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Over/Under voltage protection of electrical appliances.
CHAPTER – 1
INTRODUCTION
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Over/Under voltage protection of electrical appliances.
1.1. MOTIVATION:
Sudden fluctuation in supply is a very big problem in industries and domestic
applications. It causes a major loss for industries, offices and homes. This project gives a
low cost and powerful solution for this problem. This Circuit protects refrigerators ACs,
Microwave ovens as well as other appliances from over and under voltage fluctuations.
Operational amplifier IC LM324 is used here as a comparator. IC LM324 consists of four
operational amplifiers, of which only two operational amplifiers are used in the circuit.
The unregulated power supply is connected to the series combination of resistors and
preset. The same supply is also connected to 6.8v zener diode through a resistor. The
preset is adjusted such that for the normal supply of 180V to 240V, the voltage at non-
inverting terminal of op-amp is less than 6.8V. The same unregulated supply is given to
the second comparator through a 5.6V zener.12V SPDT relay is used to control the Load.
An NPN transistor is used to drive the relay. An LED is connected across the relay for
visual indication of relay position. This project uses regulated 12V, 500mA power
supply. 7812 three terminal voltage regulator is used for supply voltages and reference
voltages for LM324 comparator circuit. Bridge type full wave rectifier is used to rectify
the ac output of secondary of 230/12V step down transformer
1.2. STATEMENT OF PROBLEM:
Now a day the automation field gets a wide growth in the world wide. Under this
concept here the project is developed. In this project the power to the electrical
appliances turn-offs according to the over/under voltage conditions appears in the system
and this is controlled by the operational amplifier LM324.
1.3. RELATED WORK:
To complete our project we studied about the LM324 operational amplifier and its
features. We also studied about Relays, Transistors, Resistors, Regulators and Relay drivers.
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1.4. EMBEDDED SYSTEM:
Embedded System is a combination of hardware and software used to achieve
a single specific task. An embedded system is a microcontroller-based, software driven,
reliable, real-time control system, autonomous, or human or network interactive,
operating on diverse physical variables and in diverse environments and sold into a
competitive and cost conscious market.
An embedded system is not a computer system that is used primarily for
processing, not a software system on PC or UNIX, not a traditional business or scientific
application. High-end embedded & lower end embedded systems. High-end embedded
system - Generally 32, 64 Bit Controllers used with OS. Examples Personal Digital
Assistant and Mobile phones etc .Lower end embedded systems - Generally 8,16 Bit
Controllers used with an minimal operating systems and hardware layout designed for the
specific purpose. Examples Small controllers and devices in our everyday life like
Washing Machine, Microwave Ovens, where they are embedded in.
1.4.1.CHARACTERISTICS OF EMBEDDED SYSTEM:
• An embedded system is any computer system hidden inside a product other than
a computer
• There will encounter a number of difficulties when writing embedded system
software in addition to those we encounter when we write applications
– Throughput – Our system may need to handle a lot of data in a short
period of time.
– Response–Our system may need to react to events quickly
– Testability–Setting up equipment to test embedded software can be
difficult
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– Debugability–Without a screen or a keyboard, finding out what the
software is doing wrong (other than not working) is a troublesome
problem
– Reliability – embedded systems must be able to handle any situation
without human intervention
– Memory space – Memory is limited on embedded systems, and you must
make the software and the data fit into whatever memory exists
– Program installation – you will need special tools to get your software
into embedded systems
– Power consumption – Portable systems must run on battery power, and
the software in these systems must conserve power
– Processor hogs – computing that requires large amounts of CPU time can
complicate the response problem.
– Cost – Reducing the cost of the hardware is a concern in many embedded
system projects; software often operates on hardware that is barely
adequate for the job.
• Embedded systems have a microprocessor/ microcontroller and a memory. Some
have a serial port or a network connection. They usually do not have keyboards,
screens or disk drives.
1.4.2. APPLICATIONS OF EMBEDDED SYSTEM:
1. Military and aerospace embedded software applications.
2. Communication applications.
3. Industrial automation and process control software.
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1.5. POWER SYSTEM:
The power system is an interconnection of generating units to load centers
through high voltage electric transmission lines and in general is mechanically controlled.
It can be divided into three subsystems: generation, transmission and distribution
subsystems. Until recently all three subsystems were under supervision of one body
within a certain geographical area providing power at regulated rates. In order to provide
cheaper electricity the deregulation of power system, which will produce separate
generation, transmission and distribution companies, is already being performed. At the
same time electric power demand continues to grow and also building of the new
generating units and transmission circuits is becoming more difficult because of
economic and environmental reasons. Therefore, power utilities are forced to relay on
utilization of existing generating units and to load existing transmission lines close to
their thermal limits. However, stability has to be maintained at all times. Hence, in order
to operate power system effectively, without reduction in the system security and quality
of supply, even in the case of contingency conditions such as loss of transmission lines
and/or generating units, which occur frequently, and will most probably occur at a higher
frequency under deregulation, a new control strategies need to be implemented.
1.6. INTRODUCTION TO THE PROJECT:
This circuit protects refrigerators as well as other electrical appliances from over
and under voltage. By the name itself we can say that if the input voltage is more or less
than the required voltage then the electrical appliance is turned off nothing but it gets
disconnected from its respective power supply.
This circuit uses an operational amplifier IC LM324 which is used as a
comparator. Generally, the IC LM324 consists of four operational amplifiers, of which
only two operational amplifiers are used. In this IC, we have an inverting terminal and a
non-inverting terminal. The voltages at the inverting terminal and the non-inverting
terminal are 6.0v and 6.8v respectively.
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When the input AC voltage exceeds 240v, the voltage at the non-inverting
terminal increases. So now if the voltage at the non-inverting terminal increases more
than 6.8v then the output of the operational amplifier is pulled high. So the electrical
appliance is turned off by means of a relay connected to the output pin of the op-amp.
Thus the electrical appliance is now protected against over voltage. Now let us consider
under voltage condition. When the line voltage is below 180v, the voltage at the inverting
terminal is less than the voltage at the non-inverting terminal. Thus the output of the
operational amplifier now goes high and the AC supply gets disconnected and the
electrical appliance also turns off. Thus, the appliance is now protected against under
voltage.
Thus the circuit protects any electrical appliance from over and under voltages.
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BLOCK DIAGRAM
Fig 1.1: Block representation
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The total circuit consists of several units, those are,
1. INPUT unit
2. Power supply
3. IC LM324
4. Transistor driver circuit
5. Relay unit
6. OUTPUT Unit
The components used in this circuit are,
1. Step-down transformer,
2. Rectifier,
3. Filter,
4. Voltage regulator,
5. Resistors,
6. Presets,
7. LM 324 Op-amp,
8. Transistors,
9. Zener diode,
10. Relay.
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CHAPTER - 2
POWER SUPPLY
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2.1. DESCRIPTION:
The input to the circuit is applied from the regulated power supply. The a.c. input
i.e., 230V from the mains supply is step down by the transformer to 12V and is fed to a
rectifier. The output obtained from the rectifier is a pulsating d.c voltage. So in order to
get a pure d.c voltage, the output voltage from the rectifier is fed to a filter to remove any
a.c components present even after rectification. Now, this voltage is given to a voltage
regulator to obtain a pure constant dc voltage.
Fig 2.1: Power supply block diagram
Power supply unit consists of following units,
1. Step down transformer,
2. Rectifier unit,
3. Input filter,
4. Regulator unit,
5. Output filter.
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RegulatorFilter
Bridge
Rectifier
Step down
transformer
230V AC 50Hz D.C
Output
Over/Under voltage protection of electrical appliances.
2.2. STEP DOWN TRANSFORMER:
The Step down Transformer is used to step down the main supply voltage from
230V AC to lower value. This 230 AC voltage cannot be used directly, thus it is stepped
down. The Transformer consists of primary and secondary coils. To reduce or step down
the voltage, the transformer is designed to contain less number of turns in its secondary
core. The output from the secondary coil is also AC waveform. Thus the conversion from
AC to DC is essential.
This conversion is achieved by using the Rectifier Circuit/Unit.
Fig 2.2: Step down transeformer
Step down transformers can step down incoming voltage, which enables you to
have the correct voltage input for your electrical needs. For example, if our equipment
has been specified for input voltage of 12 volts, and the main power supply is 230 volts,
we will need a step down transformer, which decreases the incoming electrical voltage to
be compatible with your 12 volt equipment.
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2.3. RECTIFIER UNIT:
The Rectifier circuit is used to convert the AC voltage into its corresponding DC
voltage. There are Half-Wave, Full-Wave and bridge Rectifiers available for this specific
function. The most important and simple device used in Rectifier circuit is the diode. The
simple function of the diode is to conduct when forward biased and not to conduct in
reverse bias.
Bridge rectifier: A bridge rectifier makes use of four diodes in a bridge
arrangement to achieve full-wave rectification. This is a widely used configuration, both
with individual diodes wired as shown and with single component bridges where the
diode bridge is wired internally.
A diode bridge or bridge rectifier is an arrangement of four diodes in a bridge
configuration that provides the same polarity of output voltage for either polarity of input
voltage. When used in its most common application, for conversion of alternating current
(AC) input into 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 center-tapped transformer design.
Fig 2.3: Bridge Rectifier
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The Forward Bias is achieved by connecting the diode’s positive with positive of
the battery and negative with battery’s negative. The efficient circuit used is the Full
wave Bridge rectifier circuit. The output voltage of the rectifier is in rippled form, the
ripples from the obtained DC voltage are removed using other circuits available. The
circuit used for removing the ripples is called Filter circuit.
2.4. FILTER:
Capacitors are used as filter. The ripples from the DC voltage are removed and
pure DC voltage is obtained. And also these capacitors are used to reduce the harmonics
of the input voltage. The primary action performed by capacitor is charging and
discharging. It charges in positive half cycle of the AC voltage and it will discharge in
negative half cycle. So it allows only AC voltage and does not allow the DC voltage. The
1000µf capacitor serves as a "reservoir" which maintains a reasonable input voltage to
the 7805 throughout the entire cycle of the ac line voltage.
Fig 2.4: Capacitor filter
The four rectifier diodes keep recharging the reservoir capacitor on
alternate half-cycles of the line voltage, and the capacitor is quite capable of
sustaining any reasonable load in between charging pulses. This filter is fixed
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before the regulator. Thus the output is free from ripples. Input side the low pass
filter has been used.
2.5. REGULATOR UNIT:
Regulator regulates the output voltage to be always constant. The output voltage
is maintained irrespective of the fluctuations in the input AC voltage. As and then the AC
voltage changes, the DC voltage also changes. Thus to avoid this Regulators are used.
Also when the internal resistance of the power supply is greater than 30 ohms, the output
gets affected. Thus this can be successfully reduced here. Meanwhile it also contains
current-limiting circuitry and thermal overload protection, so that the IC won't be
damaged in case of excessive load current; it will reduce its output voltage instead. The
regulators are mainly classified for low voltage and for high voltage.
Fig 2.5: voltage regulator
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Further they can also be classified as:
1) Positive regulator
Input pin
Ground pin
Output pin
It regulates the positive voltage.
2) Negative regulator
Ground pin
Input pin
Output pin
It regulates the negative voltage.
2.6. VOLTAGE REGULATOR 7812:
1) The 7812 fixed voltage regulator is a monolithic integrated circuit in a
TO220 type package designed for use in a wide variety of applications
including local, onboard regulation. This regulator employs internal
current limiting, thermal shutdown, and safe area compensation.
2) With adequate heat-sinking it can deliver output currents in excess of 1.0
ampere. Although designed primarily as a fixed voltage regulator, this
device can be used with external components to obtain adjustable
voltages and currents.
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Fig 2.6: Voltage Regulator 7812
2.6.1. FEATURES:
1. Output current up to 1.5 amperes.
2. Output voltages of 5, 5.2, 6, 8, 8.5, 9, 12, 15, 18, 24v.
3. Thermal overload protection.
4. Short circuit protection.
5. Output transition Soa Protection.
2.6.2. VOLTAGE RANGE:
1. LM7805C 5V
2. LM7812C 12V
3. LM7815C 15V
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DESCRIPTION:
The L7800 series of three-terminal positive regulators is available in TO-220, TO-
220FP, TO-3 and D2PAK packages and several fixed output voltages, making it useful in
a wide range of applications. These regulators can provide local on-card regulation,
eliminating the distribution problems associated with single point regulation. Each type
employs internal current limiting, thermal shut-down and safe 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.
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Fig 2.7: Voltage regulators
BLOCK DIAGRAM:
Fig 2.8: Block diagram for voltage regulator
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CHAPTER - 3
RESISTORS
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3.1. INTRODUCTION:
A linear resistor is a two-terminal, linear, passive electronic component that
implements electrical resistance as a circuit element. The current flowing through a
resistor is in a direct proportion to the voltage across the resistor's terminals. Thus, the
ratio of the voltage applied across resistor's terminals to the intensity of current flowing
through the resistor is called resistance. This relation is represented with a well-known
Ohm's law:
Resistors are common elements of electrical networks and electronic circuits and
are ubiquitous in most electronic equipment. 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-chrome). Resistors are also implemented within integrated circuits,
particularly analog devices, and can also be integrated into hybrid and printed circuits.
3.2. TYPES OF RESISTORS:
3.2.1. CARBON FILM:
The carbon film type is the most popular resistor type. This resistor is made by
depositing a carbon film onto a small ceramic cylinder. A small spiral groove cut into the
film controls the amount of carbon between the leads, hence setting the resistance. Such
resistors show excellent reliability, excellent solder ability, noise stability, moisture
stability, and heat stability. Typical power ratings range from 1/4 to 2 W. Resistances
range from about 10 Ohm to 1 Mega ohm, with tolerances around 5 percent.
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Fig 3.1: Carbon film resistor
3.2.2. CARBON COMPOSITION:
This type is also popular. It is made from a mixture of carbon powder and glue
like binder. To increase the resistance, less carbon is added. These resistors show
predictable performance, low inductance, and low capacitance. Power ratings range from
about 1/4 to 2 W. Resistances range from 1 Ohm to about 100 Mega ohms, with
tolerances around +/- 5 percent.
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Fig 3.2: Carbon composition resistor
3.2.3. METAL OXIDE FILM:
This type is general purpose resistor. It uses a ceramic core coated with a metal
oxide film. These resistors are mechanically and electrically stable and readable during
high temperature operation. They contain a special paint on their outer surfaces making
them resistant to flames, solvents, heat, and humidity. Typical resistances range from 1
Ohm to 200 kilo ohm, with typical tolerances of +/- 5 percent.
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Fig 3.3: Metal oxide film resistor
3.2.4. PRECISION METAL FILM:
This type is very accurate, ultra low noise resistor. It uses a ceramic substrate
coated with a metal film, all encased in an epoxy shell. These resistors are used in
precision devices, such as test instruments, digital and analog devices, and audio and
video devices. Resistances range from about 10 Ohm to 2 Mega Ohm, with power rating
from 1/4 to about 1/2 W, and tolerances of +/- 1 percent.
Fig 3.4: Precision metal film resistor
3.2.5. FOIL RESISTORS:
Foil resistors are similar in characteristics to metal film resistors. Their main
advantages are better stability and lower temperature coefficient of resistance (TCR).
They have excellent frequency response, low TCR, good stability, and are very accurate.
They are manufactured by rolling the same wire materials as used in precision wire
wound resistors to make thin strips of foil. This foil is then bonded to a ceramic substrate
and etched to produce the value required. They can be trimmed further by abrasive
processes, chemical machining, or heat treating to achieve the desired tolerance. Their
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main disadvantage is that the maximum value is less than metal film resistors. The
accuracy is about the same as metal film resistors, the TCR and stability approaches
precision wire wounds but are somewhat less because the rolling and packaging
processes produce stresses in the foil.
The resistive materials used in precision wire wound resistors are very sensitive to
stresses, which result in instability and higher TCS. Any stresses on these materials will
result in a change in the resistance value and TCR, the greater the stresses, the larger the
change. This type can be used as strain gauges, strain being measured as a change in the
resistance. When used as a strain gauge, the foil is bonded to a flexible substrate that can
be mounted on a part where the stress is to be measured.
Fig 3.5: Foil Resistors
3.2.6. FILAMENT RESISTORS:
Filament resistors are similar to bathtub or boat resistors except that they are not
packaged in a ceramic shell (boat). The individual resistive element with the leads
already crimped is coated with an insulating material, generally a high temperature
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varnish. They are used in applications where tolerance, TCR, and stability are not
important but the cost is the governing consideration. The cost of this type is slightly
higher that of carbon composition and the electrical characteristics are better.
3.2.7. POWER FILM RESISTIOR:
Power film resistors are similar in manufacture to metal film or carbon film
resistors. They are manufactured and rated as power resistors, with the power rating
being the most important characteristic. Power film resistors are available in higher
maximum values than the power wire wound resistors and have a very good frequency
response. They are generally used in applications requiring good frequency response
and/or higher maximum values.
Generally, for power applications the tolerance is wider. The temperature rating is
changed so that under full load, the resistor will not exceed the maximum design
temperature. The physical sizes are larger and, in some cases, the core may be made from
a more head conductive material and other means employed to help radiate heat.
Fig 3.6: Power film resistors.
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3.2.8. PRECISION WIRE WOUND RESISTOR:
The precision wire wound resistor is a highly accurate resistor (within 0.005%)
with a very low TCR. A TCR as little as 3ppm/oC can be achieved. However these
components are too expensive for general use and are normally used in highly accurate dc
applications.
Fig 3.7: Precision wire wound resistors
3.2.9. HIGH POWER WIRE WOUND RESISTOR:
These resistors are used for high power applications. Types include vitreous
enamel coated, cement, and aluminum housed wire wound resistors. Resistive elements
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are made from a resistive wire that is coiled around a ceramic cylinder. These are the
most durable of the resistors, with high heat dissipation and high temperature stability.
Resistances range from 0.1 Ohm to about 150 kilo ohms, with power ratings from around
2 W to as high as 500 W, or more.
Fig 3.8: High-Power wire wound
3.2.10. PHOTORESISTORS AND THERMISTORS:
These are special types of resistors that change resistance when heat or light is
applied. Photoresistors are made from semiconductive materials, such as cadmium
sulfide. Increasing the light level will decrease the resistance. This type also called LDR
(Light Dependent Resistor). Thermistors are temperature sensitive resistors. Increasing
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Over/Under voltage protection of electrical appliances.
the temperature will decrease the resistance (in most cases). This type also called
Thermistor NTC (Negative Temperature Coefficient). The reciprocal type is Thermistor
PTC (Positive Temperature Coefficient). Increasing the temperature will increase its
resistance.
Fig 3.9: Photo resistor and Thermistor
3.2.11. VARIABLE RESISTORS:
Variable resistors provide varying degrees of resistance that can be set with the
turn of a knob. Special kinds of variable resistors include potentiometers, rheostats, and
trimmers. Potentiometers and rheostats are essentially the same thing, but rheostats are
used specially for high power AC electricity, whereas potentiometers typically are used
with lower level DC electricity. Both potentiometers and rheostats are designed for
frequent adjustment. Trimmers, on the other hand, are miniature potentiometers that are
adjusted infrequently and usually come with pins that can be inserted into PCB. They are
used for fine tuning circuits (eg. fine tuning a circuit that goes astray as it ages), and they
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are usually hidden within a circuits enclosure box. Variable resistors come with 2 or 3
terminals. There are 2 kinds of taper, ie., linear tapered and nonlinear tapered
(logarithmic). The 'taper' describes the way in which the resistance changes as the control
knob is twisted. Linear taper usually has coded as 'A' while nonlinear tapes has coded as
'B'.
Fig 3.10: Symbol of variable resistor Fig 3.11: Variable resistor
3.3. PRESET:
A preset or trimmer is a miniature adjustable electrical component. It is meant to
be set correctly when installed in some device, and never seen or adjusted by the device's
user. Trimmers can be variable resistors (potentiometers), variable capacitors, trimmable
inductors. They are common in precision circuitry like A/V components, and may need to
be adjusted when the equipment is serviced. Unlike many other variable controls,
trimmers are mounted directly on circuit boards, turned with a small screwdriver and
rated for many fewer adjustments over their lifetime. Trimmers like trimmable inductors
and trimmable capacitors are usually found in superhet radio and television receivers, in
the Intermediate frequency, oscillator and RF circuits. They are adjusted into the right
position during the alignment procedure of the receiver.
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Trimmers come in a variety of sizes and levels of precision; for example, multi-
turn trim potentiometers exist, in which it takes several turns of the adjustment screw to
reach the end value, allowing for very high degrees of accuracy.
CHAPTER – 4
LM324
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4.1. GENERAL DESCRIPTION:
The LM324 series consists of four independent, high gain, internally frequency
compensated operational amplifiers which were designed specifically to operate from a
single power supply over a wide range of voltages. Operation from split power supplies is
also possible and the low power supply current drain is independent of the magnitude of
the power supply voltage. Application areas include transducer amplifiers, DC gain
blocks and all the conventional op amp circuits which now can be more easily
implemented in single power supply systems. For example, the LM124 series can be
directly operated off of the standard +5V power supply voltage which is used in digital
systems and will easily provide the required interface electronics without requiring the
additional ±15V power supplies.
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Fig 4.1: LM 324
4.2. OPERATIONAL AMPLIFIER:
An operational amplifier, which is often called an op-amp, is a DC-coupled high-
gain electronic voltage amplifier with differential inputs and, usually, a single output.
Typically the output of the op-amp is controlled either by negative feedback, which
largely determines the magnitude of its output voltage gain, or by positive feedback,
which facilitates regenerative gain and oscillation. High input impedance at the input
terminals (ideally infinite) and low output impedance (ideally zero) are important typical
characteristics.
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Fig 4.2: Symbol for Op-Amp
Op-amps are among the most widely used electronic devices today, being used in
a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps
cost only a few cents in moderate production volume; however some integrated or hybrid
operational amplifiers with special performance specifications may cost over $100 US in
small quantities.
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Fig 4.3: Operational amplifier
Modern designs are electronically more rugged than earlier implementations and
some can sustain direct short circuits on their outputs without damage. The op-amp is one
type of differential amplifier. Other types of differential amplifier include the fully
differential amplifier, the instrumentation amplifier, the isolation amplifier, and negative
feedback amplifier.
4.3. ADVANTAGES:
1. Eliminates need for dual supplies.
2. Four internally compensated op amps in a single package.
3. Allows directly sensing near GND and VOUT also goes to GND.
4. Compatible with all forms of logic.
5. Power drain suitable for battery operation.
4.4. FEATURES:
1. Internally frequency compensated for unity gain.
2. Large DC voltage gain 100 dB.
3. Wide bandwidth (unity gain) 1 MHz (temperature compensated).
4. Wide power supply range: Single supply 3V to 32V or dual supplies ±1.5V to
±16V.
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5. Very low supply current drain (700 μA)—essentially independent of supply
voltage.
6. Low input biasing current 45 nA (temperature compensated).
7. Low input offset voltage 2 mV and offset current: 5 nA.
8. Input common-mode voltage range includes ground.
9. Differential input voltage range equal to the power supply voltage.
10. Large output voltage swing 0V to V+ − 1.5V.
Fig 4.4: Pin diagram for LM324
LM324 is a 14 pin IC consisting of four independent operational amplifiers (op-
amps) compensated in a single package. Op-amps are high gain electronic voltage
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amplifier with differential input and, usually, a single-ended output. The output voltage is
many times higher than the voltage difference between input terminals of an op-amp.
These op-amps are operated by a single power supply LM324 and need for a dual supply
is eliminated. They can be used as amplifiers, comparators, oscillators, rectifiers etc. The
conventional op-amp applications can be more easily implemented with LM324.
CHAPTER - 5
TRANSISTOR
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5.1. DESCRIPTION:
A transistor is a semiconductor device used to amplify and switch electronic
signals. It is composed of a 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, a transistor can amplify a signal. Today, some transistors are packaged
individually, but many more are found embedded in integrated circuits.
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Fig 5.1: Transistor
5.2. NPN TRANSISTOR:
NPN is one of the two types of bipolar transistors, consisting of a layer of P-
doped semiconductor (the "base") between two N-doped layers. A small current entering
in to the base is amplified to produce a large collector and emitter current. That is, an
NPN transistor is "on" when its base is pulled high relative to the emitter.
Fig 5.2: NPN Transistor
Most of the NPN current is carried by electrons, moving from emitter to collector
as minority carriers in the P-type base region. Most bipolar transistors used today are
NPN, because electron mobility is higher than hole mobility in semiconductors, allowing
greater currents and faster operation.
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5.3. TRANSISTOR DRIVER CIRCUIT:
Fig 5.3: Transistor driver circuit
The transistor driver subsystem is an electronic switch that provides an output
signal powerful enough to drive output subsystems requiring medium current. The
subsystem acts as an inverter; the output signal is the inverse of the input signal. The
output subsystem is connected between the supply rail (+Vs) and the output signal. The
output subsystem is sometimes called the load resistance.
The transistor driver circuit uses an NPN transistor, which has three legs known
as the base, emitter and collector.
5.4. ZENER DIODE:
A Zener diode is a special kind of diode which allows current to flow in the
forward direction same as an ideal diode, but will also permit it to flow in the reverse
direction when the voltage is above a certain value known as the breakdown voltage,
"Zener knee voltage" or "Zener voltage." The device was named after Clarence Zener,
who discovered this electrical property.
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Fig 5.4: Symbol of Zener diode
A conventional solid-state diode 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 due to
overheating. 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.
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Fig 5.5: Zener diode
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
close to the Zener breakdown voltage. For example, a diode with a Zener breakdown
voltage of 3.2 V will exhibit a voltage drop of very nearly 3.2 V across a wide range of
reverse currents. The Zener diode is therefore ideal for applications such as the
generation of a reference voltage (e.g. for an amplifier stage), or as a voltage stabilizer for
low-current applications.
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CHAPTER - 6
RELAYS
6.1. DESCRIPTION:
A relay is an electrically controllable switch widely used in industrial controls,
automobiles and appliances. 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. Relays were used
extensively in telephone exchanges and early computers to perform logical operations.
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The relay allows the isolation of two separate sections of a system with two
different voltage sources i.e., a small amount of voltage/current on one side can handle a
large amount of voltage/current on the other side but there is no chance that these two
voltages mix up.
Fig 6.1: Circuit symbol of a relay Fig 6.2: Relay
6.2. CONSTRUCTION:
A simple electromagnetic relay consists of a coil of wire surrounding 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.
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Fig 6.3: Construction of a Relay
6.3. OPERATION:
When an electric current is passed through the coil it generates a magnetic
field that attracts the armature and the consequent movement of the movable contact
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.
In choosing a relay, the following characteristics need to be considered:
1. The contacts can be normally open (NO) or normally closed (NC). In the NC type, the
contacts are closed when the coil is not energized. In the NO type, the contacts are closed
when the coil is energized.
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2. There can be one or more contacts. i.e., different types like SPST (single pole single
throw), SPDT (single pole double throw) and DPDT (double pole double throw) relay’s.
3. The voltage and current required to energize the coil. The voltage can vary from a few
volts to 50 volts, while the current can be from a few milliamps to 20milliamps. The relay
has a minimum voltage, below which the coil will not be energized. This minimum
voltage is called the “pull-in” voltage.
4. The minimum DC/AC voltage and current that can be handled by the contacts. This is
in the range of a few volts to hundreds of volts, while the current can be from a few amps
to 40A or more, depending on the relay.
6.4. APPLICATIONS:
Control a high-voltage circuit with a low-voltage signal, as in some types
of modems or audio amplifiers,
Control a high-current circuit with a low-current signal, as in
the starter solenoid of an automobile,
Detect and isolate faults on transmission and distribution lines by opening and
closing circuit breakers (protection relays).
Isolate the controlling circuit from the controlled circuit when the two are at
different potentials.
Logic functions.
Time delay functions.
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CHAPTER – 7
WORKING PROCEDURE
7.1. CIRCUIT DIAGRAM:
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7.2. WORKING PROCEDURE:
IC LM324 consists of four operational amplifiers, of which only two operational
amplifiers (N1 and N2) are used in the circuit. The unregulated power supply is
connected to the series combination of resistors R1 and R2 and potentiometer VR1. The
same supply is also connected to a 6.8V Zener diode (ZD1) through resistor R3. Preset
VR1 is adjusted such that for the normal supply of 180V to 240V, the voltage at the non-
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inverting terminal (pin 3) of operational amplifier N1 is less than 6.8V. Hence the output
of the operational amplifier is zero and transistor T1 remains off. The relay, which is
connected to the collector of transistor T1, also remains de-energized. As the AC supply
to the electrical appliances is given through the normally closed (N/C) terminal of the
relay, the supply is not disconnected during normal operation.
When the AC voltage increases beyond 240V, the voltage at the non-inverting
terminal (pin 3) of operational amplifier N1 increases. The voltage at the inverting
terminal is still 6.8V because of the Zener diode. Thus now if the voltage at pin 3 of the
operational amplifier is higher than 6.8V, the output of the operational amplifier goes
high to drive transistor T1 and hence energize relay RL. Consequently, the AC supply is
disconnected and electrical appliances turn off. Thus the appliances are protected against
over-voltage.
Now let’s consider the under-voltage condition. When the line voltage is below
180V, the voltage at the inverting terminal (pin 6) of operational amplifier N2 is less than
the voltage at the non-inverting terminal (6V). Thus the output of operational amplifier
N2 goes high and it energizes the relay through transistor T1. The AC supply is
disconnected and electrical appliances turn off. Thus the appliances are protected against
under-voltage. IC1 is wired for a regulated 12V supply. Thus the relay energizes in two
conditions: first, if the voltage at pin 3 of IC2 is above 6.8V, and second, if the voltage at
pin 6 of IC2 is below 6V. Over-voltage and under-voltage levels can be adjusted using
presets VR1 and VR2, respectively.
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CHAPTER – 8
CONCLUSION
8.1. CONCLUSION:
This project presents the over/under voltage protection of electrical appliances
and it is designed, implemented and tested. Experimental work has been carried out
carefully. The result shows that higher efficiency is indeed achieved using the embedded
domain.
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8.2. REFERENCES:
1. WWW. howstuffworks.com
2. Magazines
3. Electronics for you
4. Electrikindia
5. www.google.com
6. www.electronicprojects.com
7. Wikipedia
8. IEEE Papers.
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Recommended