EN2042102 วงจรไฟฟ าและอ เล กทรอน กส Circuits and Electronics บทท 1 พ นฐานทางไฟฟ า สาขาว ชาว ศวกรรมคอมพ วเตอร คณะว ศวกรรมศาสตร มหาว ทยาล ยเทคโนโลย ราชมงคลพระนคร
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POWERS OF TEN It should be apparent from the relative magnitude of the various units of measurement that very large and very small numbers are frequently encountered in the sciences. To ease the difficulty of mathematical operations with numbers of such varying size, powers of ten are usually employed. This notation takes full advantage of the mathematical properties of powers of ten.
POWERS OF TEN The notation used to represent numbers that are integer powers of ten is as follows:
FIXED-POINT, FLOATING-POINT, SCIENTIFIC, AND ENGINEERING NOTATION Scientific (also called standard) notation and engineering notation make use of powers of ten, with restrictions on the mantissa (multiplier) or scale factor (power of ten). Engineering notation specifies that all powers of ten must be 0 or multiples of 3, and the mantissa must be greater than or equal to 1 but less than 1000.
FIXED-POINT, FLOATING-POINT, SCIENTIFIC, AND ENGINEERING NOTATION Prefixes TABLE 1.2
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INTRODUCTION Now that the foundation for the study of electricity/electronics has been established, the concepts of voltage and current can be investigated. The term voltage is encountered practically every day. We are aware that most outlets in our homes are 220 volts. Although current may be a less familiar term, we know what happens when we place too many appliances on the same outlet the circuit breaker opens due to the excessive current that results.
ATOMS AND THEIR STRUCTURE A basic understanding of the fundamental concepts of current and voltage requires a degree of familiarity with the atom and its structure. The simplest of all atoms is the hydrogen atom, made up of two basic particles, the proton and the electron. The nucleus of the hydrogen atom is the proton, a positively charged particle. The orbiting electron carries a negative charge equal in magnitude to the positive charge of the proton.
ATOMS AND THEIR STRUCTURE FIG. 2.1 Hydrogen and helium atoms.
ATOMS AND THEIR STRUCTURE Copper is the most commonly used metal in the electrical/electronics industry. An examination of its atomic structure will reveal why it has such widespread application. It has 29 electrons in orbits around the nucleus, with the 29th electron appearing all by itself in the 4 th shell.
ATOMS AND THEIR STRUCTURE FIG. 2.2 The atomic structure of copper.
VOLTAGE Since it would be inconsequential to talk about the voltage established by the separation of a single electron, a package of electrons called a coulomb (C) of charge was defined as follows: One coulomb of charge is the total charge associated with 6.242 x 10 18 electrons. If a total of 1 joule (J) of energy is used to move the negative charge of 1 coulomb (C), there is a difference of 1 volt (V) between the two points.
VOLTAGE Since the potential energy associated with a body is defined by its position, the term potential is often applied to define voltage levels. For example, the difference in potential is 4 V between the two points, or the potential difference between a point and ground is 12 V, and so on.
CURRENT FIG. 2.9 Basic electric circuit.
CURRENT The unit of current measurement, ampere, was chosen to honor the efforts of André Ampère in the study of electricity in motion. In summary, therefore, the applied voltage (or potential difference) in an electrical/electronics system is the pressure to set the system in motion, and the current is the reaction to that pressure.
VOLTAGE SOURCES The term dc, used throughout this text, is an abbreviation for direct current, which encompasses all systems where there is a unidirectional (one direction) flow of charge. FIG. 2.11 Standard symbol for a dc voltage source.
VOLTAGE SOURCES In general, dc voltage sources can be divided into three basic types: Batteries (chemical action or solar energy) Generators (electromechanical), and Power supplies (rectification a conversion process to be described in your electronics courses).
VOLTAGE SOURCES Batteries General Information Primary Cells (Non-rechargeable) Secondary Cells (Rechargeable) Lead-Acid Nickel Metal Hydride (NiMH) Lithium-ion (Li-ion)
VOLTAGE SOURCES Batteries FIG. 2.12 Alkaline primary cell: (a) Cutaway of cylindrical Energizer cell; (b) various types of Eveready Energizer primary cells.
VOLTAGE SOURCES Batteries FIG. 2.13 Lithium primary batteries.
VOLTAGE SOURCES Batteries FIG. 2.14 Maintenance-free 12 V (actually 12.6 V) lead-acid battery.
VOLTAGE SOURCES Batteries FIG. 2.15 Nickel metal hydride (NiMH) rechargeable batteries.
VOLTAGE SOURCES Batteries FIG. 2.16 Dell laptop lithium-ion battery: 11.1 V, 4400 mah.
VOLTAGE SOURCES Solar Cell FIG. 2.17 Solar System: (a) panels on roof of garage; (b) system operation.
VOLTAGE SOURCES Generators The dc generator is quite different from the battery, both in construction and in mode of operation. When the shaft of the generator is rotating at the nameplate speed due to the applied torque of some external source of mechanical power, a voltage of rated value appears across the external terminals. The terminal voltage and power-handling capabilities of the dc generator are typically higher than those of most batteries, and its lifetime is determined only by its construction.
VOLTAGE SOURCES Generators FIG. 2.18 dc generator.
VOLTAGE SOURCES Power Supplies The dc supply encountered most frequently in the laboratory uses the rectification and filtering processes as its means toward obtaining a steady dc voltage. FIG. 2.19 A 0 V to 60 V, 0 to 1.5 A digital display dc power supply
CONDUCTORS AND INSULATORS Different wires placed across the same two battery terminals allow different amounts of charge to flow between the terminals. Many factors, such as the density, mobility, and stability characteristics of a material, account for these variations in charge flow. In general, however, conductors are those materials that permit a generous flow of electrons with very little external force (voltage) applied. In addition, good conductors typically have only one electron in the valence (most distant from the nucleus) ring.
CONDUCTORS AND INSULATORS TABLE 2.1 Relative conductivity of various materials
CONDUCTORS AND INSULATORS FIG. 2.26 Various types of insulators and their applications. (a) Fi-Shock extender insulator; (b) Fi-Shock corner insulator; (c) Fi-Shock screw-in post insulator.
CONDUCTORS AND INSULATORS TABLE 2.2 Breakdown strength of some common insulators.
AMMETERS AND VOLTMETERS It is important to be able to measure the current and voltage levels of an operating electrical system to check its operation, isolate malfunctions, and investigate effects impossible to predict on paper. As the names imply, ammeters are used to measure current levels; voltmeters, the potential difference between two points. If the current levels are usually of the order of milliamperes, the instrument will typically be referred to as a milliammeter, and if the current levels are in the microampere range, as a microammeter.
AMMETERS AND VOLTMETERS FIG. 2.27 Voltmeter connection for an up-scale (+) reading.
AMMETERS AND VOLTMETERS FIG. 2.28 Ammeter connection for an up-scale (+) reading.
AMMETERS AND VOLTMETERS FIG. 2.29 Volt-ohmmilliammeter (VOM) analog meter.
AMMETERS AND VOLTMETERS FIG. 2.30 Digital multimeter (DMM).
APPLICATIONS FIG. 2.32 Battery charger: (a) external appearance; (b) internal construction.
APPLICATIONS FIG. 2.33 Electrical schematic for the battery charger of Fig. 2.32.
APPLICATIONS FIG. 2.34 Answering machine/phone 9 V dc supply.
APPLICATIONS FIG. 2.35 Internal construction of the 9 V dc supply in Fig. 2.34.
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INTRODUCTION This opposition to the flow of charge through an electrical circuit, called resistance, has the units of ohms and uses the Greek letter omega (Ω) as its symbol. The graphic symbol for resistance, which resembles the cutting edge of a saw.
INTRODUCTION FIG. 3.1 Resistance symbol and notation.
INTRODUCTION This opposition, due primarily to collisions and friction between the free electrons and other electrons, ions, and atoms in the path of motion, converts the supplied electrical energy into heat that raises the temperature of the electrical component and surrounding medium. The heat you feel from an electrical heater is simply due to current passing through a high-resistance material.
RESISTANCE: CIRCULAR WIRES The resistance of any material is due primarily to four factors: Material Length Cross-sectional area Temperature of the material
RESISTANCE: CIRCULAR WIRES The first three elements are related by the following basic equation for resistance:
RESISTANCE: CIRCULAR WIRES FIG. 3.2 Factors affecting the resistance of a conductor.
RESISTANCE: CIRCULAR WIRES TABLE 3.1 Resistivity (p) of various materials.
RESISTANCE: CIRCULAR WIRES FIG. 3.3 Cases in which R2 > R1. For each case, all remaining parameters that control the resistance level are the same.
RESISTANCE: CIRCULAR WIRES Circular Mils (CM) In Eq. (3.1), the area is measured in a quantity called circular mils (CM). It is the quantity used in most commercial wire tables, and thus it needs to be carefully defined. The mil is a unit of measurement for length and is related to the inch by
WIRE TABLES The wire table was designed primarily to standardize the size of wire produced by manufacturers. As a result, the manufacturer has a larger market, and the consumer knows that standard wire sizes will always be available. The table was designed to assist the user in every way possible; it usually includes data such as the cross-sectional area in circular mils, diameter in mils, ohms per 1000 feet at 20 C, and weight per 1000 feet.
WIRE TABLES TABLE 3.2 American Wire Gage (AWG) sizes.
WIRE TABLES FIG. 3.8 Popular wire sizes and some of their areas of application.
TYPES OF RESISTORS Fixed Resistors Resistors are made in many forms, but all belong in either of two groups: fixed or variable. The most common of the low-wattage, fixedtype resistors is the film resistor.
TYPES OF RESISTORS Fixed Resistors FIG. 3.12 Film resistors: (a) construction; (b) types.
TYPES OF RESISTORS Fixed Resistors FIG. 3.13 Fixed-composition resistors: (a) construction; (b) appearance.
TYPES OF RESISTORS Fixed Resistors FIG. 3.14 Fixed metal-oxide resistors of different wattage ratings.
TYPES OF RESISTORS Fixed Resistors FIG. 3.15 Various types of fixed resistors.
TYPES OF RESISTORS Variable Resistors Variable resistors, as the name implies, have a terminal resistance that can be varied by turning a dial, knob, screw, or whatever seems appropriate for the application. They can have two or three terminals, but most have three terminals. If the two- or three-terminal device is used as a variable resistor, it is usually referred to as a rheostat.
TYPES OF RESISTORS Variable Resistors If the three-terminal device is used for controlling potential levels, it is then commonly called a potentiometer. Even though a three-terminal device can be used as a rheostat or a potentiometer (depending on how it is connected), it is typically called a potentiometer when listed in trade magazines or requested for a particular application.
TYPES OF RESISTORS Variable Resistors FIG. 3.16 Potentiometer: (a) symbol; (b) and (c) rheostat connections; (d) rheostat symbol.
TYPES OF RESISTORS Variable Resistors FIG. 3.17 Molded composition-type potentiometer. (Courtesy of Allen- Bradley Co.)
TYPES OF RESISTORS Variable Resistors FIG. 3.18 Resistance components of a potentiometer: (a) between outside terminals; (b) between wiper arm and each outside terminal.
TYPES OF RESISTORS Variable Resistors FIG. 3.19 Variable resistors: (a) 4 mm ( 5/32 in.) trimmer (courtesy of Bourns, Inc.); (b) conductive plastic and cermet elements (courtesy of Honeywell Clarostat); (c) three-point wirewound resistor.
COLOR CODING AND STANDARD RESISTOR VALUES A wide variety of resistors, fixed or variable, are large enough to have their resistance in ohms printed on the casing. Some, however, are too small to have numbers printed on them, so a system of color coding is used. For the thin-film resistor, four, five, or six bands may be used. The four-band scheme is described. Later in this section the purpose of the fifth and sixth bands will be described.
COLOR CODING AND STANDARD RESISTOR VALUES FIG. 3.21 Color coding for fixed resistors.
COLOR CODING AND STANDARD RESISTOR VALUES FIG. 3.22 Color coding.
COLOR CODING AND STANDARD RESISTOR VALUES FIG. 3.23 Example 3.11. FIG. 3.24 Example 3.12.
COLOR CODING AND STANDARD RESISTOR VALUES FIG. 3.25 Five-band color coding for fixed resistors.
COLOR CODING AND STANDARD RESISTOR VALUES TABLE 3.5 Standard values of commercially available resistors.
CONDUCTANCE By finding the reciprocal of the resistance of a material, we have a measure of how well the material conducts electricity. The quantity is called conductance, has the symbol G, and is measured in siemens.
OHMMETERS The ohmmeter is an instrument used to perform the following tasks and several other useful functions: Measure the resistance of individual or combined elements. Detect open-circuit (high-resistance) and short-circuit (lowresistance) situations. Check the continuity of network connections and identify wires of a multilead cable. Test some semiconductor (electronic) devices.
OHMMETERS FIG. 3.28 Measuring the resistance of a single element. FIG. 3.29 Checking the continuity of a connection.
OHMMETERS FIG. 3.30 Identifying the leads of a multilead cable.
SUPERCONDUCTORS The field of electricity/electronics is one of the most exciting of our time. New developments appear almost weekly from extensive research and development activities. The research drive to develop a superconductor capable of operating at temperatures closer to room temperature has been receiving increasing attention in recent years due to the need to cut energy losses. What are superconductors? Why is their development so important? In a nutshell, superconductors are conductors of electric charge that, for all practical purposes, have zero resistance.
SUPERCONDUCTORS FIG. 3.34 Rising temperatures of superconductors.
SUPERCONDUCTORS FIG. 3.35 Defining the critical temperature T c.
THERMISTORS The thermistor is a two-terminal semiconductor device whose resistance, as the name suggests, is temperature sensitive. FIG. 3.36 Thermistor: (a) characteristics; (b) symbol.
THERMISTORS FIG. 3.37 NTC (negative temperature coefficient) and PTC (positive temperature coefficient) thermistors.
THERMISTORS FIG. 3.38 Photoconductive cell: (a) characteristics. (b) symbol.
PHOTOCONDUCTIVE CELL The photoconductive cell is a two-terminal semiconductor device whose terminal resistance is determined by the intensity of the incident light on its exposed surface. As the applied illumination increases in intensity, the energy state of the surface electrons and atoms increases, with a resultant increase in the number of free carriers and a corresponding drop in resistance.
PHOTOCONDUCTIVE CELL FIG. 3.39 Photoconductive cells.
VARISTORS Varistors are voltage-dependent, nonlinear resistors used to suppress high-voltage transients; that is, their characteristics enable them to limit the voltage that can appear across the terminals of a sensitive device or system.
VARISTORS FIG. 3.40 Varistors available with maximum dc voltage ratings between 18 V and 615 V.
APPLICATIONS The following are examples of how resistance can be used to perform a variety of tasks, from heating to measuring the stress or strain on a supporting member of a structure. In general, resistance is a component of every electrical or electronic application.
APPLICATIONS Strain Gauges Any change in the shape of a structure can be detected using strain gauges whose resistance changes with applied stress or flex. FIG. 3.43 Resistive strain gauge.
OHM S LAW
OHM S LAW FIG. 4.2 Basic circuit.
OHM S LAW FIG. 4.3 Defining polarities.
OHM S LAW FIG. 4.4 Example 4.3. FIG. 4.5 Example 4.4.
PLOTTING OHM S LAW Graphs, characteristics, plots, and the like play an important role in every technical field as modes through which the broad picture of the behavior or response of a system can be conveniently displayed. It is therefore critical to develop the skills necessary both to read data and to plot them in such a manner that they can be interpreted easily.
PLOTTING OHM S LAW FIG. 4.6 Plotting Ohm s law.
PLOTTING OHM S LAW FIG. 4.7 Demonstrating on an I-V plot that the lower the resistance, the steeper is the slope.
PLOTTING OHM S LAW FIG. 4.8 Applying Eq. (4.7).
PLOTTING OHM S LAW FIG. 4.9 Example 4.5.
POWER In general, the term power is applied to provide an indication of how much work (energy conversion) can be accomplished in a specified amount of time; that is, power is a rate of doing work.
ENERGY
ENERGY Note that the energy in kilowatthours is simply the energy in watthours divided by 1000. To develop some sense for the kilowatthour energy level, consider that 1 kwh is the energy dissipated by a 100 W bulb in 10 h. The kilowatthour meter is an instrument for measuring the energy supplied to the residential or commercial user of electricity.
ENERGY FIG. 4.16 Kilowatthour meters: (a) analog; (b) digital. (Courtesy of ABB Electric Metering Systems.)
ENERGY FIG. 4.17 Cost per kwh and average kwh per customer versus time. (Based on data from Edison Electric Institute.)
ENERGY TABLE 4.1 Typical wattage ratings of some common household items.