Study Unit Understanding and Using Electronic Diagrams

1 Study UnitUnderstanding andUsing ElectronicDiagramsByThomas GregoryIn your studies so far, you ve learned the basic principles ofhow electrical circuits provide power for useful work. Basiccircuits consist of a power source, components that regulatethe flow of current, and loads such as motors, lights, heat-ers, and more complex devices like computers or and Electronic technicians may work installing,servicing, maintaining, and troubleshooting hundreds of dif-ferent electrical and electromechanical devices from manydifferent manufacturers, on a regular basis. Over the years astandard visual language has evolved that allows designers,engineers, and technicians to effectively describe these elec-trical devices functions.

2 In this unit you ll learn how circuits are described by draw-ings called schematics. These drawings use standard symbolsthat allow technicians to quickly understand how a circuit isconstructed, what function it performs, and how to trouble-shoot the equipment. Schematics are also sometimes calledprints or electrical component has a univer-sally recognized symbol, and schematic drawings typically showthe connections between the components. As you learn howthe different types of components can be connected, you llbegin to recognize common circuit configurations that occurrepeatedly in many different types of electrical these circuit conventions and configurations willhelp you quickly spot likely problems based on the type ofequipment and the symptoms you observe.

3 You ll also knowhow circuits can be modified to alter their function, add capa-bilities,or improve their behavior. iiiPreviewPreviewPreviewivWhen you complete this Study unit, you ll beable to Understand the difference between schematics, wiringdiagrams, and block Diagrams , and how each is used todescribe circuit performance and function Recognize common electrical component symbols anddrawing conventions that describe circuits Recognize and describe the configuration of common circuit building blocks such as power supplies, oscillators,amplifiers, load drivers, and digital circuits Understand how schematics can supply important information for troubleshooting electrical circuitsDRAWINGS REPRESENTING Electronic EQUIPMENT1 Schematic Diagrams1 Abbreviations5 Ladder Diagrams6 Block Diagrams9 Wiring Diagrams11 Electronic COMPONENTS IN SCHEMATICS15 Mechanical Devices16 Passive Devices19 Active Devices29 INTERPRETING DIGITAL-CIRCUIT SCHEMATICS56 Basic Digital Gates57 Common Digital Integrated Circuits63 Common Digital-Circuit Applications65 Common 555 Timer Circuits70 Reading Datasheets 72 SELF-CHECK ANSWERS 79 EXAMINATION 81vContentsContents1 DRAWINGS REPRESENTINGELECTRONIC EQUIPMENT Electrical and Electronic technicians are

4 Often called on toinstall and maintain hundreds of different types of these devices have grown in variety and complexity, a system of symbols and conventions evolved to describe thecircuits in a shorthand method of documentation. This allowsengineers, designers, and technicians to understand how thecircuits that make up a device work, and how its componentsconnect with each other. Although the schematic diagramisthe most common document for this function, there are alsoblock diagramsand wiring of these documentshas a unique function in describing the circuit to aid in under-standingand troubleshooting. Technicians encounter some differences between companyschematics and those produced in European or Asian coun-tries.

5 In this unit you ll Study mostly the schematics you llsee from American companies, but once you re accustomed to reading these, you ll recognize common characteristics inall DiagramsSchematic Diagrams document the connection points andconstruction methods of electrical and Electronic 1shows a simple schematic diagram of a power sup-ply; on it you can see some of the conventions used. Figure 2shows the symbols for such basic components as wires andUnderstanding and UsingElectronic DiagramsUnderstanding and Using Electronic Diagrams2connections, switches, power sources, transformers, fuses, andground connections. In addition to these standard symbols,you ll sometimes run across symbols that are variations ofthese, or ones that are specific to certain companies, espe-cially in older schematic Diagrams .

6 Schematic Diagrams are often read from left to right, like abook, with inputs on the left and outputs on the right. Thisisn t a universal practice, but it s a good way to begin youranalysis of the schematic. Schematic Diagrams show the con-nections of the components in a clear, easily readable format,but they don t show how the components are physicallyarranged. In the schematic in Figure 1you ll see a plug onthe left side; this means the supply (or any device with thissymbol) is powered by an AC source, which isn t shown. Thefuse is in series with the power transformer to prevent dam-age from overloads, and the switch controls the on/off statusof the supply. Note that neither of the transformer primarywires are FusePlugSwitchTI18 V2A-+2000 F781212 V Reg100 FLED330 12 VDCS ource4-Amp BridgeBR1R1 FIGURE 1 This is a simple schematic of a power supply, containing some commonly and Using Electronic Diagrams3 This schematic uses the convention of dots to indicate con-nected wires (see the left side of BR1); unconnected wiressimply cross each other.

7 To avoid confusion, unconnectedwires that cross sometimes have jumpersto show that thewires aren t connected (see Figure 2). This is an older con-vention, but you ll still find it on many Diagrams . You ll also see that the components often have text writtennext to them: the component number, component value orrating, and perhaps the catalog number. Components oftenhave standard alphanumeric designators, such as D1, D2,etc., for diodes; Q1, Q2, etc. for transistors; U1, U2, etc., forintegrated circuits; and C1, C2, etc., for capacitors. The num-bersadvance for each designator, so if there are 22 resistorsin a circuit, they ll be numbered R1through R22, and the valuesand specifications will often be listed in a bill of materials onthe side of the schematic.

8 When there are multiple deviceswithin a single physical package, as with quad op-amp pack-ages or hex inverter integrated circuits, the individual amp orgate will be labeled U1 Aor U1 Bto indicate that it s one of severalcomponents in one package. For some components such as capacitors, you have to deter-mine what standard units are being used. A capacitor C1in aschematic might be listed as C13300. The C1designation willmore often be printed simply as C1. Throughout the Study unit,we ll treat these two styles of designation as number 3300 refers to the capacity value in and other components often have voltages a voltage rating of 50 V is specified on the print, a replace-ment capacitor for C1should have a value of 3300 microfaradsand a minimum voltage rating of 50 volts.

9 A 100-volt capacitorwould be acceptable as a replacement, but a 25-volt capacitorwouldn t. In the case of the transformer, the voltage listed refers to the secondary voltage. The primary side is connected to line voltage of 120 volts, so the step-down transformer in Figure 1has a turns ratio of 120 V/18 V = :1. Since the second-ary voltage is 18 volts and the maximum current is 2 amps,you know that the maximum volt-amp (VA) rating of thetransformer is 18 V 2 A = 36 VA. A replacement trans-former would need to have the same secondary voltage, butcould have a larger VA rating if overall size isn t a factor, sincelarger VA ratings usually require larger most of thisstudy unit and in someschematics, componentsare identified with num-bers that are printed assubscripts (such as C1,representing capacitornumber 1 ).

10 While print-ing the numbers assubscripts makes themeasier to recognize, mostschematics will just placethe number immediatelyafter the letter as in should recognize thatthese two identificationmethods are interchange-ableand realize thatyou ll find both labelingstyles used throughoutdocumentation related toelectronic and Using Electronic Diagrams4 Lighting ArresterGapAir Circuit BreakerOil Circuit BreakerCapacitorAdjustable CapacitorCoilorContact (Make) - NOContact (Make) - NCContactor without Blowout CoilsContactor with Blowout CoilsDisconnect DeviceFuseFusable ElementFused CutoutHot-Line ConductorInductorMagnetic-Core InductororAdjustable InductorTapped InductorIndicating (Green, Red, Amber) LampRotating Machine (Basic)Diode/Rectifier (Half Wave)Rectifier (Full Wave)Relay (Basic)(Indicate Type by StandardDesignation and Functionby Standard Symbol.)