse Recovery Time is the time required for the current to reach a specified reverse
current (IR) after instantaneous switching from a specified forward condition (IF).
IF: Forward Current is the current flowing through the diode in the direction of lower
resistance.
Tj: Junction Operating Temperature is the range of temperatures in which the high voltage
diodes are designed to operate.
8.4.2 Fast Rectifiers
Figure 3a and b show typical styles of reverse recovery. The area within the negative portion
of each curve, , is the total reverse recovery charge Qrr and represents the charge removal
19
from the junction and the bulk regions of the diode and is effectively independent of the

30. forward current in the diode. The recovery time t2 - t1 is dependent on the size of the bulk
region thus high di/dt currents can be obtained when using fast diodes. If the di/dt of the snap
recovery is too high and stray inductance exists in the circuit then extremely high and
possibly damaging voltage spikes can be induced.
(Note: ). Qrr can be found from manufacturers specifications thus the maximum reverse
recovery current Irr is given by:
If ta is very small compared to ta then ta trr and knowing the rate of decrease of current di/dt
= Irr/ta Irr/trr leads to:
Figure 3: (a) Reverse recovery of a general purpose diode, (b) fast diode. Reverse recovery
time trr = t2 - t0.
The effect of reverse recovery on the output voltage of a rectifier feeding a resistive load is
shown in figure 4.
Figure 4: Bridge rectifier output voltage showing diode reverse recovery effects.
20
9.4.3 Ultra Fast Rectifiers

31. International Rectifier's new series of Ultra-fast recovery diodes are aimed specifically at the
12/24/48V SMPS output stage, and extend the company's current product range of Ultra-fast
recovery diodes with industry standard part number products. The new product series has
been developed to meet today's requirement of high frequency operation and power ratings,
using a technology platform flexible enough to match the performance improvement curve of
the market requirements in the years to come. The new IR Ultra-fast recovery diode series
(200-400V) adopts platinum diffusion in order to overcome the limitation of gold diffusion
and the electron irradiation technology. With this approach, the best trade off for leakage
current, forward voltage drop and reverse recovery, has been achieved with a maximum
operating junction temperature of 175 degrees Celsius and a reverse recovery time as low as
15-20ns. With this type of performance, the maximum allowable switching frequency for this
Ultra- fast diode family would be up to 500-750kHz. This assumption is verified by the diode
loss calculation used for the IR MUR1620 operating in a typical output rectification in a
forward converter.
21
9.5 RELAY

32. A relay is an electrically operated switch. Many relays use an electromagnet to mechanically
operate a switch, but other operating principles are also used, such as solid-state relays.
Relays are used where it is necessary to control a circuit by a low-power signal (with
complete electrical isolation between control and controlled circuits), or where several
circuits must be controlled by one signal. The first relays were used in long distance telegraph
circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted
it on another circuit. Relays were used extensively in telephone exchanges and early
computers to perform logical operations.
A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an
iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature,
and one or more sets of contacts (there are two in the relay pictured). The armature is hinged
to the yoke and mechanically linked to one or more sets of moving contacts. It is held in
place by a spring so that when the relay is de-energized there is an air gap in the magnetic
circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and
the other set is open. Other relays may have more or fewer sets of contacts depending on their
function. The relay in the picture also has a wire connecting the armature to the yoke. This
ensures continuity of the circuit between the moving contacts on the armature, and the circuit
track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.
9.5.1 Operation:
When an electric current is passed through the coil it generates a magnetic field that activates
the armature, and the consequent movement of the movable contact(s) either makes or breaks
(depending upon construction) a connection with a fixed contact. If the set of contacts was
closed when the relay was de-energized, then the movement opens the contacts and breaks
the connection, and vice versa if the contacts were open. When the current to the coil is
switched off, the armature is returned by a force, approximately half as strong as the
magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity
is also used commonly in industrial motor starters. Most relays are manufactured to operate
quickly. In a low-voltage application this reduces noise; in a high voltage or current
application it reduces arcing.
22
When the coil is energized with direct current, a diode is often placed across the coil to

33. dissipate the energy from the collapsing magnetic field at deactivation, which would
otherwise generate a voltage spike dangerous to semiconductor circuit components. Some
automotive relays include a diode inside the relay case. Alternatively, a contact protection
network consisting of a capacitor and resistor in series (snubbercircuit) may absorb the surge.
If the coil is designed to be energized with alternating current (AC), a small copper "shading
ring" can be crimped to the end of the solenoid, creating a small out-of-phase current which
increases the minimum pull on the armature during the AC cycle.
9.5.2 Application:
Switching to a standby power supply.
Amplifying a digital signal, switching a large amount of power with a small operating power.
Some special cases are:
A telegraph relay, repeating a weak signal received at the end of a long wire
Controlling a high-voltage circuit with a low-voltage signal, as in some types of
modems or audio amplifiers,
Controlling a high-current circuit with a low-current signal, as in the starter
solenoid of an automobile,
Detecting and isolating faults on transmission and distribution lines by opening and closing
circuit breakers (protection relays),
A DPDT AC coil relay with
"ice cube" packaging
23
Isolating the controlling circuit from the controlled circuit when the two are at different
potentials, for example when controlling a mains-powered device from a low-voltage switch.
The latter is often applied to control office lighting as the low voltage wires are easily
installed in partitions, which may be often moved as needs change. They may also be
controlled by room occupancy detectors to conserve energy.
Logic functions. For example, the boolean AND function is realised by connecting normally

34. open relay contacts in series, the OR function by connecting normally open contacts in
parallel. The change-over or Form C contacts perform the XOR (exclusive or) function.
Similar functions for NAND and NOR are accomplished using normally closed contacts. The
Ladder programming language is often used for designing relay logicnetworks.
 The application of Boolean Algebra to relay circuit design was formalized by
Claude Shannon in A Symbolic Analysis of Relay and Switching Circuits
 Early computing. Before vacuum tubes and transistors, relays were used as
logical elements in digital computers. See electro-mechanical computers such as
the ARRA, Harvard Mark II, Zuse Z2, and Zuse Z3.
 Safety-critical logic. Because relays are much more resistant than
semiconductors to nuclear radiation, they are widely used in safety-critical logic,
such as the control panels of radioactive waste-handling machinery.
Time delay functions. Relays can be modified to delay opening or delay closing a set of
contacts. A very short (a fraction of a second) delay would use a copper disk between the
armature and moving blade assembly. Current flowing in the disk maintains magnetic field
for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a
dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. The
time period can be varied by increasing or decreasing the flow rate. For longer time periods, a
mechanical clockwork timer is installed.
Vehicle battery isolation. A 12v relay is often used to isolate any second battery in cars,
4WDs, RVs and boats.
24
9.6 TRANSFORMER
A transformer consists of two coils (often called 'windings') linked by an iron core, as shown
in figure 1. There is no electrical connection between the coils, instead they are linked by a
magnetic field created in the core.
Transformers are used to convert electricity from one voltage to another with minimal loss of

35. power. They only work with AC (alternating current) because they require a changing
magnetic field to be created in their core.
Transformers can increase voltage (step-up) as well as reduce voltage (step- down).
Alternating current flowing in the primary (input) coil creates a continually changing
magnetic field in the iron core. This field also passes through the secondary (output) coil and
the changing strength of the magnetic field induces an alternating voltage in the secondary
coil. If the secondary coil is connected to a load the induced voltage will make an induced
current flow. The correct term for the induced voltage is 'induced electromotive force' which
is usually abbreviated to induced e.m.f.
The iron core is laminated to prevent 'eddy currents' flowing in the core. These are currents
produced by the alternating magnetic field inducing a small voltage in the core, just like that
induced in the secondary coil. Eddy currents waste power by needlessly heating up the core
but they are reduced to a negligible amount by laminating the iron because this increases the
electrical resistance of the core without affecting its magnetic properties.
Transformers have two great advantages over other methods of changing voltage:
1. They provide total electrical isolation between the input and output, so they can be safely
used to reduce the high voltage of the mains supply.
2. Almost no power is wasted in a transformer. They have a high efficiency (power out /
power in) of 95% or more.
25
Mains transformers are the most common type. They are designed to reduce the AC mains
supply voltage (230-240V in the UK or 115-120V in some countries) to a safer low voltage.
The standard mains supply voltages are officially 115V and 230V, but 120V and 240V are
the values usually quoted and the difference is of no significance in most cases.

37. 26

38. To allow for the two supply voltages mains transformers usually have two separate primary
coils (windings) labelled 0-120V and 0-120V. The two coils are connected in series for 240V
(figure 2a) and in parallel for 120V (figure 2b). They must be wired the correct way round as
shown in the diagrams because the coils must be connected in the correct sense (direction):
Most mains transformers have two separate secondary coils (e.g. labelled 0- 9V, 0-9V) which
may be used separately to give two independent supplies, or connected in series to create a
centre-tapped coil (see below) or one coil with double the voltage.
Some mains transformers have a centre-tap halfway through the secondary coil and they are
labelled 9-0-9V for example. They can be used to produce full-wave rectified DC with just
two diodes, unlike a standard secondary coil which requires four diodes to produce full-wave
rectified DC.
A mains transformer is specified by:
1.Its secondary (output) voltages Vs
2. Its maximum power, Pmax, which the transformer can pass, quoted in VA (volt-amp). This
determines the maximum output (secondary) current, Imax. Whereas Vs is the secondary
voltage. If there are two secondary coils the maximum power should be halved to give the
maximum for each coil.
3. Its construction - it may be PCB-mounting, chassis mounting (with solder tag
connections) or toroidal (a high quality design).
8.6.1 STEP DOWN TRANSFORMER
If the first coil has more turns that the second coil, the secondary voltage is smaller than the
primary voltage. This is called a step-down transformer. If the second coil has half as many
turns as the first coil, the secondary voltage will be half the size of the primary voltage if the
second coil has one tenth as many turns, it has one tenth the voltage.
27

39. In general:
Secondary voltage ÷ Primary voltage = Number of turns in secondary ÷ Number of turns in
primary
The current is transformed the opposite way—increased in size—in a step- down transformer:
Secondary current ÷ Primary current = Number of turns in primary ÷ Number of turns in
secondary
So a step-down transformer with 100 coils in the primary and 10 coils in the secondary will
reduce the voltage by a factor of 10 but multiply the current by a factor of 10 at the same
time. The power in an electric current is equal to the current times the voltage (watts = volts x
amps is one way to remember this), so you can see the power in the secondary coil is
theoretically the same as the power in the primary coil. (In reality, there is some loss of
power between the primary and the secondary because some of the "magnetic flux" leaks out
of the core, some energy is lost because the core heats up, and so on.)
28

40. 9.7 CAPACITOR
A capacitor (originally known as a condenser) is a passive two-terminal electrical component
used to store energyelectrostatically in an electric field. The forms of practical capacitors vary
widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e.,
insulator). The conductors can be thin films of metal, aluminum foil or disks, etc. The
'nonconducting' dielectric acts to increase the capacitor's charge capacity. A dielectric can be
glass, ceramic, plastic film, air, paper, mica, etc. Capacitors are widely used as parts of
electrical circuits in many common electrical devices. Unlike aresistor, a capacitor does not
dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field
between its plates.
When there is a potential difference across the conductors (e.g., when a capacitor is attached
across a battery), an electric field develops across the dielectric, causing positive charge (+Q)
to collect on one plate and negative charge (-Q) to collect on the other plate. If a battery has
been attached to a capacitor for a sufficient amount of time, no current can flow through the
capacitor. However, if an accelerating or alternating voltage is applied across the leads of the
capacitor, a displacement current can flow.
The symbol for a capacitor used in schematic diagrams of electronic circuits looks very much
like a parallel-plate model.
Symbol
29

41. An ideal capacitor is characterized by a single constant value for its capacitance. Capacitance
is expressed as the ratio of theelectric charge (Q) on each conductor to the potential
difference (V) between them. The SI unit of capacitance is the farad (F), which is equal to
one coulomb per volt (1 C/V). Typical capacitance values range from about 1 pF (10−12
F) to
about 1 mF (10−3
F).
The capacitance is greater when there is a narrower separation between conductors and when
the conductors have a larger surface area. In practice, the dielectric between the plates passes
a small amount of leakage current and also has an electric field strength limit, known as the
breakdown voltage. The conductors and leads introduce an undesired inductance and
resistance.
Capacitors are widely used in electronic circuits for blocking direct current while allowing
alternating current to pass. In analog filternetworks, they smooth the output of power
supplies. In resonant circuits they tune radios to particular frequencies. In electric power
transmission systems they stabilize voltage and power flow

42. 9.7.1 Operation:
A capacitor consists of two conductors separated by a non-conductive region.[10]
The non-
conductive region is called the dielectric. In simpler terms, the dielectric is just an electrical
insulator. Examples of dielectric media are glass, air, paper, vacuum, and even a
semiconductordepletion region chemically identical to the conductors. A capacitor is
assumed to be self-contained and isolated, with no net electric charge and no influence from
any external electric field. The conductors thus hold equal and opposite charges on their
facing surfaces,[11]
and the dielectric develops an electric field. In SI units, a capacitance of
one farad means that one coulomb of charge on each conductor causes a voltage of one volt
across the device.[12]
An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of
charge ±Q on each conductor to the voltage V between them:[10]
30
Because the conductors (or plates) are close together, the opposite charges on the conductors
attract one another due to their electric fields, allowing the capacitor to store more charge for
a given voltage than if the conductors were separated, giving the capacitor a large
capacitance.
Sometimes charge build-up affects the capacitor mechanically, causing its capacitance to
vary. In this case, capacitance is defined in terms of incremental changes:

43. 31
9.8 TRANSISTOR
The pnp transistor works essentially the same as the npn transistor. However, since the
emitter, base, and collector in the pnp transistor are made of materials that are different from
those used in the npn transistor, different current carriers flow in the pnp unit. The majority
current carriers in the pnp transistor are holes. This is in contrast to the npn transistor where
the majority current carriers are electrons. To support this different type of current (hole
flow), the bias batteries are reversed for the pnp transistor.

44. 32

45. Notice that the procedure used earlier to properly bias the npn transistor also applies here to
the pnp transistor. The first letter (p) in the pnp sequence indicates the polarity of the voltage
required for the emitter (positive), and the second letter (n) indicates the polarity of the base
voltage (negative). Since the base-collector junction is always reverse biased, then the
opposite polarity voltage (negative) must be used for the collector. Thus, the base of the pnp
transistor must be negative with respect to the emitter, and the collector must be more
negative than the base. Remember, just as in the case of the npn transistor, this difference in
supply voltage is necessary to have current flow (hole flow in the case of the pnp transistor)
from the emitter to the collector. Although hole flow is the predominant type of current flow
in the pnp transistor, hole flow only takes place within the transistor itself, while electrons
flow in the external circuit. However, it is the internal hole flow that leads to electron flow in
the external wires connected to the transistor.

46. 33
Now let us consider what happens when the emitter-base junction is forward biased. With the
bias setup shown, the positive terminal of the battery repels the emitter holes toward the base,
while the negative terminal drives the base electrons toward the emitter. When an emitter
hole and a base electron meet, they combine. For each electron that combines with a hole,
another electron leaves the negative terminal of the battery, and enters the base. At the same
time, an electron leaves the emitter, creating a new hole, and enters the positive terminal of
the battery. This movement of electrons into the base and out of the emitter constitutes base
current flow (IB), and the path these electrons take is referred to as the emitter-base circuit.
In the reverse-biased junction , the negative voltage on the collector and the positive voltage
on the base block the majority current carriers from crossing the junction.
However, this same negative collector voltage acts as forward bias for the minority current
holes in the base, which cross the junction and enter the Collector.
The minority current electrons in the collector also sense forward bias-the positive base
voltage-and move into the base. The holes in the collector are filled by electrons that flow
from the negative terminal of the battery. At the same time the electrons leave the negative
terminal of the battery, other electrons in the base break their covalent bonds and enter the
positive terminal of the battery. Although there is only minority current flow in the reverse-
biased junction, it is still very small because of the limited number of minority current
carriers.
The interaction between the forward- and reverse-biased junctions in a pnp transistor is very
similar to that in an npn transistor, except that in the pnp transistor, the majority current
carriers are holes. In the pnp transistor shown in figure 4, the positive voltage on the emitter
repels the holes toward the base. Once in the base, the holes combine with base electrons. But
again, remember that the base region is made very thin to prevent the recombination of holes
with electrons. Therefore, well over 90 percent of the holes that enter the base become
attracted to the large negative collector voltage and pass right through the base. However, for
each electron and hole that combine in the base region, another electron leaves the negative

47. terminal of the base battery (V BB) and enters the base as base current (IB). At the same time
an electron leaves the negative terminal of the battery, another electron leaves the emitter as
IE (creating a new hole) and enters the positive terminal of VBB.
34
Meanwhile, in the collector circuit, electrons from the collector battery (VCC) enter the
collector as Ic and combine with the excess holes from the base. For each hole that is
neutralized in the collector by an electron, another electron leaves the emitter and starts its
way back to the positive terminal of VCC.
Although current flow in the external circuit of the pnp transistor is opposite in direction to
that of the npn transistor, the majority carriers always flow from the emitter to the collector.
This flow of majority carriers also results in the formation of two individual current loops
within each transistor. One loop is the base-current path, and the other loop is the collector-
current path. The combination of the current in both of these loops (IB + IC) results in total
transistor current (IE). The most important thing to remember about the two different types of
transistors is that the emitter-base voltage of the pnp transistor has the same controlling effect
on collector current as that of the npn transistor. In simple terms, increasing the forward-bias
voltage of a transistor reduces the emitter-base junction barrier. This action allows more
carriers to reach the collector, causing an increase in current flow from the emitter to the
collector and through the external circuit. Conversely, a decrease in the forward-bias voltage
reduces collector current.

48. 35
9.9 VOLTAGE REGULATOR
A voltage regulator is designed to automatically maintain a constant voltage level. A voltage
regulator may be a simple "feed-forward" design or may include negative feedback control
loops. It may use an electromechanical mechanism, or electronic components. Depending on
the design, it may be used to regulate one or more AC or DC voltages.
Electronic voltage regulators are found in devices such as computer power supplies where
they stabilize the DC voltages used by the processor and other elements. In automobile
alternators and central power station generator plants, voltage regulators control the output of
the plant. In an electric power distribution system, voltage regulators may be installed at a
substation or along distribution lines so that all customers receive steady voltage independent
of how much power is drawn from the line.
9.9.1 Electronic Regulator circuits
A simple voltage regulator can be made from a resistor in series with a diode§ (or series of
diodes). Due to the logarithmic shape of diode V-I curves, the voltage across the diode
changes only slightly due to changes in current drawn or changes in the input. When precise
voltage control and efficiency are not important, this design may work fine.
Feedback voltage regulators operate by comparing the actual output voltage to some fixed
reference voltage. Any difference is amplified and used to control the regulation element in
such a way as to reduce the voltage error. This forms a negative feedback§ control loop§;
increasing the open-loop gain§ tends to increase regulation accuracy but reduce stability
(stability is avoidance of oscillation, or ringing, during step changes). There will also be a
trade-off between stability and the speed of the response to changes. If the output voltage is
too low (perhaps due to input voltage reducing or load current increasing), the regulation
element is commanded, up to a point, to produce a higher output voltage–by dropping less of
the input voltage (for linear series regulators and buck§ switching regulators§), or to draw
input current for longer periods (boost-type switching regulators§); if the output voltage is
too high, the regulation element will normally be commanded to produce a lower voltage.
However, many regulators have over-current protection, so that they will entirely stop
sourcing current (or limit the current in some way) if the output current is too high, and some
regulators may also shut down if the input voltage is outside a given range (see also: crowbar
circuits§).

49. 36

50. 9.9.2 IC 7812:
The 78xx (sometimes L78xx, LM78xx, MC78xx...) is a family of self-contained fixed linear
voltage regulator§ integrated circuits§. The 78xx family is commonly used in electronic
circuits requiring a regulated power supply due to their ease-of-use and low cost. For ICs
within the family, the xx is replaced with two digits, indicating the output voltage§ (for
example, the 7805 has a 5 volt output, while the 7812 produces 12 volts). The 78xx line are
positive voltage regulators: they produce a voltage that is positive relative to a common
ground. There is a related line of 79xx devices which are complementary negative voltage
regulators. 78xx and 79xx ICs can be used in combination to provide positive and negative
supply voltages in the same circuit.
78xx ICs have three terminals and are commonly found in the TO220§ form factor, although
smaller surface-mount and larger TO3§ packages are available. These devices support an
input voltage anywhere from a few volts over the intended output voltage, up to a maximum
of 35 to 40 volts depending on the make, and typically provide 1 or 1.5 amperes§ of current§
(though smaller or larger packages may have a lower or higher current rating).
9.9.3 Advantages:
78xx series ICs do not require additional components to provide a constant, regulated source
of power, making them easy to use, as well as economical and efficient uses of space. Other
voltage regulators may require additional components to set the output voltage level, or to
assist in the regulation process. Some other designs (such as aswitched-mode power supply§)
may need substantial engineering expertise to implement.
These ICs have built-in protection against a circuit drawing too much power. They have
protection against overheating and short-circuits, making them quite robust in most
applications. In some cases, the current-limiting features of the 78xx devices can provide
protection not only for the 78xx itself, but also for other parts of the circuit.
9.9.4 Disadvantages:-
The input voltage must always be higher than the output voltage by some minimum amount
(typically 2.5 volts). This can make these devices unsuitable for powering some devices from
certain types of power sources (for example, powering a circuit that requires 5 volts using 6-
volt batteries will not work using a 7805).
37

51. As they are based on a linear regulator§ design, the input current required is always the same
as the output current. As the input voltage must always be higher than the output voltage, this
means that the total power (voltage multiplied by current) going into the 78xx will be more
than the output power provided. The extra input power is dissipated as heat. This means both
that for some applications an adequate heatsink§ must be provided, and also that a (often
substantial) portion of the input power is wasted during the process, rendering them less
efficient than some other types of power supplies. When the input voltage is significantly
higher than the regulated output voltage (for example, powering a 7805 using a 24 volt power
source), this inefficiency can be a significant issue.
Voltage
Regulator IC
38

52. 9.10 OPTOCOUPLER
In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical
isolator, is a component that transfers electrical signals between two isolated circuits by
using light.[1]
Opto-isolators prevent high voltages from affecting the system receiving the
signal.[2]
Commercially available opto-isolators withstand input-to-output voltages up to 10
kV[3]
and voltage transients with speeds up to 10 kV/μs.[4]
A common type of opto-isolator consists of an LED and a phototransistor in the same opaque
package. Other types of source-sensor combinations include LED-photodiode, LED-LASCR,
and lamp-photoresistor pairs. Usually opto-isolators transfer digital (on-off) signals, but some
techniques allow them to be used with analog signals.
8.10.1 Operation:-
An opto-isolator contains a source (emitter) of light, almost always a near infrared light-
emitting diode (LED), that converts electrical input signal into light, a closed optical channel
(also called dielectrical channel[7]
), and a photosensor, which detects incoming light and
either generates electric energy directly, or modulates electric current flowing from an
external power supply.opto-isolator can transfer the light signal not transfer the electrical
signal . The sensor can be a photoresistor, a photodiode, a phototransistor, a silicon-
controlled rectifier (SCR) or a triac. Because LEDs can sense light in addition to emitting it,
construction of symmetrical, bidirectional opto-isolators is possible. An optocoupledsolid
state relay contains a photodiode opto-isolator which drives a power switch, usually a
complementary pair of MOSFETs. A slotted optical switch contains a source of light and a
sensor, but its optical channel is open, allowing modulation of light by external objects
obstructing the path of light or reflecting light into the sensor.
39

53. Symbolic diagram

54. 40

55. 10. PCB DESIGN
A Printed Circuit Board mechanically supports and electrically connects electronic
components§ using conductive§ tracks, pads and other features etched from copper sheets
laminated onto a non-conductive substrate . PCBs can be single sided (one copper layer),
double sides (two copper layers) or multi-layer. Conductors on different layers are connected
with plated-through holes calledvias. Advanced PCBs may contain components - capacitors,
resistors or active devices - embedded in the substrate.
Printed circuit boards are used in all but the simplest electronic products. Alternatives to
PCBs include wire wrap and point-to-point construction. PCBs require the additional design
effort to lay out the circuit but manufacturing and assembly can be automated. Manufacturing
circuits with PCBs is cheaper and faster than with other wiring methods as component are
mounted and wired with one single part. Furthermore, operator wiring errors are eliminated.
10.1 PCB DESIGN PROCEDURE
• PCB preparation can be done using the following steps:
• Prepare the PCB layout of the circuit in a graph sheet
• Cut the copper clad sheet in proper dimension and wash it.
• Trace the PCB layout on the copper clad sheet
• Prepare the ferric chloride solution.
• Dip the PCB in to ferric chloride solution for etching non printed surfaces.
• Wash cleanly with detergents.
• Drill the holes in necessary positions.
41

56. 10.2 PCB PREPARATION
• You need to generate a positive (copper black) UV translucent artwork film. You will never
get a good board without good artwork, so it is important to get the best possible quality at
this stage the most important thing is to get a clear sharp image with a very solid opaque
black. Art work is done using ORCAD software. It is absolutely essential that your PCB
software prints holes in the middle of pads, which will act as centre marks when drilling. It is
virtually impossible to accurately hand-drill boards without these holes. If you are looking to
buy PCB software at any cost level and want to do hand-pro-typing of boards before
production, check that this facility is available when defining pad and line shapes, the
minimum size recommended (through-linking holes) for reliable result is 50 mil, assuming
0.8mm drill size; 1 mil=(1/1000)d‘ of an inch.
You can go smaller drill sizes, but through linking will be harder. 65 mil round or square
pads for normal components.
ICs, with 0.8 mm hole, will allow a 12.5 down to 10mil if you really need to. Centre-to-centre
spacing of12.5 mil tracks should be 25 mil-slightly less may be possible if your printer can
manage it. Take care to preserve the correct diagonal track-track spacing on mitered corners;
grid is 25 mil and track width 12.5mil. the art work must be printed such that the printed side
is in contact with PCB surface when exposing, to avoid blurred edges. In practice, this means
that if you design the board as seen from the component side, the bottom (solder side) layer
should be printed the "correct" way round, and top side of the double-sided board must be
printed mirrored.
42

57. 10.3 Eching:
Ferric chloride etchant is a messy stuff, but easily available and cheaper than most
alternatives. It attacks any metal including stainless steel. So when setting up a PCB etching
area, use a plastic or ceramic sink, with plastic fitting and screws wherever possible, and seal
any metal screws with silicon. Copper water pipes may be splashed or dripped-on, so sleeve
or cover them in plastic; heat-shrink sleeving is great if you are installing new pipes. Fumes
extraction is not normally required, although a cover over the tank or tray when not in use is a
good idea. You should always use the hex hydrate type of ferric chloride, which should be
dissolved in warm water until saturation. Adding a teaspoon of table salt helps to make the
enchant clearer for easier inspection. Avoid anhydrous ferric chloride. It creates a lot of heat
when dissolved. So always add the powder very slowly to water; do not add water to the
powder, and use gloves and safety glasses. The solution made from anhydrous ferric chloride
doesn’t etch at all, so you need to add a small amount of hydrochloric acid and leave it for a
day or two.
Always take extreme care to avoid splashing when dissolving either type of ferric chloride,
acid tends to clump together and you often get big chunks coming out of the container and
splashing into the solution. It can damage eyes and permanently stain clothing if you are
making PCBs in a professional environment where time is money you should get heated
bubble-etch tank.With fresh hot ferric chloride, the PCB will etch in well under 5 min. Fat
etching produces better edge-quality and consistent line widths. If you aren’t using a bubble
tank, you need to agitate frequently to ensure ever etching. Warm the etchant by putting the
etching tray inside a larger tray filled with boiling water.
43

58. 10.4 DRILLING
If you have fibre glass (FR4) board, you must use tungsten carbide drill bits. Fibre glass eats
normal high-speed steel (HSS) bits very rapidly, although HSS drills are alright for older
larger sizes (>2mm). Carbide drill bits are available as straight-shank or thick-shank. In
straight shank. In straight shank, the hole bit is the diameter of the hole, and in thick shank, a
standard size (typically about 3.5mm) shank tapers down to the hole size.
The straight hank drills are usually preferred because they break less easily and are usually
cheaper. The longer thin section provides more flexibility. Small drills for PCB use usually
come with either a set collets of various sizes or a three-jaw chuck. Sometimes the 3-jaw
chuck is and optional extra and is worth getting for the time it saves on changing collets. For
accuracy, however, 3-jaw chucks are not brilliant, and small drill sixes below 1 mm quickly
formed grooves in the jaws, preventing good grip. Below l mm, you should use collets, and
buy a few extra of the smallest ones; keeping one collect per drill size as using a larger should
use collets, and buy a tew extra of the smallest ones; keeping one collect per drill size as
using a larger drill in a collect will open it out and it no longer grips smaller drills well.
You need a good strong light on the board when drilling, to ensure accuracy. A di-chloric
halogen lamp, under run at 9v to reduce brightness, can be useful to raise the working surface
above 15 cm above the normal desk height for more comfortable viewing. Dust extraction is
nice, but not essential and occasional blow does the trick! A foot-pedal control to switch the
drill ‘off’ and ‘on’ is very convenient, especially when frequently changing bits. Avoid hole
sizes less than 0.8 mm unless you really need them. When making two identical boards, drill
them both together to save time. To do this, carefully drill a 0.8mm whole in the pad near
each comer of each of the two boards, drill a hole near the centre of each side as well. Lay the
boards on the top of each other and insert a 0.8 mm track pin in two opposite comers, using
the pins as pegs to line the PCBs up. Squeeze or hammer the pins into boards, and then into
the remaining holes. The two PCBs are now ‘nailed’ together accurately and can be drilled
together.
44

59. 10.5 SOLDERING
Soldering is the process of joining two or more similar or dissimilar metals by melting
another metal having lower melting point.
10.5.1 SOLDERING FLUXES
In order to make the surfaces accept the solder readily,the component ter1ninals should be
free from oxides and other obstructing films.Soldering flux cleans the oxides from the surface
of the metal. Zinc chloride ammonium chloride and rosin are commonly used fluxes.
10.5.2 SOLDER HEATING
Solder is used for joining two or more metals at temperature below their melting point.The
popularly used solders are alloys of tin(60%) and lead(40%) that melts at 375F and it
solidifies when it cools.
11.3 SOLDERING IRON
It is used to melt the solder and apply at the joints in the circuit. Many temperature control
soldering iron designed for electronics have a power rating of around 40 to 50 watt. They will
heat fast and give enough power for operation but are mechanically small.
You will occasionally see gas-powered soldering irons which use butane rather than the main
electrical supply to operate. They have a catalytic element which once warmed up, continues
to glow hot when gas passes over them. Gas powered soldering irons are designed for
occasional ‘on the spot’ used for quick repairs, rather than for main stream construction or for
assembly work.
Currently, the best commonly available, workable, and safe solder alloy is 63/37. That is,
63% lead, 37% tin. It is also known as eutectic solder. Its most desirable characteristic is that
is solids (‘pasty’) state, and its liquid state occur at the same temperature -361 degree
Fahrenheit. The combination of 63% lead and 37% tin melts at the lowest possible
temperature. Nowadays there is tendency to move to use lead free solders, but it will take
years until they catch on normal soldering work. Lead free solders are nowadays available,
but they are generally more expensive or harder to work on than traditional solder that they
have lead in them.
The metals involved are not the only things to consider in a solder. Flux is vital to good
solder joint. Flux is an aggressive chemical that removes oxide and impurities from the parts
to be soldered. The chemical reactions at points if connection must take place for the metal to
fuse. RMA type flux (Rosin Mildly Active) is the least corrosive of the readily available
materials, and provides an adequate oxide removal.
45

60. In electronics, a 60/40 fixed core solder is used. This consists of 60% lead and 40% tin, with
flux cores added to the length of solder.
There are certain safety measures which you should keep in mind when soldering. The tin
material used soldering contains dangerous substance like lead (40-60% of typical soldering
tins are lead and lead poisonous). Also the various fumes from the soldering flux can be
dangerous. while it is true that lead do not vaporize at the temperature at which soldering is
typically done.
46

61. 11.COST ESTIMATION

62. s.no Component Component details quantity Unit
cost(in
Rs.)
Total (in
Rs.)
1. LED Light emitting
diode
2 1.00 2.00
2. Resistors 10k,470 ohm 7 1.00 7.00
3. Diode 1n4007 13 1.00 13.00
4. Optocoupler(1) Mc2te, 2410 1 30.00 30.00
5. Optocoupler(2) Mct2e,2430 1 40.00 40.00
6. Voltage regulator 7812 1 15.00 15.00
7. Transistor 1 7.00 7.00
8. Relay 1 20.00 20.00
9. Comparator Lm358p 1 17.00
17.00
10. Transformers 220kv/12v 2 60.00 120.00
11. Capacitor (1-3) Electrolytic
(big)1000 microF
3 7.00 21.00
12 Capacitor Electrolytic
(small)100 microF
1 4.00 4.00
13 Wires simple 3 15.00 45.00
14 PCB Printed circuit
board
1 15 15
15. bulb 10 watt 1 20.00 20.00
16. switches 3 4.00 12.00
17. IC bases 8 pin 3 6.00 18.00
Total Amt: 406.00

63. 48
12. ADVANTAGES AND APPLICATIONS
ADVANTAGES:
1) More reliable
2) Less maintenance
3) Low cost
APPLICATIONS:
1) Residential. 
2) Commercial offices. 
3) Factories operating with 1 phase machineries. 
4) Hospitals/Banks/Institutions.

64. 5) It automatically supplies voltage in case of power failure or low voltage in up to 2 of the
3 incoming phases.
6) Automatic Phase Changer automatically cuts supply during low voltage thus, protects
equipment from the harmful affects of unhealthily low voltage.
49

65. 13. FUTURE SCOPE AND CONCLUSION
FUTURE SCOPE:
Automatic phase changer finds wide application in modem world.During earlier days,if there
is a power failure in any one of three phase,we have to manually switch to the available
phase.By implementing automatic phase changer it automatically shifts to the phase where
correct voltage is available.It can be used in residences,small offices,buildings etc.
CONCLUSION:
In three phase application if low voltage is available in any one phase,and you want your
equipment to work on normal voltage, this automatic phase changer will solve your problem.
This device is more reliable, is of less cost and maintenance free.
50

66. REFERENCES
 http://www.allbookez.com/
 en.wikipedia.org/
 http://www.roshanengineeringcorporation.com/
 en.wikipedia.org/wiki/Voltage_regulator_module