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AN
INDUSTRIAL ORIENTED MINI PROJECT
ON
“SELF SWITCHING POWER SUPPLY”
A Dissertation submitted in partial fulfillment of the academic
requirements for the award of the degree of
BACHELOR OF TECHNOLOGY
IN
ELECTRICAL AND ELECTRONICS ENGINEERING
SUBMITTED
BY
K.SAI KRISHNA GUPTA (15R91A0220)
M.MALLESH (16R95A0217)
Y.SHASHIDHAR REDDY (16R95A0216)
Under the esteemed guidance of
Mr.A.NAGASRIDHAR
Assistant professor
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
TEEGALA KRISHNA REDDY ENGINEERING COLLEGE
(Affiliated to JNTUH, Approved by AICTE, Accredited by NBA & NAAC with „A‟ Grade)
MEERPET, SAROORNAGAR, HYDERABAD – 500097
2018-19

(Affiliated to JNTUH, Approved by AICTE Accredited by NBA & NAAC with „A‟ Grade)
MEERPET, SAROORNAGAR, HYDERABAD - 500097
CERTIFICATE
This is to certify that the Dissertation entitled "SELF SWITCHING POWER
SUPPLY" is a bonafide work done by K.SAI KRISHNA GUPTA (15R91A0220),
M.MALLESH (16R95A0217) , Y.SHASHIDHAR REDDY (16R95A0216) in
partial fulfillment of the academic requirements for the award of the degree of
Bachelor of Technology in ELECTRICAL AND ELECTRONICS ENGINEERING ,
submitted to the Department of Electrical and Electronics Engineering, TEEGALA
KRISHNA REDDY ENGINEERING COLLEGE, Hyderabad during the academic
year 2018-19.
Internal Guide Name
Mr.A.NAGASRIDHAR
Assistant professor
TKREC
Coordinators
Mr. C.SREENIVASULU
Associate Professor
TKREC
HOD-EEE
Mr.T.MADHUBABU
Assistant Professor
TKREC

(Affiliated to JNTUH, Approved by AICTE Accredited by NBA & NAAC with „A‟ Grade)
MEERPET, SAROORNAGAR, HYDERABAD - 500097
CERTIFICATE
This is to certify that the Dissertation entitled "SELF SWITCHING POWER
SUPPLY" is a bonafide work done by K.SAI KRISHNA GUPTA (15R91A0220),
M. MALLESH (16R95A0217), Y.SHASHIDHAR REDDY (16R95A0216) in
partial fulfillment of the academic requirements for the award of the degree of
Bachelor of Technology in ELECTRICAL AND ELECTRONICS ENGINEERING ,
submitted to the Department of Electrical and Electronics Engineering, TEEGALA
KRISHNA REDDY ENGINEERING COLLEGE, Hyderabad during the academic
year 2018-19.
External Examiner :………………….
(Signature with date)
Internal Examiner :………………….
(Signature with date)

DECLARATION
The work presented in this dissertation entitled “SELF SWITCHING POWER
SUPPLY” is original and has been carried out under the supervision of
Mr.A.NAGASRIDHAR Assistant Professor, TKREC
Also, we declare that the matter embodied is thesis has not been submitted in any
full or any part there for the award of any degree of any other institution.
MINI PROJECT ASSOCIATES

ACKNOWLEDGMENT
With great pleasure I want to take this opportunity to express my heartfelt gratitude
to all the people who helped in making this major project work a grand success.
I am grateful to Mr.A.NAGASRIDHAR for his valuable suggestions and
guidance given by him during the execution of this major project work.
I would like to thank Mr.T.MADHUBABU Head of the Department of Electrical
& Electronics Engineering, for being moral support throughout the period of my study in
TKREC.
I am highly indebted to Principal Dr.J.B.V.SUBRAHMANYAM, for giving me
the permission to carry out this Major project.
I would like to thank the Teaching & Non- teaching staff of Department of
Electrical & Electronics Engineering for sharing their knowledge with me.
.
MINI PROJECT ASSOCIATES

ABSTRACT
Embedded system requires a regulated power supply. This power supply circuit
gives a variable regulated supply and switches off in no load condition.
Through an arrangement of voltage regulator 7805 and a potentiometer the output
voltage is varied from 3.7V to 8.7V. Another feature of this power supply is, when no load is
there it automatically switches off. It is achieved through an arrangement of transistors and
relay.
i

INDEX
CHAPTER NO CONTENTS PAGE NO
ABSTRACT i
LIST OF FIGURES ii
LIST OF TABLES iii
1 INTRODUCTION 1
1.1 Introduction 1
2 BLOCK DIAGRAM DISCRIPTION 2
2.1 Block diagram 2
2.2 Block diagram operation 2
3 HARD WARE REQUIREMENTS 4
3.1 List of components 4
3.2 Resister 5
3.3 Capacitor 6
3.4 Voltage regulator 7805 7
3.5 Light Emitting Diode 9
3.6 Transistor 11
3.7 Relay 13
3.8 Potentiometer 14
3.9 Diode 15
3.10 Filter 17
3.11 PCB 18
3.12 Transformer 20

3.13 Rectifier 21
4 SCHEMETIC DIAGRAM 24
4.1 Schematic diagram 24
4.2 Operation 24
5 HARD WARE TESTING 26
6 ADVANTAGES and DISADVANTAGES 28
7 APPLICATIONS 29
8 HARDWARE MODEL 30
CONCLUSION 31
REFERENCES 32

LIST OF FIGURES
FIG. NO FIGURE NAME PAGE NO
2.1 Block diagram of self-switching power supply 2
3.1 Resistor 5
3.2 Capacitor 7
3.3 Voltage regulator 7
3.4 Block diagram LM7805 9
3.5 LED 10
3.6 Transistors 11
3.7 Symbol of transistor 12
3.8 Relay 13
3.9 Relay pin configuration 14
3.10 Potentiometer 14
3.11 Diodes 15
3.12 1N4007 Diode 16
3.13 PCB 19
3.14 Transformer 20
3.15 Diode bridge in various packages 21
3.16 Handmade Diode bridge 21
4.1 Schematic diagram 24
8.1 Hardware model 30
ii

LIST OF TABLES
TABLE NO TABLE NAME PAGE NO
3.1 LIST OF COMPONENTS 4
3.2 MACHINE RATINGS 8
3.3 VOLTAGES OF DIFFERENT LEDS 11
iii

SELF SWITCHING POWER SUPPLY
TKREC 1 DEPARTMENT OF EEE
CHAPTER 1
INTRODUCTION
1.1 INTRODUCTION
The main objective of this project is to develop a circuit which can give a variable
voltage and to switch off itself automatically when no load is connected.
Embedded system requires a regulated power supply. This power supply circuit
gives a variable regulated supply and switches off in no load condition.
Through an arrangement of voltage regulator 7805 and a potentiometer the output
voltage is varied from 3.7V to 8.7V. Another feature of this power supply is, when no load is
there it automatically switches off. It is achieved through an arrangement of transistors and
relay.
In this we will study about components which are using in construction of this circuit
like transformer, voltage regulator, diodes, resistors, capacitors, relays, potentiometer etc.
We also study about construction, operation, advantages, disadvantages, applications
of this circuit.

CHAPTER 2
BLOCK DIAGRAM DISCRIPTION
2.1 BLOCK DIAGRAM
Fig. 2.1 BLOCK DIAGRAM OF SELF SWITCHING POWER SUPPLY
2.2 BLOCK DIAGRAM OPERATION
A regulated power supply circuit that through a fixed voltage regulator U1-LM7805
not only gives a variable but also auto switch off features this is achieved by connecting a
potentiometer between common terminal of regulator IC and Ground for every 100Ω
increment in the in circuit value of the resistance of potentiometer RV1, the output voltage
increases by 1 volt.
Thus, the output varies from 3.7 to 8.7v (taking into account 1.3 volts drop across
diodes D7 and D8).

Another important feature of the supply is that it switches itself off when no load is
connected across its output terminals.
This is achieved with the help of transistors Q1 and Q2, diodes D7and D8 and
capacitor C2. When a load is connected at the output potential drop across diodes D7 and D8
(approximately 1.3v) is sufficient for transistor of Q2 and Q1 to conduct.
As a result, the relay gets energized and remains in that state as long as the load
remains connected. At the same time capacitor C2 gets charged to around 7-8volt potential
through transistor Q2. But when the load (here in series with S2 ) is disconnected, transistors
Q2 is cut off. However, capacitor C2 is still charged and it starts discharging through base of
the transistor Q1.
After some time (which is basically determined by value of C2), relay RL1 is de
energized, which switches off the mains input to primary of transformer TR1.
To resume the power again switch S1 push button should be present momentarily.
Higher the value of capacitor C2 more will be the delay in switching off the power supply on
disconnection of the load, and vise versa.
A transformer which a secondary voltage 12v-0v, 250mA was used it can nevertheless
be changed as per users requirement (up to 30v maximum, and 1A current rating). For
drawing more 300mA current, the regulator IC must be fitted with a small heat sink over a
mica insulator. When the transformers secondary voltage increases beyond 12v RMS,
potentiometer RV1 must be re-dimensioned.
Also, the relay voltage rating should be predetermined.

CHAPTER 3
HARDWARE REQUIREMENTS
3.1 LIST OF COMPONENTS
TABLE 3.1 LIST OF COMPONENTS
COMPONENT NAME QUANTITY
RESISTORS
1KΩ 2
6.2KΩ 1
6.8KΩ 1
CAPACITORS
1000µF 1
100µF 1
10µF 1
MISCELLENEOUS
7805 REGULATOR 2
LED 1
TRANSISTOR (NPN,PNP) EACH 1
12V RELAY 1
POTENTIOMETER (1KΩ) 1
DIODE (1N4007) 3
FILTER 1
PUSH BUTTON SWITCH 2
RECTIFIER 1
DC STEP DOWN TRANSFORMER 1
DIGITAL MULTIMETER 1
PCB 1
CONNECTING WIRES
As per
requirement

3.2 RESISTOR
A resistor is a two-terminal electronic component designed to oppose an electric
current by producing a voltage drop between its terminals in proportion to the current, that is,
in accordance with Ohm's law.
Resistors are used as part of electrical networks and electronic circuits. Practical
resistors can be made of various compounds and films, as well as resistance wire (wire made
of a high-resistivity alloy, such as nickel).
FIG 3.1 RESISTOR
The primary characteristics of resistors are their resistance and the power they can
dissipate. Other characteristics include temperature coefficient, noise, and inductance. Less
well-known is critical resistance, the value below which power dissipation limits the
maximum permitted current flow, and above which the limit is applied voltage. Critical
resistance depends upon the materials constituting the resistor as well as its physical
dimensions; it's determined by design.
Power dissipation:
The power P dissipated by a resistor (or the equivalent resistance of a resistor network
) is calculated as:

The total amount of heat energy released over a period of time can be determined from
the integral of the power over that period of time:
∫ ( ) ( )
Practical resistors are rated according to their maximum power dissipation. Resistors
required to dissipate substantial amounts of power, particularly used in power supplies, power
conversion circuits, and power amplifiers, are generally referred to as power resistors; this
designation is loosely applied to resistors with power ratings of 1 watt or greater. Power
resistors are physically larger and tend not to use the preferred values, color codes, and
external packages described below.
If the average power dissipated by a resistor is more than its power rating, damage to
the resistor may occur, permanently altering its resistance; this is distinct from the reversible
change in resistance due to its temperature coefficient when it warms.
3.3 CAPACITOR
A capacitor or condenser is a passive electronic component consisting of a pair of
conductors separated by a dielectric, used for storing electric charge.
An ideal capacitor is characterized by a single constant value, capacitance, which is
measured in farads. This is the ratio of the electric charge on each conductor to the potential
difference between them. When a voltage potential difference exists between the conductors,
an electric field is present in the dielectric. This field stores energy and produces a mechanical
force between the plates. The effect is greatest between wide, flat, parallel, narrowly separated
conductors.
The properties of capacitors in a circuit may determine the resonant frequency and
quality factor of a resonant circuit, power dissipation and operating frequency in a digital
logic circuit, energy capacity in a high-power system, and many other important aspects.
Capacitors are widely used in electronic circuits for blocking direct current while
allowing alternating current to pass, in filter networks, for smoothing the output of power

supplies, in the resonant circuits that tune radios to particular frequencies and for many other
purposes. The capacitance is therefore greatest in devices made from materials with a high
permittivity.
FIG 3.2 CAPACITORS
3.4 VOLTAGE REGULATOR 7805
A voltage regulator is an electrical regulator 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
FIG 3.3 VOLTAGE REGULATOR
FEATURES:
1) Output Current up to 1A.

2) Output Voltages of 5, 6, 8, 9, 10, 12, 15, 18, 24V.
3) Thermal Overload Protection.
4) Short Circuit Protection.
5) Output Transistor Safe Operating Area Protection.
DESCRIPTION:
The LM78XX/LM78XXA series of three-terminal positive regulators are available in
the TO-220/D-PAK package and with several fixed output voltages, making them useful in a
Wide range of applications. Each type employs internal current limiting, thermal shutdown
and safe operating area protection, making it essentially indestructible. If adequate heat
sinking is provided, they can deliver over 1A output Current. Although designed primarily as
fixed voltage regulators, these devices can be used with external components to obtain
adjustable voltages and currents.
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.
MACHINE RATINGS:
TABLE 3.2 MACHINE RATINGS
PARAMETER SYMBOL VALUE UNIT
Input Voltage a) For V0=5V to 18V
b)For V0=24V
V1
V1
35
40
V
V
Thermal Resistance Junction-Cases(TO-220) RƟJC 5 ˚C/W
Thermal Resistance Junction-Air(TO-220) RƟJA 65 ˚C/W
Operating Temperature Range(KA78XX/A/R) TOPR 0~+125 ˚C

Storage Temperature Range TSTG -65~+150 ˚C
BLOCK DIAGRAM:
FIG 3.4 BLOCK DIAGRAM OF LM7805
3.5 LIGHT EMITTING DIODE
LEDs are semiconductor devices. Like transistors, and other diodes, LEDs are made
out of silicon. What makes an LED give off light are the small amounts of chemical
impurities that are added to the silicon, such as gallium, arsenide, indium, and nitride. When
current passes through the LED, it emits photons as a byproduct
When the diode is forward biased (switched on), electrons are able to recombine with
holes and energy is released in the form of light. This effect is called electroluminescence and
the color of the light is determined by the energy gap of the semiconductor. The LED is
usually small in area (< 1mm2) with integrated optical components to shape its radiation
pattern & assist in reflection.

FIG 3.5 LED
LEDs present many advantages over traditional light sources including lower energy
consumption, longer lifetime, improved robustness, smaller size and faster switching.
However, they are relatively expensive and require more precise current and heat
management than traditional light sources.
Applications of LEDs are diverse. They are used as low-energy and also for
replacements for traditional light sources in well-established applications such as indicators
and automotive lighting. The compact size of LEDs has allowed new text and video displays
and sensors to be developed, while their high switching rates are useful in communications
technology. So here the role of LED is to indicate the status of the components like relays and
power circuit etc.
LED Circuits:
A LED circuit needs some resistance in it, so that it isn't a short circuit. Among the
specifications for LEDs, a maximum forward current rating is usually given. This is the
most current that can pass through the LED without damaging it, and also the current at which
the LED will produce the most light. A specific value of resistor is needed to obtain this exact
current. LEDs consume a certain voltage known as the "forward voltage drop", and are
usually given with the specs for that LED. This must be taken into account when calculating
the correct value of resistor to use. So to drive an LED using a voltage source and a resistor in
series with the LED, use the following equation to determine the needed Resistance:
Ohm's = (Source Voltage - LED Voltage Drop) / Amps
For example, to drive an LED from your car's 12v system, use the following values:

TABLE 3.3 VOLTAGES FOR DIFFERENT LEDS
3.6 TRANSISTOR
A transistor is a semiconductor device used to amplify and switch electronic signals. It
is made of a solid piece of semiconductor material, with at least three terminals for connection
to an external circuit. A voltage or current applied to one pair of the transistor's terminals
changes the current flowing through another pair of terminals. Because the controlled (output)
power can be much more than the controlling (input) power, the transistor provides
amplification of a signal. Today, some transistors are packaged individually, but many more
are found embedded in integrated circuits.
FIG 3.6 TRANSISTORS
The transistor's low cost, flexibility, and reliability have made it a ubiquitous device.
Transistorized mechatronic circuits have replaced electromechanical devices in controlling
Source Voltage =
13.4 volts (12v car systems
aren't really 12v in most
cases)
Voltage Drop =
3.6 volts (Typical for a
blue or white LED)
Desired Current =
30 milliamps (again, a
typical value)

appliances and machinery. It is often easier and cheaper to use a standard microcontroller and
write a computer program to carry out a control function than to design an equivalent
mechanical control function.
The essential usefulness of a transistor comes from its ability to use a small signal
applied between one pair of its terminals to control a much larger signal at another pair of
terminals. This property is called gain. A transistor can control its output in proportion to the
input signal; that is, it can act as an amplifier. Alternatively, the transistor can be used to turn
current on or off in a circuit as an electrically controlled switch, where the amount of current
is determined by other circuit elements.
VCC
Vout
COLLECTOR
Vin BASE
EMITTER
FIG 3.7 SYMBOL OF TRANSISTOR
The two types of transistors have slight differences in how they are used in a circuit. A
bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base
terminal (that is, flowing from the base to the emitter) can control or switch a much larger
current between the collector and emitter terminals. For a field-effect transistor, the terminals
are labeled gate, source, and drain, and a voltage at the gate can control a current between
source and drain.

3.7 RELAY
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 separate low-power
signal, 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.
FIG 3.8 RELAY
A type of relay that can handle the high power required to directly control an electric
motor or other loads is called a contactor. Solid-state relays control power circuits with
no moving parts, instead using a semiconductor device to perform switching. Relays with
calibrated operating characteristics and sometimes multiple operating coils are used to protect
electrical circuits from overload or faults; in modern electric power systems these functions
are performed by digital instruments still called "protective relays".
Magnetic latching relays require one pulse of coil power to move their contacts in one
direction, and another, redirected pulse to move them back. Repeated pulses from the same
input have no effect. Magnetic latching relays are useful in applications where interrupted
power should not be able to transition the contacts.
Magnetic latching relays can have either single or dual coils. On a single coil device,
the relay will operate in one direction when power is applied with one polarity, and will reset
when the polarity is reversed. On a dual coil device, when polarized voltage is applied to the

reset coil the contacts will transition. AC controlled magnetic latch relays have single coils
that employ steering diodes to differentiate between operate and reset commands.
PIN CONFIGURATION:
FIG 3.9 RELAY PIN CVONFIGURATION
3.8 POTENTIOMETER
FIG 3.10 POTENTIOMETER
A potentiometer is a three-terminal resistor with a sliding or rotating contact that
forms an adjustable voltage divider.[1]
If only two terminals are used, one end and the wiper, it
acts as a variable resistor or rheostat.
The measuring instrument called a potentiometer is essentially a voltage divider used
for measuring electric potential (voltage); the component is an implementation of the same
principle, hence its name.

Potentiometers are commonly used to control electrical devices such as volume
controls on audio equipment. Potentiometers operated by a mechanism can be used as
position transducers, for example, in a joystick. Potentiometers are rarely used to directly
control significant power (more than a watt), since the power dissipated in the potentiometer
would be comparable to the power in the controlled load.
3.9 DIODE
A diode is a two-terminal electronic component that conducts current primarily in
one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction,
and high (ideally infinite) resistance in the other. A diode vacuum tube or thermionic
diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons
can flow in only one direction, from cathode to plate.
FIG 3.11 DIODES
A semiconductor diode, the most common type today, is a crystalline piece
of semiconductor material with a p–n junction connected to two electrical terminals.
Semiconductor diodes were the first semiconductor electronic devices. The discovery of
asymmetric electrical conduction across the contact between a crystalline mineral and a metal
was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made
of silicon, but other materials such as gallium arsenide and germanium are used.

IN4007:
Diodes are used to convert AC into DC; these are used as half wave rectifier or full
wave rectifier. Three points must he kept in mind while using any type of diode.
1. Maximum Forward Current Capacity
2. Maximum Reverse Voltage Capacity
3. Maximum Forward Voltage Capacity
FIG 3.12 1N4007 DIODE
Electrons Holes
I (amps)
Forward Bias
Reverse Bias 0.7
N-Type P-Type V (volts)
(Not Pointing) (Pointing)
Cathode Anode Breakdown
SYMBOL INPUT – OUTPUT CHARACTERISTICS
Current Flow in the N-Type Material:
The positive potential of the battery will attract the free electrons in the crystal. These
electrons will leave the crystal and flow into the positive terminal of the battery. As an result,
N
-
P
+

an electron from the negative terminal of the battery will enter the crystal, thus completing the
current path. So, the majority current carriers in the N-type material (electrons) are repelled
by the negative side of the battery & move through the crystal toward the positive side of the
battery.
Current Flow in the P-Type Material:
Conduction in the P material is by positive holes, instead of negative electrons. A hole
moves from the positive terminal of the P material to the negative terminal. Electrons from
the external circuit enter the negative terminal of the material and fill holes in the vicinity of
this terminal. At the positive terminal, electrons are removed from the covalent bonds, thus
creating new holes. This process continues as the steady stream of holes (Hole Current)
moves toward the negative terminal.
3.10 FILTER
Electronic filters are electronic circuits which perform signal processing functions,
specifically to remove unwanted frequency components from the signal, to enhance wanted
ones, or both.
Electronic filters can be categorized as:
1) Passive or Active.
2) Analog or Digital.
3) High-Pass, Low-Pass, Band Pass, Band-Reject (Notch), or All-Pass.
4) Discrete-Time (Sampled) or Continuous-Time.
5) Linear or Non-Linear.
6) Infinite Impulse Response (IIR type) or Finite Impulse Response (FIR type).
The most common types of electronic filters are linear filters, regardless of other
aspects of their design. See the article on linear filters for details on their design and analysis.
Capacitive filter is used in this project. It removes the ripples from the output of
rectifier and smoothens the D.C. Output received from this filter is constant until the mains

voltage and load is maintained constant. However, if either of the two is varied, D.C. voltage
received at this point changes. Therefore a regulator is applied at the output stage.
The simple capacitor filter is the most basic type of power supply filter. The use of this
filter is very limited. It is sometimes used on extremely high-voltage, low-current power
supplies for cathode-ray and similar electron tubes that require very little load current from
the supply. This filter is also used in circuits where the power-supply ripple frequency is not
critical and can be relatively high.
3.11 PCB
A printed circuit board (PCB) mechanically supports and electrically
connects electronic components or electrical components using conductive tracks, pads and
other features etched from one or more sheet layers of copper laminated onto and/or between
sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB
to both electrically connect and mechanically fasten them to it.
Printed circuit boards are used in all but the simplest electronic products. They are also
used in some electrical products, such as passive switch boxes.
Alternatives to PCBs include wire wrap and point-to-point construction, both once popular
but now rarely used. PCBs require additional design effort to lay out the circuit, but
manufacturing and assembly can be automated. Specialized CAD software is available to do
much of the work of layout. Mass-producing circuits with PCBs is cheaper and faster than
with other wiring methods, as components are mounted and wired in one operation.
Large numbers of PCBs can be fabricated at the same time, and the layout only has to
be done once. PCBs can also be made manually in small quantities, with reduced benefits.
PCBs can be single-sided (one copper layer), double-sided (two copper layers on both
sides of one substrate layer), or multi-layer (outer and inner layers of copper, alternating with
layers of substrate). Multi-layer PCBs allow for much higher component density, because
circuit traces on the inner layers would otherwise take up surface space between components.
The rise in popularity of multilayer PCBs with more than two, and especially with more than
four, copper planes was concurrent with the adoption of surface mount technology. However,
multilayer PCBs make repair, analysis, and field modification of circuits much more difficult
and usually impractical.

A basic PCB consists of a flat sheet of insulating material and a layer of copper foil,
laminated to the substrate. Chemical etching divides the copper into separate conducting lines
called tracks or circuit traces, pads for connections, vias to pass connections between layers
of copper, and features such as solid conductive areas for EM shielding or other purposes. The
tracks function as wires fixed in place, and are insulated from each other by air and the board
substrate material. The surface of a PCB may have a coating that protects the copper from
corrosion and reduces the chances of solder shorts between traces or undesired electrical
contact with stray bare wires. For its function in helping to prevent solder shorts, the coating
is called solder resist.
FIG 3.13 PCB
A printed circuit board can have multiple copper layers. A two-layer board has copper
on both sides; multi layer boards sandwich additional copper layers between layers of
insulating material. Conductors on different layers are connected with vias, which are copper-
plated holes that function as electrical tunnels through the insulating substrate. Through-hole
component leads sometimes also effectively function as vias. After two-layer PCBs, the next
step up is usually four-layer. Often two layers are dedicated as power supply and ground
planes, and the other two are used for signal wiring between components.

3.12 TRANSFORMER
A transformer is a static electrical device that transfers electrical energy between two
or more circuits through electromagnetic induction. A varying current in one coil of the
transformer produces a varying magnetic field, which in turn induces a varying electromotive
force (emf) or "voltage" in a second coil.
Power can be transferred between the two coils, without a metallic connection
between the two circuits. Faraday's law of induction discovered in 1831 described this effect.
Transformers are used to increase or decrease the alternating voltages in electric power
applications.
Since the invention of the first constant-potential transformer in 1885, transformers
have become essential for the transmission, distribution, and utilization of alternating current
electrical energy.[3]
A wide range of transformer designs is encountered in electronic and
electric power applications. Transformers range in size from RF transformers less than a cubic
centimeter in volume to units interconnecting the power grid weighing hundreds of tons.
FIG 3.14 TRANSFORMER

3.13 RECTIFIER
FIG 3.15 Diode bridge in various packages
FIG 3.16 A hand-made diode bridge.
The wide silver band on the diodes indicates the cathode side of the diode.
A diode bridge is an arrangement of four (or more) diodes in a bridge
circuit configuration that provides the same polarity of output for either polarity of input.
When used in its most common application, for conversion of an alternating-
current (AC) input into a direct-current (DC) output, it is known as a bridge rectifier. A
bridge rectifier provides full-wave rectification from a two-wire AC input, resulting in lower
cost and weight as compared to a rectifier with a 3-wire input from a transformer with
a center-tapped secondary winding.
RECTIFICATION:
In the diagrams below, when the input connected to the left corner of the diamond
is positive, and the input connected to the right corner is negative, current flows from

the upper supply terminal to the right along the red (positive) path to the output and returns to
the lower supply terminal through the blue (negative) path.
When the input connected to the left corner is negative, and the input connected to the
right corner is positive, current flows from the lower supply terminal to the right along
the red (positive) path to the output and returns to the upper supply terminal through
the blue(negative) path.
In each case, the upper right output remains positive, and lower right output negative.
Since this is true whether the input is AC or DC, this circuit not only produces a DC output
from an AC input, it can also provide what is sometimes called "reverse-polarity protection".
That is, it permits normal functioning of DC-powered equipment when batteries have been

installed backwards, or when the leads (wires) from a DC power source have been reversed,
and protects the equipment from potential damage caused by reverse polarity.
Alternatives to the diode-bridge full-wave rectifiers are the center-tapped transformer
and double-diode rectifier, and voltage doubler rectifier using two diodes and two capacitors
in a bridge topology.

CHAPTER 4
SCHEMETIC DIAGRAM
4.1 SCHEMETIC DIGARAM
FIG 4.1 SCHEMATIC DIAGRAM
4.2 OPERATION
A regulated power supply circuit that through a fixed voltage regulator U1-LM7805
not only gives a variable but also auto switch off features this is achieved by connecting a
potentiometer between common terminal of regulator IC and Ground for every 100Ω
increment in the in circuit value of the resistance of potentiometer RV1, the output voltage
increases by 1 volt.
Thus, the output varies from 3.7 to 8.7v (taking into account 1.3 volts drop across
diodes D7 and D8).

Another important feature of the supply is that it switches itself off when no load is
connected across its output terminals.
This is achieved with the help of transistors Q1 and Q2, diodes D7and D8 and
capacitor C2. When a load is connected at the output potential drop across diodes D7 and D8
(approximately 1.3v) is sufficient for transistor of Q2 and Q1 to conduct.
As a result, the relay gets energized and remains in that state as long as the load
remains connected. At the same time capacitor C2 gets charged to around 7-8volt potential
through transistor Q2. But when the load (here in series with S2 ) is disconnected, transistors
Q2 is cut off. However, capacitor C2 is still charged and it starts discharging through base of
the transistor Q1.
After some time (which is basically determined by value of C2), relay RL1 is de
energized, which switches off the mains input to primary of transformer TR1.
To resume the power again switch S1 push button should be present momentarily.
Higher the value of capacitor C2 more will be the delay in switching off the power supply on
disconnection of the load, and vise versa.
A transformer which a secondary voltage 12v-0v, 250mA was used it can nevertheless
be changed as per users requirement (up to 30v maximum, and 1A current rating). For
drawing more 300mA current, the regulator IC must be fitted with a small heat sink over a
mica insulator. When the transformers secondary voltage increases beyond 12v RMS,
potentiometer RV1 must be re-dimensioned.
Also, the relay voltage rating should be predetermined.

CHAPTER 5
HARDWARE TESTING
CONTINUITY TEST:
In electronics, a continuity test is the checking of an electric circuit to see if current
flows i.e.in fact a complete circuit. A continuity test is performed by placing a small voltage
(wired in series with an LED or noise-producing component such as a piezoelectric speaker)
across the chosen path. If electron flow is inhibited by broken conductors, damaged
components, or excessive resistance, the circuit is "open".
This test is the performed just after the hardware soldering and configuration has been
completed. This test aims at finding any electrical open paths in the circuit after the soldering.
Many a times, the electrical continuity in the circuit is lost due to improper soldering, wrong
and rough handling of the PCB, improper usage of the soldering iron, component failures and
presence of bugs in the circuit diagram.
We use a multimeter to perform this test. We keep the multimeter in buzzer mode and
connect the ground terminal of the multimeter to the ground. We connect both the terminals
across the path that needs to be checked. If there is continuation then you will hear the beep
sound.
POWER ON TEST:
This test is performed to check whether the voltage at different terminals is according
to the requirement or not. We take a multi meter and put it in voltage mode. This test is
performed without ICs because if there is any excessive voltage, this may lead to damaging
the ICs.
Firstly, if we are using a transformer we check the output of the transformer; whether
we get the required 12V AC voltage. If we use a battery then we check if the battery is fully
charged or not according to the specified voltage of the battery by using multimeter.
Then we apply this voltage to the power supply circuit. If a circuit consists of voltage
regulator then we check for the input to the voltage regulator i.e. are we getting an input of
12V and a required output depending on the regulator used in the circuit if we are using 7805
we get output of 5V and if using 7809 we get 9V at output pin and so on.

This output from the voltage regulator is given to the power supply pin of specific ICs.
Hence we check for the voltage level at those pins whether we are getting required voltage.
Similarly, we check for the other terminals for the required voltage. In this way we can assure
that the voltage at all the terminals is as per the requirement.

CHAPTER 6
ADVANTAGES and DISADVANTAGES
ADVANTAGES:
 Used as protector for electronic circuit.
 Can obtain different voltages.
 High accuracy.
 Low cost.
 Easy to construct.
DISADVANTAGE:
 If once the supply breaks then it is powered on manually.

CHAPTER 7
APPLICATIONS
 This circuit is used in different electronic circuits, electronic equipments.
 This circuits is used for damage less uses.
 By this circuit we can provide various voltages.
 It can be used as supply voltages for different electronic equipments.

CHAPTER 8
HARDWARE MODEL
FIG 8.1 HARD WARE MODEL

CONCOLUSION
Almost all electronic equipments include a circuit that converts AC voltage of main
supply into DC voltage. This part of equipment is called power supply. In general, at the input
of the power supply there is a power transformer. A diode circuit called rectifier follows it.
The output of the rectifier goes to a smoothing filter and then to a voltage regulator circuit.
The rectifier is the heart of the power supply.
This power supply is used as a variable one. In laboratory applications we need
variable voltage in different level. So this circuit is very much helpful in electronic
experimental applications. Also, the auto switching facility less the energy wastage.

REFERENCES
 “The 8051 Microcontroller and Embedded systems” by Md. Ali Mazidi and Janice
Gillispien Mazidi, Pearson Education.
 ATMEL 89S52 Data Sheets.
 www.wikipedia.org
 www.beyondlogic.org
 www.edgefxkits.in

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