For these values, the third stripe is either gold or silver, indicating a fractional decimal multiplier. When the third stripe is gold, multiply the first two digits by 0.1.
In Fig. 6-6c, the R value is 25 x 0.1 = 2.5 Ω Silver means a multiplier of 0.01. If the third band in Fig. 6-6c were silver, the R value would be 25 x 0.01 = 0.25 Ω
It is important to realize that the gold and silver colors are used as decimal multipliers only in the third stripe. However, gold and silver are used most often as a fourth stripe to indicate how accurate the R value is.
The amount by which the actual R can be different from the color-coded value is the tolerance, usually given in percent. For instance, a 2000 Ω resistor with ± 10 percent tolerance can have resistance 10 percent above or below the coded value. This R, therefore, is between 1800 and 2200Ω. The calculations are as follows:
10 percent of 2000 is 0.1 x 2000 = 200
For + 10 percent, the value is 2000 + 200 = 2200 Ω
For -10 percent, the value is 1000 – 200 = 1800 Ω
As illustrated in Fig. 2.5, silver in the fourth band indicates a tolerance of ± 10 percent; gold indicates 5 percent. If there is no color band for tolerance, it is ±20 percent. The inexact value of carbon resistors is a disadvantage of their economical construction. They usually cost only about 10 to 50 cents each, or less in larger quantities. In most circuits, though, a small difference in resistance can be tolerated.
It should be noted that some resistors have five stripes, instead of four. In this case, the first three stripes give three digits, followed by the decimal multiplier in the fourth stripe and tolerance in the fifth stripe. These resistors have more precise values, with tolerances of 0.1 to 2 percent. More details are given in App. G.
- Wire-Wound-Resistor Marking
Usually, wirewound resistors are big enough physically to have the R value printed on the insulating case. The tolerance is generally ±5 percent, except for precision resistors, which have a tolerance of ± 1 percent or less.
Some small wire-wound resistors may be colorcoded with stripes, however, like carbon resistors. In this case, the first stripe is double the width of the others to indicate a wire-wound resistor. This type may have a wattage rating of 3 or 4 W.
- Preferred Resistance Values
In order to minimize the problem of manufacturing different R values for an almost unlimited variety of circuits, specific values are made in large quantities so that they are cheaper and more easily available than unusual sizes. For resistors of ±10 percent, the preferred values are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, and 82 with their decimal multiples. As examples, 47, 470, 4700, and 47,000 are preferred values. In this way, there is a preferred value available within 10 percent of any R value needed in a circuit.
For more accurate resistors of lower tolerance, there are additional preferred values. They are listed in Table G-3 in App. G.
Answers at End of Chapter
- Give the color for 4.
- What tolerance does a silver stripe show?
- Give the multiplier for red in the third stripe.
- Give R and the tolerance for a resistor coded with yellow, violet, brown, and gold stripes.
Variable resistors can be wire-wound, as in Fig. 2.2b, or the carbon type, illustrated in Fig. 2.7. Inside the metal case of Fig. 2.7a, the control has a circular disc, shown in Fig. 2.7b that is the carbon-composition resistance element. It can be a thin coating on pressed paper or a molded carbon disc. Joined to the two ends are the external soldering-lug terminals 1 and 3. The middle terminal is connected to the variable arm that contacts the resistor element by a metal spring wiper. As the shaft of the control is turned, the variable arm moves the wiper to make contact at different points on the resistor element. The same idea applies to the slide control in Fig. 2.8, except that the resistor element is long instead of circular.
When the contact moves closer to one end, the R decreases between this terminal and the variable arm.