Resistor values

Chapter Resistor values

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

  • Resistor valuesIn theory, a resistor can have any value from the lowest possible (such as a shaft of solidsilver) to the highest (open air). In practice, it is unusual to find resistors with valuesless than about 0.1 Ω, or more than about 100 MΩ.Resistors are manufactured in standard values that might at first seem rather oddto you. The standard numbers are 1.0, 1.2, 1.5,1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2.Units are commonly made with values derived from these values, multiplied by somepower of 10. Thus, you will see units of 47 Ω, 180 Ω, 6.8 KΩ, or 18 MΩ, but not 380 Ωor 650 KΩ. Maybe you’ve wondered at some of the resistor values that have been usedin problems and quiz questions in previous chapters. Now you know that these choicesweren’t totally arbitrary; they were picked to represent values you might find in real cir-cuits.In addition to the above values, there are others that are used for resistors madewith greater precision, or tighter tolerance. These are power-of-10 multiples of 1.1, 1.3,1. 6, 2.0, 2.4, 3.0, 3.6, 4.3, 5.1, 6.2, 7.5, and 9.1.You don’t have to memorize these numbers. They’ll become familiar enough overtime, as you work with electrical and electronic circuits.ToleranceThe first set of numbers above represents standard resistance values available in tol-erances of plus or minus 10 percent. This means that the resistance might be asmuch as 10 percent more or 10 percent less than the indicated amount. In the caseof a 470-ohm resistor, for example, the value can be off by as much as 47 ohms andstill be within tolerance. That’s a range of 423 to 517 ohms. The tolerance is calcu-lated according to the specified value of the resistor, not the actual value. You mightmeasure the value of a 470-ohm resistor and find it to be 427 ohms, and it would bewithin 10 percent of the specified value; if it measures 420 ohms, it’s outside the10-percent range and is a “reject.”The second set, along with the first set, of numbers represents standard resistancevalues available in tolerances of plus or minus 5 percent. A 470-ohm, 5-percent resistorwill have an actual value of 470 ohms plus or minus 24 ohms, or a range of 446 to 494ohms.Some resistors are available in tolerances tighter than 5 percent. These precisionunits are employed in circuits where a little error can make a big difference. In most au-dio and radio-frequency oscillators and amplifiers, 10-percent or 5-percent tolerance isgood enough. In many cases, even a 20-percent error is all right.Power ratingAll resistors are given a specification that determines how much power they can safelydissipate. Typical values are 1/4 W, 1/2 W, and 1 W. Units also exist with ratings of 1/8 Wor 2 W. These dissipation ratings are for continuous duty.You can figure out how much current a given resistor can handle, by using the for-mula for power (P) in terms of current (I) and resistance (R): P = I 2R. Just work this110 Resistors