The potentiometer

Chapter The potentiometer

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

  • into a couple of ICs, or chips, whose total volume is about the same as that of the tip ofyour little finger. In 1930, a similar receiver would have been as large as a television set.Resistors can be fabricated onto the semiconductor chip that makes up an IC. Thethickness, and the types and concentrations of impurities added, control the resistanceof the component.IC resistors can only handle a tiny amount of power because of their small size. Butbecause IC circuits in general are designed to consume minimal power, this is not aproblem. The small signals produced by ICs can be amplified using circuits made fromdiscrete components if it is necessary to obtain higher signal power.The potentiometerAll of the resistors mentioned are fixed in value. It is impossible to change or adjust theirresistances. Of course, their values will change if they overheat, or if you chip pieces ofthem out, but they’re meant to provide an unchanging opposition to the flow of electriccurrent.It might have occurred to you that a variable resistor can be made by hooking up abunch of fixed resistors in series or parallel, and then switching more or fewer of themin and out. This is almost never done in electronic circuits because there’s a better wayto get a variable resistance: use a potentiometer.The construction of a potentiometer is shown in simplified form in Fig. 6-8. A re-sistive strip is bent into a nearly complete circle, and terminals are connected to eitherend. This forms a fixed resistance. To obtain the variable resistance, a sliding contact isattached to a rotatable shaft and bearing, and is connected to a third terminal. The re-sistance between this middle terminal, and either of the end terminals, can vary fromzero up to the resistance of the whole strip.Some potentiometers use a straight strip of resistive material, and the controlmoves up and down, or from side to side. This type of variable resistor, called a slide po-tentiometer, is used in graphic equalizers, as the volume controls in some stereo ampli-fiers, and in some other applications when a linear scale is preferable to a circular scale.Potentiometers are made to handle only very low levels of current, at low voltage.Linear taperOne type of potentiometer uses a strip of resistive material whose density is constant allthe way around. This results in a linear taper. The resistance between the center ter-minal and either end terminal changes at a steady rate as the control shaft is turned.Suppose a linear taper potentiometer has a value of zero to 280 Ω. In most units theshaft rotates about 280 degrees, or a little more than three-quarters of a circle. Then theresistance between the center and one end terminal will increase right along with thenumber of degrees that the shaft is turned. The resistance between the center and theother end terminal will be equal to 280 minus the number of degrees the shaft is turned.Engineers say that the resistance is a linear function of the shaft position.Linear taper potentiometers are commonly used in electronic test instruments andin various consumer electronic devices. A graph of resistance versus shaft displacementfor a linear taper potentiometer is shown in Fig. 6-9.The potentiometer105