Modified Sine wave UPS

Submitted by:

Ali Ibrahim 2007-Elect-204 Ahsan Mubashir 2007-Elect-203 Bilal Ahmed 2007-Elect-132

Supervised by:

Mr. Syed Ali Mohsin

Department of Electrical EngineeringUniversity of Engineering and Technology Lahore

Modified Sine wave UPS

Submitted to the faculty of the Electrical Engineering Department of the University of Engineering and Technology Lahore in partial fulfillment of the requirements for the Degree of

Bachelor of Science

In

Electrical Engineering.

________________ _____________________

Internal Examiner External Examiner

________________________

DirectorUndergraduate Studies

Department of Electrical EngineeringUniversity of Engineering and Technology Lahore

i

Declaration

We declare that the work contained in this thesis is our own, except where explicitly stated otherwise. In addition this work has not been submitted to obtain another degree or professional qualification.

Signed by group member 1: _____________Signed by group member 2: _____________Signed by group member 3: _____________

Date: _____________

ii

AcknowledgmentsIn the name of Allah, who is the most merciful, the most compassionate; the one and only supreme power, the one whose will makes everything possible, and the one without whose will the simplest is impossible.

We are all thankful and highly obliged to all our family members who have been a source of great courage and support throughout this work and especially to our project advisor Mr. Syed Ali Mohsin who has provided us the guidance and technical route to make our work more smooth and timely.

iii

Dedications

We dedicate our work to our beloved parents, families and all our hardworking and skilled teachers who guided us, polished our rough knowledge about technical terms and finally made us capable enough to achieve this far.

ivTABLE OF CONTENTS

ACKNOWLEDGEMENTS..........iii DEDICATIONS.....iv LIST OF FIGURES....................................................................................................viiiABBREVIATIONS....................................................................................................ix ABSTRACT................................................................................................. xChapter 1 Introduction .................................................. 11.1 Inverter......................................................................................................... 1 1.1.1 Applications........... 1.1.1.1 Dc Power source utilization 1.1.1.2 Uninterruptable power supplies 1.1.1.3 Induction Heating 1.1.1.4 HVDC Power transmission 1.1.1.5 Variable frequency drives 1.1.1.6 Electric vehicle drives 1.1.1.7 Air conditioning 1.1.1.8 The General case 1.1.2 Types of inverters 1.1.2.1 The Pure sine wave inverters 1.1.2.2 Modified sine wave inverters 1.1.2.3 Advantages of sine wave inverters 1.1.3 Problem statement1.2 Pulse width modulation

Chapter 2 Operational Analysis......................5

2.1Selection between Mains and inverter ..............................................................5

2.2 Adjustment variables .......................................................................6

2.3 Trickle charging................................................6

2.4 Display & LEDs...........................................................7

2.5 Working..........................................................................................................7

Chapter 3 MOSFETs and H-bridge........................................9

3.1 MOSFETSs..................................................................................................9

3.2 H-Bridge.....................................................................................................9

Chapter 4 Optocouplers..........................................................................13

4.1 Introduction.................................................................................................13

4.2 Functions of optocouplers...........................................................................14

4.3 Special response of silicon..........................................................................

14

4.4 Construction............................................................................................. 15 4.5 Principle of operation............................................................................... 16 4.6 Outputs. 16 4.6.1 Photo diode output. 16 4.6.2 Photo transistor output. 17 4.6.3 Photo darlington output 18 4.6.4 Photo SCR output. 18 4.7 Key Parameters.. 19 4.8 How theyre used. 19

Chapter 5 PIC microcontroller ........................................................... 215.1 Introduction............................................................................................. 215.2 Core Architecture..................................................................................... 215.3 Data space (RAM).................................................................................... 225.4 Code Space.............................................................................................. 235.5 Word Size. 235.6 Stacks. 235.7 Instruction Set 235.8 Performance 245.9 Advantages.. 255.10 Limitations.. 25

Chapter 6 Transformer........................................................................ 266.1 Introduction........................................................................................... 266.2 Basic Principle........................................................................................ 27

6.2.1 Induction Law..........................................................................28

6.3 Applications............................................................................................28

Chapter 7 Useful Components............................................................ 307.1 Heat Sink................................................................................................. 30 7.1.1 Basic heat sink operating principle............................................ 30 7.1.2 Design factors which influence the thermal performance of heat sink 32 7.1.2.1 Material. 33

7.1.2.2 Fin efficiency 33 7.1.2.3 Spreading resistance. 34 7.1.2.4 Fin arrangements 34 7.1.2.5 Surface color 357.2 Buzzer 367.3 Fuses. 36 7.3.1 Operation.. 37 7.3.2 Characteristic parameters..... 38 7.3.2.1 Rated current IN .......... 38 7.3.2.2 Speed.. 38 7.3.2.3 The I2t value 38 7.3.2.4 Breaking capacity. 39 7.3.2.5 Rated voltage.. 39 7.3.2.6 Voltage drop.. 39 7.3.2.7 Temperature derating 40

References...........................................................................................................41

viiLIST OF FIGURES

Fig. 1.1: PWM techniqueFig. 3.1: MOSFET symbolsFig. 3.2: How a MOSFET is turned ONFig 3.3: structure of H-bridgeFig 4.1 Response of siliconFig. 4.2: Opto-coupler cross-section and schematicFig. 4.3: OptocouplersFig. 4.4: optocoupler with Photo Diode OutputFig. 4.5: optocoupler with Photo Darlington OutputFig 4.6: optocoupler with Photo SCR OutputFig. 4.7 operational view of optocouplerFig. 7.1: Sketch of a heat sink in a duct used to calculate the governing equations from conservation of energy and Newtons law of cooling

viii

ABBREVIATIONS

UPS-uninterruptible power supplyPWM-pulse width modulationDC-direct currentAC-alternate currentWAPDA-water and power development authorityLED-light emitting diodeMOSFET-metal oxide semiconductor field effect transistorIGFET-insulated gate field effect transistorSCR-silicon controlled rectifierIC-integrated circuit

ixABSTRACT

The project is confined to single phase modified sine wave UPS (1kW, 220V output) which aims to efficiently transform a DC power source (12V battery) to a high voltage AC source, similar to power that would be available at an electrical wall outlet and vice versa. This close to sine wave UPS is able to run more sensitive devices that a simple square wave may cause damage to such as: laser printers, laptop computers, power tools, digital clocks and medical equipment. This form of AC power also reduces audible noise in devices such as fluorescent lights and runs inductive loads, like motors, faster and quieter due to the low harmonic distortion. Another edge which this type of modification achieves is to provide a constant voltage of 220V irrespective of load burden and less fluctuations are observed as compared to square wave inverter.Prominent features of this UPS to be highlighted compromise of trickle charging circuit which reduces the current to a very small value when the battery is fully charged hence enhancing both the performance and life time of the battery attached. This circuit operates on 160V taken directly by the secondary of transformer by a different number of turns for those used in 220V. UPS includes a segment display which consistently displays the present terminal voltage of the battery available and thus can be operated accordingly. Besides the battery status several LEDs indicate the current state of operation like mains, UPS, full battery, battery charging etc. Another distinguishing feature that puts it ahead is the overload tripping action. At overload it trips and turns an LED on the front panel to ON state thus protecting any damage to the connected equipment. It then automatically re-accesses for an overload every 20 seconds and if the load drops to normal value only then the system retains its original functionality.The operation is roughly a straight line approximation of sine wave. The inversion is carried out by an H bridge comprising of N-type IRF150 MOSFETs. The charging is carried out by diodes which can handle high ratings of power and current up to 4A to prevent any abnormal behavior. These two circuits are placed on separate boards close to transformer for compact packing. An extra 1k resistor is employed to protect short circuit currents.Speaking of the class of sine wave inverters and their modifications our initial work explored pure sine wave UPS but the shortage of time and factors regarding the components unavailability, we switched our focus to this form of modification which mainly covers all the positive prospects a sine wave UPS can offer.

xChapter 1

INTRODUCTION

1.1 Inverter:An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.1.1.1 Applications:

1.1.1.1 DC power source utilization:

Inverter designed to provide 115 VAC from the 12 VDC source provided in an automobile. The unit shown provides up to 1.2 amperes of alternating current, or enough to power two sixty watt light bulbs.An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.Grid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as supplied by the distribution system. They can also switch off automatically in the event of a blackout.Micro-inverters convert direct current from individual solar panels into alternating current for the electric grid. They are grid tie designs by default.

1.1.1.2 Uninterruptible power supplies:An uninterruptible power supply (UPS) uses batteries and an inverter to supply AC power when main power is not available. When main power is restored, a rectifier supplies DC power to recharge the batteries.1.1.1.3 Induction heating:Inverters convert low frequency main AC power to higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.

1.1.1.4 HVDC power transmission:With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC.

1.1.1.5Variable-frequency drives:A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters.

1.1.1.6 Electric vehicle drives:Adjustable speed motor control inverters are currently used to power the traction motors in some electric and diesel-electric rail vehicles as well as some battery electric vehicles and hybrid electric highway vehicles such as the Toyota Prius and Fisker Karma. Various improvements in inverter technology are being developed specifically for electric vehicle applications.[2] In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries.

1.1.1.7Air conditioning:An air conditioner bearing the inverter tag uses a variable-frequency drive to control the speed of the motor and thus the compressor.

1.1.1.8 The general case:A transformer allows AC power to be converted to any desired voltage, but at the same frequency. Inverters, plus rectifiers for DC, can be designed to convert from any voltage, AC or DC, to any other voltage, also AC or DC, at any desired frequency. The output power can never exceed the input power, but efficiencies can be high, with a small proportion of the power dissipated as waste heat1.1.2 Types of Inverter:The two basic types of inverters include: Pure sine inverters Modified sine inverters

1.1.2.1 The Pure Sine Inverter:The pure sine inverter, which is also referred to as a "true" sine wave, utilizes sine wave in order to provide your appliances with power. A sine wave, which is produced by rotating AC machinery, is the type of wave that is generally provided by the utility company with the help of a generator.The benefits of using a pure sine inverter include:- All equipment currently on the market is designed for use with sine waves.- Some appliances, particularly microwaves and variable speed motors, will not produce full output if they do not use sine wave power.- Some appliances, such as light dimmers and bread makers, will not work at all without sine wave power.On the downside, pure sine wave inverters are more expensive than the other two types of inverters. By looking out for discount inverters that are on sale, however, you can keep your costs down.

1.1.2.2 The Modified Sine Inverter:The modified sine inverter is different from a pure sine power inverter because the wave is in more of a step wave and because appliances are not specifically designed to work with this type of inverter. Although many appliances will still work with a modified sine inverter, some may not work as efficiently. As such, it may take more power to run appliances with a modified sine inverter.Using fluorescent lighting can be problematic when using modified sine power inverters. For example, they may not get as bright and some may make buzzing noises when on. Certain appliances with digital clocks or electronic timers may also work improperly with this type of inverter because the waves are rougher and cause extra "noise" to be created in the line. Furthermore, appliances that use electronic temperature controls will not be able to properly control the temperature when using modified sine wave marine inverters, RV inverters or car inverters.Despite the few problems associated with them, most equipment can run just fine with modified sine wave inverters. In addition, since they are less expensive than the pure sine wave inverters, they are the most commonly used inverter. In fact, it can sometimes be difficult to find options other than modified sine wave inverters.Regardless of the type of inverter you use, you will also need to acquire the proper power inverter cables. Your power inverter cables need to be the proper size in order to guarantee damage does not occur. For most people, pure sine wave inverters are the best choice for meeting all of their on-the-road needs.

1.1.2.3 Advantages of Sine Wave InvertersPure and modified sine wave inverters have many advantages over the conventional square wave inverters. Some of the major advantages they provide are as followsa) They have very low harmonic distortion in the output voltage waveform.b) The power generated by these inverters is similar in nature to the utility companies (in our country WAPDA)c) They greatly reduce noise in amplifiers, TV etc.d) They run inductive loads smoothly and at the rated speed.e) They are able to run more sensitive devices that a simple square wave may cause damage to such as: laser printers, laptop computers, power tools, digital clocks and medical equipment.

1.1.3 Problem statement:The high end pure sine wave inverters tend to incorporate very expensive, high power capable digital components. The square wave units can be very efficient, as there is not much processing being performed on the output waveform, but this results in a waveform with a high number of harmonics, which can affect sensitive equipment such as medical monitors. Many of the very cheap devices output a square wave, perhaps a slightly modified square wave, with the proper RMS voltage, and close to the right frequency. Our goal is to fill a niche which seems to be lacking in the power inverters market, one for a fairly efficient, inexpensive inverter with a pure sine wave output. Utilizing PWM and analog components, the output will be a modified sinusoid, with very little switching noise, combined with the inexpensive manufacturing that comes with an analog approach.

1.2 Pulse Width Modulation:

In electronic power converters and motors, PWM is used extensively as a means of powering alternating current (AC) devices with an available direct current (DC) source or for advanced DC/AC conversion. Variation of duty cycle in the PWM signal to provide a DC voltage across the load in a specific pattern will appear to the load as an AC signal, or can control the speed of motors that would otherwise run only at full speed or off. This is further explained in this section. The pattern at which the duty cycle of a PWM signal varies can be created through simple analog components, a digital microcontroller, or specific PWM integrated circuits. Analog PWM control requires the generation of both reference and carrier signals that feed into a comparator which creates output signals based on the difference between the signals. The referencesignal is sinusoidal and at the frequency of the desired output signal, while the carrier signal is often either a saw tooth or triangular wave at a frequency significantly greater than the reference. When the carrier signal exceeds the reference, the comparator output signal is at one state, and when the reference is at a higher voltage, the output is at its second state. In order to source an output with a PWM signal, transistor or other switching technologies are used to connect the source to the load when the signal is high or low. Full or half bridge configurations are common switching schemes used in power electronics. Full bridge configurations require the use of four switching devices and are often referred to as H-Bridges due to their orientation with respect to a load.

Fig 1.1Chapter 2 Operational Analysis

2.1 Selection between mains and inverter:In order to determine whether the UPS should operate on its own or take input from main supply from a utility company depends on the appropriate selection which in normal operating conditions acts as follows: When supply is coming from mains (wapda), it turns the inverter off and puts the battery in a charging state if not charged fully. When supply is cut off as in the case of a load shedding the basic operation lies in turning the inverter to operating condition and simultaneously putting battery from charge to a discharge state.Whole of this selection process is carried out by two relays one is named as mains and other is charging relay. Basic operation of main relay is focused in connecting the output of UPS to main supply or the one from the inverter depending on the disconnection of supply from wapda the terminals of this relay operates accordingly and in the former case of supply connects the main supply to charging relay as well. The charging relay now getting the supply from main operates in such a manner that it connects the supply from wapda to an idle pin for a predefined amount of time .this intentional time delay is required to wash away any residual charge on the wires of inverter plates.This operation carries on for a predefined period of time depending on relay settings and manufacturer operatability limits , after which it connects the main supply to transformers primary .here several loops are taken out for safety purposes because voltage in most realized cases do not remain stable and change with load fluctuations, different loops taken out are for 0,220,240,260,280 volts where over voltage conditions are taken care of transformer then steps down this voltage to 12v which then rectified is used to charge the battery.In the inverse process of switching the mains off forces the main relay to cut off the supply to charging relay and connecting the output of UPS to the inverter bridge.

2.2 Adjustment Variables:Another prominent feature which our UPS offers is several adjustments are made flexible which can be changed according to the requirement and operating conditions .some of them to mention are:

Overload Since the ups trips in an overloading condition as specified by the user, this particular value of the overload can be set during the testing period of the device by connecting loads of several watts. a simple rotating coil can be used to set the value where it trips to prevent further damage and thus a lower value for a safe operation can be selected depending on the life time condition this value can be decreased later on if some heating or other issues relating to components of lower power handling capabilities occur. MainsTo make our UPS operate at some nominal value of voltage from the supply and in order to prevent it from damaging in under voltage conditions we have provided an adjustment to main voltage under which it will break the normal operation and shifts the output to inverter. This action prevents the sensitive devices such as computers and medical equipment to stay in a safe zone of operation since lower voltages have equal damaging effects as those of over voltages. The ups in this particular operation completely cut off the main supply and operate on the power being delivered by the battery. ChargingThis adjustment is particularly important for the level up to which battery should be charged .the ideal case for a 12v battery should be 14.4v. thus this variable may be used to change the battery charging level and can be set to any value depending on the required gravity of the acid to be achieved for an efficient process. Frequency Another quantity that can be slightly matched with that coming from a power utility company is frequency and voltage level which deviates from the standard 50 hertz.

2.3 Trickle charging:

A distinguishing feature that sets this UPS apart from its class is the charging that enhances the battery life time, protection of its plates and the current level with which it is being charged, it charges the battery at a rate equal to its self discharging rate.For a faster charging operation we require a higher current level that can be availed by this variable. Also when the battery is charged to full required voltage level this circuit maintains a constant minimum level of current pulses which keep that charged level constant and allows the attached battery to be used for a longer period. LM324N containing dual operational amplifiers are used for changing the required current level.

2.4 Display & LEDs8 segment digital displays are used to show the current voltage level of battery .PIC controller is used to interface the segment display. Only battery terminals are directly connected to the respective board. A heat sink is attached as well for any damage due to overheating.Several LEDs showing the current the current status of operation of the device are used as follows:1. Main2. UPS 3. Charging 4. Full battery5. Overload tripping

2.5 Working:MOSFETs forming an H bridge for the inversion process are connected to two plates B1 and B2 which simultaneously operate to create a 2 pole operation used for the main operation of the circuit. Controller then performs the required PWM which then drives the gates of MOSFETs... Two plates are provided with negative battery voltage. One winding of the transformer is connected at both ends to these two plates thus getting 12V.this winding is tapped from center to give the positive output and negative ground of output is common to whole circuitry.As soon as the voltage from utility company deviates the respective loop of transformer is selected and then stepped down. The MOSFETs selected are of 240W each and each of them has a built in diode for rectification purposes though in our circuit separate diode for rectification are provided for larger power handling purposes. Two optocouplers are used for isolation purposes and to convert the required voltage level in order to operate the controller at 5V. a differential comparator is employed in order to achieve the operation of PWM which then later on is sent to MOSFETs plates.

Chapter 3

MOSFETs and H-bridge

3.1 MOSFETs:The metaloxidesemiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first proposed by Julius Edgar Lilienfeld in 1925. In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common.The 'metal' in the name is now often a misnomer because the previously metal gate material is now often a layer of polysilicon (polycrystalline silicon). Aluminium had been the gate material until the mid 1970s, when polysilicon became dominant, due to its capability to form self-aligned gates. Metallic gates are regaining popularity, since it is difficult to increase the speed of operation of transistors without metal gates.IGFET is a related term meaning insulated-gate field-effect transistor, and is used almost synonymously with MOSFET, being more accurate since many "MOSFETs" use a gate that is not metal and a gate insulator that is not oxide. Another synonym is MISFET for metalinsulatorsemiconductor FET.Usually the semiconductor of choice is silicon, but some chip manufacturers, most notably IBM and Intel, recently started using a chemical compound of silicon and germanium (SiGe) in MOSFET channels. Unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide, do not form good semiconductor-to-insulator interfaces, thus are not suitable for MOSFETs. Research continues on creating insulators with acceptable electrical characteristics on other semiconductor material.In order to overcome power consumption increase due to gate current leakage, high- dielectric replaces silicon dioxide for the gate insulator, while metal gates return by replacing polysilicon (see Intel announcement).The gate is separated from the channel by a thin insulating layer, traditionally of silicon dioxide and later of silicon oxynitride. Some companies have started to introduce a high- dielectric + metal gate combination in the 45 nanometer node.When a voltage is applied between the gate and body terminals, the electric field generated penetrates through the oxide and creates an "inversion layer" or "channel" at the semiconductor-insulator interface. The inversion channel is of the same type, P-type or N-type, as the source and drain, thus it provides a channel through which current can pass. Varying the voltage between the gate and body modulates the conductivity of this layer and allows to control the current flow between drain and source.A variety of symbols are used for the MOSFET as shown below

Figure 3.1 MOSFET SymbolsWhen a voltage is applied across a MOS structure (as shown in the figure below), it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with NA the density of acceptors, p the density of holes; p = NA in neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If VGB is high enough, a high concentration of negative charge carriers forms in an inversion layer located in a thin layer next to the interface between the semiconductor and the insulator. Unlike the MOSFET, where the inversion layer electrons are supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much more slowly by thermal generation through carrier generation and recombination centers in the depletion region. Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage.This structure with p-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regions.

Figure 3.2 How a MOSFET is turned ONMOSFETs have very wide range of applications. But in our project we are using them for purpose of power inverter (DC to AC) by applying appropriate gate signals to them.MOSFETs when used in power inverters 1) Can be used with a wide range of supply voltages by using appropriate transformers. 2) Can be used to deliver a wide range of output voltages by using appropriate turns ratio. 3) Output Frequency is Adjustable and Stable. 4) A Standard Step Down transformer (Reverse connected) can be used with Excellent Results. 5) With the addition of "Parallel Output Fets" and a Large Transformer, Power can be GREATLY Increased.

3.2 H-BridgeWe have used IRF150 N-type MOSFETs in our power inverter. These MOSFETs are connected in an H-Bridge configuration running a motor load as shown in the figure below

Figure 3.3 structure of H-bridge is highlighted in redAn H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards. H bridges are available as integrated circuits, or can be built from discrete components.The term H-bridge is derived from the typical graphical representation of such a circuit. An H bridge is built with four switches (solid-state or mechanical). When the switches S1 and S4 (according to the first figure) are closed (and S2 and S3 are open) a positive voltage will be applied across the motor. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor. Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches S3 and S4. This condition is known as shoot-through.A common use of the H-bridge is an inverter (which we have used in our project). The arrangement is sometimes known as a single or three phase bridge inverter (our project is single phase). The H-bridge with a DC supply will square wave voltage waveform across the load. For a purely inductive load, the waveform would be a triangle wave, with its peak depending on the inductance, switching frequency, and input voltage.Because our UPS gives modified sinusoidal output we have used PWM to generate the gate signals for the MOSFETs in the H-Bridge using a PIC controller.

Chapter 4 Optocouplers:

4.1 Introduction

An optocoupler, also called opto-isolator, is an electronic component that transfers an electrical signal or voltage from one part of a circuit to another or from one circuit to another, while electrically isolating the two circuits from each other. It consists of an infrared emitting LED chip that is optically in-line with a light-sensitive silicon semiconductor chip, all enclosed in the same package. The silicon chip could be in the form of a photo diode, photo transistor, photo Darlington, or photo SCR. There are many situations where signals and data need to be transferred from one subsystem to another within a piece of electronics equipment, or from one piece of equipment to another, without making a direct ohmic electrical connection. Often this is because the source and destination are (or may be at times) at very different voltage levels, like a microprocessor which is operating from 5V DC but being used to control a triac which is switching 240V AC. In such situations the link between the two must be an isolated one, to protect the microprocessor from overvoltage damage. Relays can of course provide this kind of isolation, but even small relays tend to be fairly bulky compared with ICs and many of todays other miniature circuit components. Because theyre electro-mechanical, relays are also not as reliable and only capable of relatively low speed operation. Where small size, higher speed and greater reliability are important, a much better alternative is to use an optocoupler. These use a beam of light to transmit the signals or data across an electrical barrier, and achieve excellent isolation.

4.2 Functions of optocouplers: The optocouplerapplicationor function in the circuit is to:

Monitor high voltage Output voltage sampling for regulation System control micro for power on/off Ground isolation

4.3 SPECTRAL RESPONSE OF SILICON: Since silicon has a response to light (spectral response) that peaks at infrared wavelengths (between 800 and 950 nanometers), silicon devices are preferred as the photo detector section in optocouplers in conjunction with an infrared LED emitter. Matching the infrared LED to the silicon chip provides a maximum transfer of the desired electrical signal. Different types of optocouplers have specific characteristics that determine suitability for each unique application. The simplest type is the optocoupler with a photo diode output section. The optocoupler output is often connected to an amplifier (or series of amplifiers) to change a low-level input voltage into an appropriate higher signal level.

Fig 4.1 Response of silicon4.4 CONSTRUCTION: The input section of an optocoupler is an infrared LED chip. It is separated from the output silicon diode chip by a thin, transparent, Mylar plate embedded in clear silicone (a derivative of silicon). The assembly is sealed in a package keyed to designate pin #1. The most commonly used optocoupler package is the plastic DIP (dual-in-line package).

Fig 4.2

Optocouplers typically come in a small 6-pin or 8-pin IC package as shown in fig 4.3, but are essentially a combination of two distinct devices: an optical transmitter, typically a gallium arsenide LED (light-emitting diode) and an optical receiver such as a phototransistor or light-triggered diac. The two are separated by a transparent barrier which blocks any electrical current flow between the two, but does allow the passage of light.

(Fig 4.3)

Usually the electrical connections to the LED section are brought out to the pins on one side of the package and those for the phototransistor or diac to the other side, to physically separate them as much as possible. This usually allows optocouplers to withstand voltages of anywhere between 500V and 7500V between input and output. Optocouplers are essentially digital or switching devices, so theyre best for transferring either on-off control signals or digital data. Analog signals can be transferred by means of frequency or pulse-width modulation.

4.5 PRINCIPLE OF OPERATION: When a forward bias voltage is applied to the input terminals of the LED (positive to the anode), an input current,I (in), limited by the series resistor,RS,will flow in the LED circuit. The current produces the infrared light emission at about 900 nanometers that impinges on the photo-sensitive silicon chip.

4.6 OUTPUTS:4.6.1 PHOTO DIODE OUTPUT: With light impinging on the silicon diode in Figure 4.4, its photovoltaic characteristic will create photo current,ILorIOUT, to flow in the silicon diode. With a load resistor,RL, connected to the output terminals of the coupler, the photo current,IOUT, will develop a voltage,VL, across the load.VL=IOUTxRL.

optocoupler with Photo Diode OutputFigure 4.4

As the input signal,VIN, varies, it will vary the intensity of the infrared light. The output current,IOUT, will also change, causing the output voltage,VL, to change in the same manner. As output current increases, output voltages will also increase, and vice-versa. A small change in input current will produce a proportionate change in output current. This characteristic of the optocoupler will act to couple low-level analog signals or small DC voltage variations with little or no distortion.In the circuit of Figure 13.3, both signal coupling and input-to-output isolation is achieved, however, thecurrent transferratio(CTR) of a diode output optocoupler is extremely low about 10% to 15%. The term current transfer ratio (CTR) defines the relationship of output current,IOUT, to input current,IIN.

The output voltage,VL, can be coupled to the input of an amplifier to increase its amplitude to an appropriate level. The input section of all optocouplers is an infrared LED; however, the output section can be different depending on the required application. The basic principle of operation is the same, regardless of the particular output section selected.

4.6.2 PHOTO TRANSISTOR OUTPUT: Since the CTR of an optocoupler with a photo diode output is so low (10 to 15%) a preferred approach is to replace the diode chip with a silicon bipolar photo transistor. The bipolar transistor, with its inherent current gain,b,will provide a considerably higher CTR (between 50% to 100%) depending on the beta of the photo transistor. The base lead of the transistor can be reverse biased to reduce sensitivity, or forward biased to increase sensitivity, or left "floating" (disconnected).

4.6.3 PHOTO DARLINGTON OUTPUT If a still higher CTR is needed, the bipolar transistor can be replaced with a Darlington transistor configuration to serve as the photo-detector output section.

optocoupler with Photo Darlington OutputFigure 4.5http://teacher.en.rmutt.ac.th/ktw/04-710-409/OPTOCOUPLER%20APPLICATIONS.htmIn the circuits\ of Figures 4.5, the output current,IC, of the single bipolar photo transistor or the photo Darlington will develop an output voltage,VL, across the load resistance,RL. This voltage is the product of the output current,IC, and the load resistance,RL. The optocoupler can be operated either as a linear amplifier or as a digital switch, depending on the forward bias voltage applied to the base of the transistor.

4.6.4 PHOTO SCR OUTPUT: If the output section of an optocoupler is a photo SCR, the coupler functions to switch the positive half of the AC voltage across the load, operating under the same principle as an ordinary SCR circuit.

optocoupler with Photo SCR OutputFigure 4.64.7 Key Parameters:

The most important parameter for most optocouplers is their transfer efficiency, usually measured in terms of their current transfer ratio or CTR. This is simply the ratio between a current change in the output transistor and the current change in the input LED which produced it. Typical values for CTR range from 10% to 50% for devices with an output phototransistor and up to 2000% or so for those with a Darlington transistor pair in the output. Note, however that in most devices CTR tends to vary with absolute current level. Typically it peaks at a LED current level of about 10mA, and falls away at both higher and lower current levels. Other optocoupler parameters include the output transistors maximum collector-emitter voltage rating VCE(max), which limits the supply voltage in the output circuit; the input LEDs maximum current rating IF(max), which is used to calculate the minimum value for its series resistor; and the optocouplers bandwidth, which determines the highest signal frequency that can be transferred through it --- determined mainly by internal device construction and the performance of the output phototransistor. Typical opto-couplers with a single output phototransistor may have a bandwidth of 200 - 300 kHz, while those with a Darlington pair are usually about 10 times lower, at around 20 - 30 kHz.

4.8 How theyre used:

Basically the simplest way to visualize an optocoupler is in terms of its two main components: the input LED and the output transistor or diac. As the two are electrically isolated, this gives a fair amount of flexibility when it comes to connecting them into circuit. All we really have to do is work out a convenient way of turning the input LED on and off, and using the resulting switching of the phototransistor/ diac to generate an output waveform or logic signal that is compatible with our output circuitry. For example just like a discrete LED, you can drive an optocouplers input LED from a transistor or logic gate/buffer. All thats needed is a series resistor to set the current level when the LED is turned on. And regardless of whether you use a transistor or logic buffer to drive the LED, you still have the option of driving it in pull down or pull up mode. This means you can arrange for the LED, and hence the optocoupler to be either on or off for a logic high (or low) in the driving circuitry. In some circuits, there may be a chance that at times the driving voltage fed to the input LED could have reversed polarity (due to a swapped cable connection, for example). This can cause damage to the device, because optocoupler LEDs tend to have quite a low reverse voltage rating: typically only 3 - 5V. So if this is a possibility, a reversed polarity diode should be connected directly across the LED. On the output side, there are again a number of possible connections even with a typical optocoupler of the type having a single phototransistor receiver (such as the 4N25 or 4N28). In most cases the transistor is simply connected as a light-operated switch, in series with a load resistor RL. The base of the transistor is left unconnected, and the choice is between having the transistor at the top of the load resistor or at the bottom i.e. in either pull-up or pull-down mode. This again gives plenty of flexibility for driving either logic gates or transistors. If a higher bandwidth is needed, this can be achieved by using only the collector and base connections, and using the transistor as a photodiode. This lowers the optocouplers CTR and transfer gain considerably, but can increase the bandwidth to 30MHz or so. An alternative approach is still to use the output device as a phototransistor, but tie the base down to ground (or the emitter) via a resistor Rb, to assist in removal of stored charge as shown in fig 7. This can extend the optocouplers bandwidth usefully (although not dramatically), without lowering the CTR and transfer gain any more than is necessary. Typically you would start with a resistor value of 1MW, and reduce it gradually down to about 47kW to see if the desired bandwidth can be reached.

Fig 4.7http://lyricsdog.eu/s/optocoupler%20circuits

A variation on the standard optocoupler with a single output phototransistor is the type having a photo- Darlington transistor pair in the output, such as the 6N138. As mentioned earlier this type of device gives a much higher CTR and transfer gain, but with a significant penalty in terms of bandwidth. Connecting a base tieback resistor can again allow a useful extension of bandwidth without sacrificing too much in terms of transfer gain. Reference: http://www.jaycar.com.au/images_uploaded/optocoup.pdfhttp://teacher.en.rmutt.ac.th/ktw/04-710-409/OPTOCOUPLER%20APPLICATIONS.htm

CHAPTER 5PIC Microcontroller

5.1 Introduction: PICis a family ofHarvard architecturemicrocontrollersmade byMicrochip Technology, derived from the PIC1650originally developed byGeneral Instrument's Microelectronics Division. The name PIC initially referred to "Peripheral Interface Controller". PICs are popular with both industrial developers and hobbyists alike due to their low cost, wide availability, large user base, extensive collection of application notes, availability of low cost or free development tools, and serial programming (and re-programming with flash memory) capability.

5.2 Core Architecture: The PIC architecture is characterized by its multiple attributes: Separate code and data spaces (Harvard architecture) for devices other than PIC32, which has aVon Neumann architecture. A small number of fixed length instructions Most instructions are single cycle execution (2 clock cycles, or 4 clock cycles in 8bit models), with one delay cycle on branches and skips Oneaccumulator(W0), the use of which (as source operand) is implied (i.e. is not encoded in the opcode) All RAM locations function as registers as both source and/or destination of math and other functions. A hardware stack for storing return addresses A fairly small amount of addressable data space (typically 256 bytes), extended through banking Data space mapped CPU, port, and peripheral registers The program counter is also mapped into the data space and writable (this is used to implement indirect jumps).There is no distinction between memory space and register space because the RAM serves the job of both memory and registers, and the RAM is usually just referred to as the register file or simply as the registers.

5.3 Data space (RAM): PICs have a set of registers that function as general purpose RAM. Special purpose control registers for on-chip hardware resources are also mapped into the data space. The addressability of memory varies depending on device series, and all PIC devices have somebanking mechanismto extend addressing to additional memory. Later series of devices feature move instructions which can cover the whole addressable space, independent of the selected bank. In earlier devices, any register move had to be achieved via the accumulator.To implement indirect addressing, a "file select register" (FSR) and "indirect register" (INDF) are used. A register number is written to the FSR, after which reads from or writes to INDF will actually be to or from the register pointed to by FSR. Later devices extended this concept with post- and pre- increment/decrement for greater efficiency in accessing sequentially stored data. This also allows FSR to be treated almost like a stack pointer (SP).External data memory is not directly addressable except in some high pin count PIC18 devices.

5.4 Code space: The code space is generally implemented asROM,EPROMorflash ROM. In general, external code memory is not directly addressable due to the lack of an external memory interface. The exceptions are PIC17 and select high pin count PIC18 devices.

5.5 Word size: All PICs handle (and address) data in 8-bit chunks. However, the unit of addressability of the code space is not generally the same as the data space. For example, PICs in the baseline and mid-range families have program memory addressable in the same word size as the instruction width, i.e. 12 or 14 bits respectively. In contrast, in the PIC18 series, the program memory is addressed in 8-bit increments (bytes), which differ from the instruction width of 16 bits.In order to be clear, the program memory capacity is usually stated in number of (single word) instructions, rather than in bytes.

5.6 Stacks: PICs have a hardwarecall stack, which is used to save return addresses. The hardware stack is not software accessible on earlier devices, but this changed with the 18 series devices.Hardware support for a general purpose parameter stack was lacking in early series, but this greatly improved in the 18 series, making the 18 series architecture friendlier to high level language compilers.

5.7 Instruction set: A PIC's instructions vary from about 35 instructions for the low-end PICs to over 80 instructions for the high-end PICs. The instruction set includes instructions to perform a variety of operations on registers directly, theaccumulatorand a literal constant or the accumulator and a register, as well as for conditional execution, and program branching. Some operations, such as bit setting and testing, can be performed on any numbered register, but bi-operand arithmetic operations always involve W (the accumulator), writing the result back to either W or the other operand register. To load a constant, it is necessary to load it into W before it can be moved into another register. On the older cores, all register moves needed to pass through W, but this changed on the "high end" cores. PIC cores have skip instructions which are used for conditional execution and branching. The skip instructions are 'skip if bit set' and 'skip if bit not set'. Because cores before PIC18 had only unconditional branch instructions, conditional jumps are implemented by a conditional skip (with the opposite condition) followed by an unconditional branch. Skips are also of utility for conditional execution of any immediate single following instruction. The 18 series implemented shadow registers which save several important registers during an interrupt, providing hardware support for automatically saving processor state when servicing interrupts.In general, PIC instructions fall into 5 classes:1. Operation on working register (WREG) with 8-bit immediate ("literal") operand. E.g.movlw(move literal to WREG),andlw(AND literal with WREG). One instruction peculiar to the PIC isretlw, load immediate into WREG and return, which is used with computed branchesto producelookup tables.2. Operation with WREG and indexed register. The result can be written to either the Working register (e.g.addwfreg, w). or the selected register (e.g.addwfreg, f).3. Bit operations. These take a register number and a bit number, and perform one of 4 actions: set or clear a bit, and test and skip on set/clear. The latter are used to perform conditional branches. The usual ALU status flags are available in a numbered register so operations such as "branch on carry clear" are possible.4. Control transfers. Other than the skip instructions previously mentioned, there are only two:gotoandcall.5. A few miscellaneous zero-operand instructions, such as return from subroutine, andsleepto enter low-power mode.

5.8 Performance: The architectural decisions are directed at the maximization of speed-to-cost ratio. The PIC architecture was among the first scalar CPU designs and is still among the simplest and cheapest. The Harvard architecturein which instructions and data come from separate sourcessimplifies timing and microcircuit design greatly, and this benefits clock speed, price, and power consumption. The PIC instruction set is suited to implementation of fast lookup tables in the program space. Such lookups take one instruction and two instruction cycles. Many functions can be modeled in this way. Optimization is facilitated by the relatively large program space of the PIC (e.g. 4096 x 14-bit words on the 16F690) and by the design of the instruction set, which allows for embedded constants. For example, a branch instruction's target may be indexed by W, and execute a "RETLW" which does as it is named - return with literal in W. Execution time can be accurately estimated by multiplying the number of instructions by two cycles; this simplifies design of real-time code. Similarly, interrupt latency is constant at three instruction cycles. External interrupts have to be synchronized with the four clock instruction cycle; otherwise there can be a one instruction cycle jitter. Internal interrupts are already synchronized. The constant interrupt latency allows PICs to achieve interrupt driven low jitter timing sequences. An example of this is a video sync pulse generator. This is no longer true in the newest PIC models, because they have a synchronous interrupt latency of three or four cycles.

5.9 Advantages: The PIC architectures have these advantages: Small instruction set to learn RISCarchitecture Built in oscillator with selectable speeds Easy entry level, in circuit programming plus in circuit debuggingPICKitunits available from Microchip.com for less than $50 Inexpensive microcontrollers Wide range of interfaces including I2C, SPI, USB, USART, A/D, programmable Comparators, PWM, LIN, CAN, PSP, and Ethernet.

5.10 Limitations: The PIC architectures have these limitations: Oneaccumulator Register-bank switchingis required to access the entire RAM of many devices Operations and registers are notorthogonal; some instructions can address RAM and/orimmediateconstants, while others can only use the accumulator

The following limitations have been addressed in thePIC18series, but still apply to earlier cores: Stack:1. The hardware call stack is not addressable, so preemptivetask switchingcannot be implemented2. Software-implementedstacksare not efficient, so it is difficult to generatereentrantcode and supportlocal variablesWith paged program memory, there are two page sizes to worry about: one for CALL and GOTO and another for computed GOTO (typically used for table lookups). For example, on PIC16, CALL and GOTO have 11 bits of addressing, so the page size is 2048 instruction words. For computed GOTOs, where you add to PCL, the page size is 256 instruction words. In both cases, the upper address bits are provided by the PCLATH register. This register must be changed every time control transfers between pages. PCLATH must also be preserved by any interrupt handler.

Chapter 6Transformer:

6.1 Introduction Atransformer usually has two windings i.e. primary winding and secondary winding. It works on principle of mutual induction. According to this principle if the current through the primary winding is carried, then due to this change, a changing magnetic flux is induced in the transformer core which results a change in magnetic field through the secondary of the transformer and a voltage is induced in the secondary. If we connect a load at the secondary winding of the transformer then an electric current will flow through the secondary and energy will be transferred from primary to the secondary of the transformer and from secondary to the load. In case of an ideal transformer ratio between the primary and secondary of the transformer is given as:

So by selecting appropriate turns ratio a transformer can be used to step up or step down the voltage. If we want to step up the voltage number of turns of the secondary are kept higher than the number of turns of the primary and f we want to step down the voltage number of turns of secondary are kept lower than the number of turns of primary. If we consider the size of the transformer then a size of the transformer varies from a small size transformer which can even fit into a microphone to large size transformer which weighs hundreds of tons. Due to the introduction of new technologies the use of transformer has been eliminated from some places but transformer still finds effective application in almost all the electrical systems.

6.2 Basic Principle:The transformer is based on two principles which are listed blow Electromagnetism Electromagnetic inductionAccording to electromagnetism, varying electric current can produce a magnetic field and according to electromagnetic induction a varying magnetic field can induce a voltage across the coil. If the current through the primary winding is carried, then due to this change, a changing magnetic flux is induced in the transformer core which results a change in magnetic field through the secondary of the transformer and a voltage is induced in the secondary.

6.2.3 Induction law: Faradays law helps us calculate the voltage that is induced in the secondary of the transformer. According to this law secondary voltage is equal to

WhereVsrepresents the voltage induced across the secondary of the transformer,Nsrepresents the number of turns of the secondary and represents the magnetic flux through the coil. According to this relation we can see that the secondary voltage of the transformer depends upon the number of turns of the secondary and magnetic flux that is induced across the coil. Greater the number of turns greater will the secondary voltage. Similarly greater the magnetic flux induced across the coil greater will be the secondary voltage. Since in case of an ideal transformer the same magnetic flux passes through both the primary and secondary coils,voltage across the primary will b equal to

Taking the ratio of the two equations forVsandVpgives the basic equation for stepping up or stepping down the voltage

Np/Nsis known as theturns ratio.

6.3 Applications: Transformers find vast applications in electrical systems. One of its major applications is in the transmission of electrical energy over long distances. Voltage is usually stepped up using a step up transformer before electrical energy is being transferred over long distances. Transmission lines have resistance and dissipate electrical energy as the square of current that flows through them so energy is being dissipated as power is transferred through the wires so by transformingelectrical powerto a high-voltage which results in lower current, transformers enable economicaltransmission of powerover long distances. Transformer are also used in electronic products that we daily use. Transformer there steps down the supply voltage i.e. 220v to the desired voltage at which that equipment operates. It also helps in electrically isolating the end user from contact with the supply voltage. Characterization of soft magnetic cores is usually achieved with the principle of open-circuit transformers, for example in the internationally standardizedEpstein framemethod.

Chapter 7

Useful Components

7.1 Heat Sink: Aheat sinkis basically an element or a object whose function is to allow the transfer of heat from solid to a fluid medium. Examples include the radiator in the car and the ones used in air conditioning systems andrefrigeration. They also assist in cooling down of optoelectronic and electronic devices, such as LEDs and high power lasers.A heat sink is contrived in such a way so that the surface area which is in contact with the cooling fluid enlarges. Thermal performance of heat sink depends upon a number of factors some of which are listed below. Material Fin efficiency Spreading resistance Fin arrangements Surface color If we consider the engineering applications of heat sink then one of its major application is in the thermal management of electronics, often graphics processors or computercentral processing unit(CPU).

7.1.1 Basic Heat Sink Operating Principle: A heat sink is an assembly or a component that allows the transfer of heat energy from a higher temperature to a lower temperaturefluid medium. The fluid medium is usually air, water, refrigerants or oil. If its water, then the heat sink is frequently called a cold plate.Fourier's law of heat conduction helps us understand the principle of a hat sink.Joseph Fourier a French mathematician has critical part in the analytical treatment of heat conduction. Fourier's law of heat conduction shows that whenever there is temperature difference between two bodies, heat will be transferred from the body of higher temperature to the lower body temperature. The rate at which heat is transferred by conduction,qk, is proportional to the product of cross-sectional area through which heat is transferred and the temperature gradient.

Fig 7.1Consider a heat sink in a duct, where air flows through the duct, as shown in Figure 7.1. It is presumed that the base of heat is at higher temperature than the air. Applying law of conservation of energy, we obtain the following equations(2)whereThe above equations show that As the flow of air through the heat sink decreases, average temperature of air increases, this increases the temperature of heat sink base. This in turn would cause the thermal resistance of the heat sink to rise. The overall result is a higher heat sink base temperature. Base temperature of heat sink is strongly related to the inlet temperature of air. For instance, if there is recirculation of air in an object, the inlet air temperature is not the ambient air temperature and is therefore higher, which also results in a higher base temperature of heat sink. So, in case of no air or fluid flow around theheat sink, the heat sink would work poorly as the energy dissipated to the air cannot be transferred to the ambient air.

Other examples of situations in which a heat sink has impaired efficiency: Though Pin fins have a large surface area, but as they are very close, air cannot flow through them easily. Adjusting a heat sink in such a way so that the fins are not in the direction of flow. For a natural convection heat sink, adjusting the fins horizontally. In case of no centrifugal forces and artificial gravity, we can achieve convective cooling as the air that is warmer than the ambient temperaturealwaysflows upward;

7.1.2 Design factors which influence the thermal performance of heat sink:

7.1.2.1 Material: Aluminium is the most common hat sink material. Aluminium alloys are preferred in the manufacture of heat sinks rather than pure aluminium. Aluminium alloy has one of the higher thermal conductivity values at 229 W/mK. but as it is relatively soft t is not used in machining. Aluminium alloys 6061 and 6063 are most commonly used, with thermal conductivity values of 166 and 201 W/mK, respectively. Copper can also be used in the manufacture of heat sinks as its conductivity is higher than aluminium but it has disadvantages too which are listed below Copper is three times heavier than aluminium. Copper is expensive than aluminium Copper cannot be extruded while aluminium can be extruded.

7.1.2.2 Fin efficiency:Fin efficiency is one the major factor which influences the thermal performance of heat sink. We can consider a fin as a flat plate with heat flowing in one end and as it travels to the other end, it is being dissipated into the surrounding fluid.As heat flows through the fin, due to the heat lost due to convention and thermal resistance of heat sink, the heat transfer to the fluid would reduce. This factor is called the fin efficiency. It can be defined as the heat transferred by the fin, divided by the heat transfer was the fin to be isothermal. Following equations are applicable for straight fins.[8][8]Where: k is thethermal conductivityof the fin material. hfis theconvection coefficientof the fin. tfis the fin thickness (m). Lfis the fin height (m). Fin efficiency can be increased by two ways which are listed below By decreasing the finaspect ratio which can achieved by either Decreasing the fin length or Increasing the fin thickness By enhancing the thermal conductivity of the fins.7.1.2.3 Spreading resistance: Spreading resistance is another important factor which influences the thermal performance of heat sink. It occurs, if in a substance with finite thermal conductivity, thermal energy is transferred from a small area to a larger area. So heat does not distribute uniformly through the base of the heat sink. The phenomenon of spreading resistance can be shown if we observe that a large temperature gradient between the edges of the heat sink and the heat source exists when heat travels from the heat source location. Meaning there are fins at a lower temperature than if the heat source were uniform across the heat sink base. Due to this non-uniformity heat sink's effective thermal resistance enhancesWe can decrease the spreading resistance of the heat sink base by the following listed ways: By increasing the base thickness By choosing a material which has more thermal conductivity By using a vapor chamberin the base of heat sink.

7.1.2.4 Fin Arrangements: There are different types of fin arrangements available in the market. Some of them are listed below

A pin fin heat sink A straight fin heat sink cross cut heat sink flared fin heat sink In a pin fin heat sink, pins extend from the base. The pins can be of any shape i.e. square, cylindrical, and elliptical. It is very commonly available in the market.In the straight fin arrangement the pins extend through the entire length of the heat sink.If we compare the Pin fin heat sink with the straight fin then pin fin is better than straight fins when used in their intended application where the fluid flows axially along the pins rather than only tangentially across the pins. In flared fin heat sinks, fins are not parallel to each other. This arrangement causes more air to go through the heat sink fin channel by decreasing flow resistance; otherwise, more air would bypass the fins. Tilting them offers longer fins by keeping the overall dimensions the same. Forghan, et al.and Lasance and Eggink made no of observations and test and reached a conclusion that flared heat sink performed better than the other heat sinks tested. The performance of a heat sink depends upon its surface area. Greater the surface area, better the performance. However, this is not always true. The concept of a pin fin heat sink is to try to pack as much surface area into a given volume as possible.

7.1.2.5 Surface Color: Thetransfer of heatfrom the heat sink is interceded by the following two effects convection via the coolant thermal radiation.Transfer of heat by radiation depends upon two factors Heat sink temperature Temperature of the surroundings.When both of these temperatures are of the order of 0 C to 100 C, the effect of radiation compared to convection is negligible, and it is often neglected.Radiative coolingcan be a major factor when convection is low. Surface properties play an important role here. Matte-black surfaces will radiate much more efficiently than shiny bare metal.This is because a matte-black due to high emissivity radiates and absorbs radiant heat highly whereas a shiny metal surface due to low emissivity radiates and absorbs only a small amount of radiant heat.In environments where there is no convective heat transfer such as a vacuum or inouter space, radiation is the only factor controlling the flow of heat between the environment and the heat sink.

7.2 Buzzer: Abuzzeris a electromechanical, mechanical,orPiezoelectricordevice. It is used for audio signaling. Typical uses of buzzers and beepers includealarms,timersand confirmation of user input such as a mouse click or keystroke. The buzzer in our project will operate under following conditions:

Full Battery. Low Battery. Overload. Main Off. Main On.

7.3 Fuses: A fuseisbasically over currentprotection device. It is a metal wire or strip which melts when excessive current due to short circuit, overload or device failure flow interrupting thecircuitin which it is connected.With the help of a fuse further damage by overheating orfireis usually prevented. Current ratings of important equipment are usually specified and fuses are selected according to those ratings. They allow the flow of normal current. They also allow excessive current to flow but only for short periods.

7.3.2 Operation: Fuse consists of a metal wire or strip which melts when excessive current due to short circuit, overload or device failure flow interrupting thecircuitin which it is connected. The fuse is always connected inseries with the equipment which it is protecting so the same amount of current flows through the fuse and it can detect over current. Under normal conditions, fuse does not operate but if the current exceeds its normal value above a specified period then it operates removing the equipment for the system thus protecting it. When the current exceeds it normal value, anelectric arcis established between the un-melted ends of the fuse. The arc continues to grow in length until the voltage required to sustain the arc is higher than the available voltage, terminating current flow. Incase of alternating currentcircuits, the speed of fuse interruption is greatly enhanced due to the natural reversal of current during each cycle. The fuse element is made of zinc, copper, silver, aluminum, or alloys to provide stable and predictable characteristics. Ideally the fuse can carry rated current for unlimited period of time and melt quickly on a small excess. The characteristics of the element should not be changed due to minor harmless surges of current. The fuse elements may be designed in such a way so as to enhance its heating effect. In case of large fuses, current is usually divided between multiple strips of metal. Fuse elements may be backed by steel or nichrome wires, so that no strain is placed on the element. The fuse element may be surrounded by air, or by materials intended to speed the quenching of the arc.Silicasand or non-conducting liquids may be used. .

7.3.3 Characteristic parameters:

7.3.3.1 Rated current IN:Rated current is the maximum current that can be carried continuously by the fuse without interrupting the circuit.

7.3.3.2 Speed: The speed at which a fuse operates depends upon the magnitude of current and the material of the fuse element. Operating time is inversely proportional to current i.e. greater the current, early the fuse will blow. Fuses can be classified into following categories depending upon characteristics of operating time compared to current fast-blow slow-blow time-delay A standard fuse usually takes 1 second to operate for twice the rated current. A fast-blow fuse takes 0.1 seconds to operate for twice the rated current and a slow-blow fuse may require twice its rated current for tens of seconds to blow. Selection of fuse depends upon the characteristics of load. In case of semiconductor devices w require a fast operating fuse since these devices heat rapidly when excessive current flows. Sensitive equipments are protected by fast blowing fuses because in case of sensitive equipment we cant take risks. We want immediate action in case a fault occurs near our sensitive equipment so fast blowing fuses are installed there. Normal fast-blow fuses are the most general purpose fuses. The time delay fuses are those fuses which operate after a specified period of time after the occurrence of fault. These types of fuses find application in motors which takes a large amount of current, much above the rated current during starting for several seconds.

7.3.3.3 The I2t value: It is the measure of energy required to blow the fuse element. It is also known as the known as the let-through energy. Unique I2t parameters are provided by charts in manufacturer data sheets for each fuse family. The energy is mainly dependent on current and time for fuses.

7.3.3.4 Breaking capacity: Maximum current that can be safely interrupted by the fuse is termed as the breaking capacity. Breaking capacity is usually higher than the prospective. There are different fuses that have high interrupting capability such as Miniature fuses These fuses have high interrupting capability. They can interrupt ten times their rated current. HRC (High Rapture Capacity) These fuses are have very high rupturing capability and are usually flled with sand or a similar material.

7.3.3.5 Rated voltage: It is common practice to set the voltage rating of the fuse greater than or equal to what would become theopen circuit voltage. If this is not done then anarcmay result and plasma will be formed. Plasma may continue to conduct current until plasma reverts to an insulating gas so it rated voltage should always be kept larger than the maximumvoltage sourceit would have to disconnect. This requirement applies to every type of fuse.It is not advisable to use Medium-voltage fuses on low voltage circuits because they cannot properly clear the circuit when operating at very low voltages.

7.3.3.6 Voltage drop: Manufacture usually provides the voltage drop across the fuse. Due to dissipation of energy at high currents resistance may change when a fuse becomes hot. In low-voltage applications this resulting voltage drop must be taken care of particularly when using a fuse.

7.3.3.7 Temperature derating: Ambient temperature will change a fuse's operational parameters. A fuse rated for 1A at 25C may conduct up to 10% or 20% more current at 40C and may open at 80% of its rated value at 100C. Operating values will vary with each fuse family and are provided in manufacturer data sheets.

References:http://www.Wikipedia.comhttp://engr.nmsu.edu/~etti/spring97/electronics/cmos/FIG1.GIFhttp://static.electro-tech-online.com/imgcache/3304-idea.JPGhttp://www.jaycar.com.au/images_uploaded/optocoup.pdfhttp://teacher.en.rmutt.ac.th/ktw/04-710-409/OPTOCOUPLER%20APPLICATIONS.html