The color code
Some resistors have color bands that indicate their values and tolerances. You’ll 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, the bands (first, second, third, fourth, fifth) are
arranged as shown in Fig. 6-12A. On resistors with radial leads, the bands are arranged
as shown in Fig. 6-12B. The first two bands represent numbers 0 through 9; the third
band represents a multiplier of 10 to some power. For the moment, don’t worry about
the fourth and fifth bands. Refer to Table 6-1.
Suppose you find a resistor whose first three bands are yellow, violet, and red, in
that order. Then the resistance is 4,700 Ω or 4.7 KΩ. Read yellow = 4, violet = 7, red = × 100.
As another example, suppose you stick your hand in a bag and pull out a unit 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 Ω = 68 KΩ.
After a few hundred real-life experiences with this color code, you’ll have it memorized.
If you aren’t going to be using resistors that often, you can always keep a copy of
Table 6-1 handy and use it when you need it.
The fourth band, if there is one, indicates tolerance. If it’s silver, it means the resistor
is rated at plus or minus 10 percent. If it’s gold, the resistor is rated at plus or minus
5 percent. If there is no fourth band, the resistor is rated at plus or minus 20 percent.
The fifth band, if there is one, indicates the percentage that the value might change
in 1,000 hours of use. A brown band indicates a maximum change of 1 percent of the
rated value. A red band indicates 0.1 percent; an orange band indicates 0.01 percent; a
yellow band indicates 0.001 percent. If there is no fifth band, it means that the resistor
might deviate by more than 1 percent of the rated value after 1,000 hours of use.
A good engineer always tests a resistor with an ohmmeter before installing it. If the
unit happens to be labeled wrong, it’s easy to catch while assembling a complex electronic
circuit. But once the circuit is all together, and it won’t work because some resistor
is mislabeled (and this happens), it’s a gigantic pain to find the problem.
