Resistive loads in general

Chapter Resistive loads in general

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • component fails, and then a third. It’s hard to predict the current and power distribu-tion in an array when its resistor values are all different. So it’s hard to know whetherany of the components in such a matrix are going to burn out.If you need a resistance with a certain power-handling capacity, you must be surethe network can handle at least that much power. If a 50-W rating is required, and a cer-tain combination will handle 75 W, that’s alright. But it isn’t good enough to build a cir-cuit that will handle only 48 W. Some extra tolerance, say 10 percent over the minimumrating needed, is good, but it’s silly to make a 500-W network using far more resistorsthan necessary, unless that’s the only convenient combination given the parts available.Nonsymmetrical series-parallel networks, made up from identical resistors, will in-crease the power-handling capability. But in these cases, the total resistance will not bethe same as the value of the single resistors. The overall power-handling capacity will al-ways be multiplied by the total number of resistors, whether the network is symmetri-cal or not, provided all the resistors are the same. In engineering work, cases sometimesdo arise where nonsymmetrical networks fit the need just right.Resistive loads in generalThe circuits you’ve seen here are good for illustrating the principles of dc. But some ofthe circuits shown here have essentially no practicality. You’ll never find a resistor con-nected across a battery, along with a couple of meters, as shown in Fig. 4-7, for exam-ple. The resistor will get warm, maybe even hot, and it will eventually drain the batteryin an unspectacular way. Aside from its educational value, the circuit does nothing ofany use.In real life, the ammeter and voltmeter readings in an arrangement such as thatshown in Fig. 4-7 would decline with time. Ultimately, you’d be left with a dead, coldbattery, a couple of zeroed-out meters, a potentiometer, and some wire.The resistances in the diagrams like Fig. 4-7 are always put to some use in electri-cal and electronic circuits. Instead of resistors, you might have light bulbs, appliances(60-Hz utility ac behaves much like dc in many cases), motors, computers, and radios.Voltage division is one important way in which resistors are employed. This, along withmore details about current, voltage, and resistance in dc circuits, is discussed in thenext chapter.QuizRefer to the text in this chapter if necessary. A good score is at least 18 correct answers.The answers are in the back of the book.1. Suppose you double the voltage in a simple dc circuit, and cut the resistance inhalf. The current will become:A. Four times as great.B. Twice as great.C. The same as it was before.D. Half as great.Quiz77