Over/Under voltage protection of electrical appliances.

CHAPTER – 1

INTRODUCTION

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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

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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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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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