Circuit Analysis天TheoryandPractice2ndEdition&MillerRobbins
IntroductionMeterOBJECTIVESNewtonAfter studying this chapter,you will bePictorial Diagramabletodescribe the SI system of measurement,PowerofTenNotationPrefixesconvert between various sets of units.Programming Language.use power of ten notation to simplifyhandling of large and small numbers.Resistanceexpress electrical units using standardSchematic Diagramprefix notation such as μA,kV,mW,etc.Scientific Notationuse a sensible number of significant dig-SIUnitsits in calculations,Significant Digitsdescribe whatblock diagrams are andSPICEwhy they are used,Voltconvert a simple pictorial circuit to itsWattschematic representation,.describe generallyhow computersfit inOUTLINEthe electrical circuit analysis picture.IntroductionKEYTERMSTheSISystemof UnitsConverting UnitsAmperePowerof TenNotationBlock DiagramPrefixesCircuitSignificant Digits and Numerical AccuracyConversion FactorCircuit DiagramsCurrentEnergyCircuit Analysis Using ComputersJoule
OBJECTIVES After studying this chapter, you will be able to • describe the SI system of measurement, • convert between various sets of units, • use power of ten notation to simplify handling of large and small numbers, • express electrical units using standard prefix notation such as mA, kV, mW, etc., • use a sensible number of significant digits in calculations, • describe what block diagrams are and why they are used, • convert a simple pictorial circuit to its schematic representation, • describe generally how computers fit in the electrical circuit analysis picture. KEY TERMS Ampere Block Diagram Circuit Conversion Factor Current Energy Joule Meter Newton Pictorial Diagram Power of Ten Notation Prefixes Programming Language Resistance Schematic Diagram Scientific Notation SI Units Significant Digits SPICE Volt Watt OUTLINE Introduction The SI System of Units Converting Units Power of Ten Notation Prefixes Significant Digits and Numerical Accuracy Circuit Diagrams Circuit Analysis Using Computers Introduction 1
n electrical circuit is a system of interconnected components such as resis-CHAPTERPREVIEWtors,capacitors,inductors,voltage sources,and so on.The electrical behav-ior of these components is described by a few basic experimental laws.Theselaws and the principles,concepts,mathematical relationships,and methods ofanalysis that have evolved from them are known as circuit theory.Much of circuit theory deals with problem solving and numerical analysisWhen you analyze a problem or design a circuit, for example, you are typicallyrequired to computevaluesfor voltage,current,and power.In additiontoanumerical value, your answer must include a unit. The system of units used forthis purpose is the SI system (Systeme International).The SI system is a unifiedsystem of metric measurement: it encompasses not only the familiar MKS(meters, kilograms, seconds) units for length, mass, and time, but also units forelectrical andmagneticquantities as well.Quite frequently, however, the SI units yield numbers that are either toolarge or too small for convenient use.To handle these, engineering notation anda set of standard prefixes have been developed. Their use in representation andcomputation is described and illustrated.The question of significant digits is alsoinvestigatedSince circuit theory is somewhat abstract, diagrams are used to help presentideas. We look at several types--schematic,pictorial, and block diagrams-andshow how to use them to represent circuits and systems.We conclude the chapter with a brief look at computer usage in circuit analy-sis and design. Several popular application packages and programming languagesare described.Special emphasis is placed on OrCADPSpice and ElectronicsWorkbench,the two principal software packages used throughout this book.Hints on Problem SolvingPUTTINGITINPERSPECTIVEDURING THEANALYSIS ofelectric circuits,you will find yourself solving quite afewproblems.Anorganizedapproachhelps.Listedbeloware someuseful guidelines:1. Make a sketch (e.g., a circuit diagram), mark on it what you know, then iden-tify what it is that you are trying to determine.Watch for"implied data"suchas the phrase"the capacitor is initially uncharged".(As you will find outlater, this means that the initial voltage on the capacitor is zero.) Be sure toconvertallimplieddatatoexplicitdata.2. Think through the problem to identify the principles involved, then look forrelationships that tie together the unknown and known quantities.3.Substitute the known information into the selected equation(s)and solve forthe unknown. (For complex problems, the solution may require a series ofsteps involving several concepts.If you cannot identify the complete set ofsteps before you start,start anyway.As each piece ofthe solution emerges,youare one step closer to the answer.You may make falsestarts.However, evenexperienced people do not get it right on the firsttry every time.Note also thatthere is seldom one “right"way to solve a problem. You may therefore comeup with an entirely different correct solution method than the authors do.)4. Check the answer to see that it is sensible-that is, is it in the “right ball-park"? Does it have the corect sign? Do the units match?3
An electrical circuit is a system of interconnected components such as resistors, capacitors, inductors, voltage sources, and so on. The electrical behavior of these components is described by a few basic experimental laws. These laws and the principles, concepts, mathematical relationships, and methods of analysis that have evolved from them are known as circuit theory. Much of circuit theory deals with problem solving and numerical analysis. When you analyze a problem or design a circuit, for example, you are typically required to compute values for voltage, current, and power. In addition to a numerical value, your answer must include a unit. The system of units used for this purpose is the SI system (Systéme International). The SI system is a unified system of metric measurement; it encompasses not only the familiar MKS (meters, kilograms, seconds) units for length, mass, and time, but also units for electrical and magnetic quantities as well. Quite frequently, however, the SI units yield numbers that are either too large or too small for convenient use. To handle these, engineering notation and a set of standard prefixes have been developed. Their use in representation and computation is described and illustrated. The question of significant digits is also investigated. Since circuit theory is somewhat abstract, diagrams are used to help present ideas. We look at several types—schematic, pictorial, and block diagrams—and show how to use them to represent circuits and systems. We conclude the chapter with a brief look at computer usage in circuit analysis and design. Several popular application packages and programming languages are described. Special emphasis is placed on OrCAD PSpice and Electronics Workbench, the two principal software packages used throughout this book. 3 CHAPTER PREVIEW Hints on Problem Solving DURING THE ANALYSIS of electric circuits, you will find yourself solving quite a few problems.An organized approach helps. Listed below are some useful guidelines: 1. Make a sketch (e.g., a circuit diagram), mark on it what you know, then identify what it is that you are trying to determine. Watch for “implied data” such as the phrase “the capacitor is initially uncharged”. (As you will find out later, this means that the initial voltage on the capacitor is zero.) Be sure to convert all implied data to explicit data. 2. Think through the problem to identify the principles involved, then look for relationships that tie together the unknown and known quantities. 3. Substitute the known information into the selected equation(s) and solve for the unknown. (For complex problems, the solution may require a series of steps involving several concepts. If you cannot identify the complete set of steps before you start, start anyway. As each piece of the solution emerges, you are one step closer to the answer. You may make false starts. However, even experienced people do not get it right on the first try every time. Note also that there is seldom one “right” way to solve a problem. You may therefore come up with an entirely different correct solution method than the authors do.) 4. Check the answer to see that it is sensible—that is, is it in the “right ballpark”? Does it have the correct sign? Do the units match? PUTTING IT IN PERSPECTIVE
4Chapter1Introduction1.1IntroductionTechnologyisrapidlychangingthewaywedothings;wenowhavecomput-ers inourhomes,electronic control systems in our cars,cellularphonesthatNWWcan beused just about anywhere,robots that assemble products on produc-tionlines,and soon.Afirst steptounderstandingthesetechnologies is electric circuit theory.Circuit theory provides you with the knowledge of basic principles that youneed to understand the behavior of electric and electronic devices, circuits,and systems.In this book,we develop and explore itsbasic ideas.Before We BeginBeforewebegin, let us look ata few examples of thetechnologyat work(As you go through these, you will see devices, components, and ideas thathave not yet been discussed.You will learn about theselater.For themoment,justconcentrateonthegeneral ideas.)As a first example, consider Figure 1-1, which shows a VCR. Its designis based on electrical, electronic,and magnetic circuit principles.For exam-ple,resistors, capacitors,transistors,and integrated circuits areused to con-trol the voltages and currents that operate its motors and amplify the audioand video signals that are the heart of the system. A magnetic circuit (theread/write system)performs the actual tape reads and writes.It creates,shapes,and controls themagnetic field that records audio and video signalson thetape.Another magneticcircuit, thepowertransformer,transforms theacvoltagefromthe120-voltwalloutletvoltagetothelowervoltagesrequiredby the system.FIGURE1-1AVCR isa familiarexample ofan electrical/electronic system
1.1 Introduction Technology is rapidly changing the way we do things; we now have computers in our homes, electronic control systems in our cars, cellular phones that can be used just about anywhere, robots that assemble products on production lines, and so on. A first step to understanding these technologies is electric circuit theory. Circuit theory provides you with the knowledge of basic principles that you need to understand the behavior of electric and electronic devices, circuits, and systems. In this book, we develop and explore its basic ideas. Before We Begin Before we begin, let us look at a few examples of the technology at work. (As you go through these, you will see devices, components, and ideas that have not yet been discussed. You will learn about these later. For the moment, just concentrate on the general ideas.) As a first example, consider Figure 1–1, which shows a VCR. Its design is based on electrical, electronic, and magnetic circuit principles. For example, resistors, capacitors, transistors, and integrated circuits are used to control the voltages and currents that operate its motors and amplify the audio and video signals that are the heart of the system. A magnetic circuit (the read/write system) performs the actual tape reads and writes. It creates, shapes, and controls the magnetic field that records audio and video signals on the tape. Another magnetic circuit, the power transformer, transforms the ac voltage from the 120-volt wall outlet voltage to the lower voltages required by the system. 4 Chapter 1 ■ Introduction FIGURE 1–1 A VCR is a familiar example of an electrical/electronic system
5Section1.1IntroductionFigure 1-2 shows another example.In this case,a designer, using a per-sonal computer,is analyzingtheperformance of a powertransformer.Thetransformermustmeetnotonlythevoltageandcurrentrequirementsoftheapplication,but safety-and efficiency-related concerns as well.A softwareapplicationpackage,programmedwithbasicelectricalandmagneticcircuitfundamentals,helpstheuserperformthistask.Figure 1-3 shows another application, a manufacturing facility wherefine pitch surface-mount (SMT)components are placed on printed circuitboards at high speed using laser centering and optical verification.Thebot-tomrowofFigure1-4showshow small thesecomponentsare.Computercontrolprovidesthehighprecisionneededtoaccuratelypositionparts astiny as these.BeforeWeMoveOnBefore wemoveon,we should note that,as diverseas these applicationsare,they all have one thing in common: all are rooted in the principles of circuittheory.FIGURE1-2A transformerdesigner usinga 3-D electromagnetic analysis program tocheck the design and operation of a power transformer.Upper inset: Magnetic field pat-tern.(Courtesy Carte International Inc.)
Figure 1–2 shows another example. In this case, a designer, using a personal computer, is analyzing the performance of a power transformer. The transformer must meet not only the voltage and current requirements of the application, but safety- and efficiency-related concerns as well. A software application package, programmed with basic electrical and magnetic circuit fundamentals, helps the user perform this task. Figure 1–3 shows another application, a manufacturing facility where fine pitch surface-mount (SMT) components are placed on printed circuit boards at high speed using laser centering and optical verification. The bottom row of Figure 1–4 shows how small these components are. Computer control provides the high precision needed to accurately position parts as tiny as these. Before We Move On Before we move on, we should note that, as diverse as these applications are, they all have one thing in common: all are rooted in the principles of circuit theory. Section 1.1 ■ Introduction 5 FIGURE 1–2 A transformer designer using a 3-D electromagnetic analysis program to check the design and operation of a power transformer. Upper inset: Magnetic field pattern. (Courtesy Carte International Inc.)