Standard Color-Coded Bands for Evaluating Resistance
Practical resistors are manufactured in standard values, various resistance tolerances, several power ratings (as will be explained shortly), and in a number of different forms of construction. The three basic construction techniques are composition type, which uses carbon or graphite and is molded into a cylindrical shape, wire-wound type, in which a length of enamel-coated wire is wrapped around an insulating cylinder, and metal-film type, in which a thin layer of metal is vacuum deposited. Table illustrates the standard color-coded bands used for evaluating resistance and their interpretation for the common carbon composition type. Sometimes a fifth band is also present to indicate reliability. Black is the least reliable color and orange is 1000 times more reliable than black.
For resistors ranging from 1 to 9.1 , the standard resistance values are listed in Table. Other available values can be obtained by multiplying the values shown in Table by factors
of 10 ranging from 10 to about 22 × 106 . For example, 8.2 Ω ,82Ω , 820Ω , . . . , 820Ω k are standard available values.
The maximum allowable power dissipation or power rating is typically specified for commercial resistors. A common power rating for resistors used in electronic circuits is 1/4 W; other ratings such as 1/8, 1/2, 1, and 2 W are available with composition-type resistors, whereas larger ratings are also available with other types. Variable resistors, known as potentiometers, with a movable contact are commonly found in rotary or linear form. Wire-wound potentiometers may have higher power ratings up to 1000 W.