When choosing a resistor for a particular application in an electrical or electronic device, it’s important to get a unit that has the correct properties, or specifications.Here are some of the most important specifications to watch for.
In theory, a resistor can have any ohmic value from the lowest possible (such as a shaft of solid silver) to the highest (dry air). In practice, it is unusual to find resistors with values less than about 0.1 Ω or more than about 100 MΩ.
Resistors are manufactured with ohmic values in power-of-10 multiples of 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. Thus, you will often see resistors with values of 47 Ω, 180 Ω, 6.8 kΩ, or 18 MΩ, but hardly ever with values such as 384 Ω, 4.54 kΩ, or 7.297 MΩ. In addition to these standard values, there are others that are used for resistors made with 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.
The first set of numbers above represents standard resistance values available in tolerances of plus or minus 10 percent (10%). This means that the resistance might be as much as 10 percent more or 10 percent less than the indicated amount. In the case of a 470-Ω resistor, for example, the value can be larger or smaller than the rated value by as much as 47 Ω, and still be within tolerance. That’s a range of 423 to 517 Ω.
Tolerance is calculated according to the specified value of the resistor, not the actual value. You might measure the value of a 470-Ω resistor and find it to be 427 Ω, and it would be within 10% of the specified value. But if it measures 420 Ω, it’s outside the rated range, and is therefore a reject. The second set, along with the first set, of numbers represents standard resistance values available in tolerances of plus or minus 5 percent (5%). A 470-Ω, 5 percent resistor will have an actual value of 470 Ω plus or minus 24 Ω, or a range of 446 to 494 Ω.
Some resistors are available in tolerances tighter than 5%. These precision units are employed in circuits where a little error can make a big difference. In most audio and radio-frequency oscillators and amplifiers, the 10% or 5% tolerance is good enough. In many cases, even a 20% tolerance is satisfactory.
All resistors are given a specification that determines how much power they can safely dissipate. Typical values are 1⁄ 4 W, 1⁄ 2W, and 1 W. Units also exist with ratings of 1⁄ 8 W or 2 W. These dissipation ratings are for continuous duty, meaning they can dissipate this amount of power constantly and indefinitely.
You can figure out how much current a given resistor can handle by using the formula for power (P) in terms of current (I ) and resistance (R). That formula, you should recall, is P = I 2R.Work this formula backward, plugging in the power rating in watts for P and the resistance in ohms for R, and solve for the current I in amperes. Alternatively, you can find the square root of P/R. The power rating for a given resistor can, in effect, be increased by using a network of 2 × 2, 3 × 3, 4 × 4, or more units in series-parallel. If you need a 47-Ω, 45-W resistor, but all you have is a bunch of 47-Ω, 1-W resistors, you can make a 7 × 7 network in series-parallel, and this will handle 49 W.
Resistor power dissipation ratings are specified with a margin for error. A good engineer never tries to take advantage of this and use, say, a 1⁄ 4-W unit in a situation that needs to draw 0.27 W. In fact, good engineers usually include their own safety margin. Allowing 10 percent, a 1⁄ 4-W resistor should not be called upon to handle more than about 0.225 W.
All resistors change value when the temperature changes dramatically. And because resistors dissipate power, they can get hot just because of the current they carry. Often, this current is so tiny that it doesn’t appreciably heat the resistor. But in some cases it does, and the resistance will change. Then a circuit might behave differently than it did when the resistor was still cool.
There are various ways to approach problems of resistors changing value when they get hot. One method is to use specially manufactured resistors that do not appreciably change value when they get hot. Such units are called temperature-compensated. But one of these can cost several times
as much as an ordinary resistor. Another approach is to use a power rating that is much higher than the actual dissipated power in the resistor. This will keep the resistor from getting very hot. Still another scheme is to use a series-parallel network of identical resistors to increase the power dissipation rating. Alternatively, you can take several resistors, say three of them, each with about three times the intended resistance, and connect them all in parallel. Or you can take several resistors, say four of them, each with about one-fourth the intended resistance, and connect them in series. It is unwise to combine resistors with different values. This can result in one of them taking most of the load while the others “loaf,” and the combination will be no better than the single hot resistor you started with.
How about using two resistors with half (or twice) the value you need, but with opposite resistance- versus-temperature characteristics, and connecting them in series or parallel? It is tempting to suppose that if you do this, the component whose resistance decreases with heat (negative temperature coefficient) will have a canceling-out effect on the component whose resistance goes up ( positive temperature coefficient). This can sometimes work, but in practice it’s difficult to find a pair of resistances that will do this job just right.
The Color Code for Resistors
Some resistors have color bands that indicate their values and tolerances. Youfll see three, four, or five bands around carbon-composition resistors and film resistors. Other units are large enough so that the values can be printed on them in ordinary numerals.
On resistors with axial leads (wires that come straight out of both ends), the first, second, third, fourth, and fifth bands are arranged as shown in following Figure. On resistors with radial leads (wires that come off the ends at right angles to the axis of the component body), the colored regions are arranged as shown in Fig. 6-12B. The first two regions represent numbers 0 through 9, and the third region represents a multiplier of 10 to some power. (For the moment, donft worry about the fourth and fifth regions.) Refer to following Table.
Suppose you find a resistor whose first three bands are yellow, violet, and red, in that order. Then the resistance is 4700 ƒ¶. Read yellow = 4, violet = 7, red = ~100. As another example, suppose you find a resistor with bands of blue, gray, orange. Refer to Table 6-1 and determine blue = 6, gray = 8, orange = ~1000. Therefore, the value is 68,000 ƒ¶ = 68 kƒ¶. The fourth band, if there is one, indicates tolerance. If itfs silver, it means the resistor is rated at 10%. If itfs gold, the resistor is rated at 5%. If there is no fourth band, the resistor is rated at 20%.
The fifth band, if there is one, indicates the maximum percentage that the resistance can be expected to change after 1000 hours of use. A brown band indicates a maximum change of 1% of the rated value. A red band indicates 0.1%. An orange band indicates 0.01%. A yellow band indicates 0.001%. If there is no fifth band, it means that the resistor might deviate by more than 1% of the rated value after 1000 hours of use.
At A, locations of color-code bands on a resistor with axial leads. At B, locations of color code designators on a resistor with radial leads.
|Color of band||Numeral (first and second bands)||Multiplier(third band)|
|Orange||3||1000 (1 k)|
|Yellow||4||104 (10 k)|
|Green||5||105 (100 k)|
|Blue||6||106 (1 M)|
|Violet||7||107 (10 M)|
|Gray||8||108 (100 M)|
|White||9||109 (1000 M or 1 G)|