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Aviation Maintenance Technician Handbook–General

FAA-H-8083-30B Version 2023
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Chapter 12

Fundamentals of Electricity & Electronics

  • Carbon film
  • Metal-oxide
  • Metal film
  • Metal glaze

The construction of a film resistor is accomplished by depositing a resistive material evenly on a ceramic rod. This resistive material can be graphite for the carbon film resistor, nickel chromium for the metal film resistor, metal and glass for the metal glaze resistor, and metal and an insulating oxide for the metal-oxide resistor.

Resistor Ratings

Color Code

It is very difficult to manufacture a resistor to an exact standard of ohmic values. Fortunately, most circuit requirements are not extremely critical. For many uses, the actual resistance in ohms can be 20 percent higher or lower than the value marked on the resistor without causing difficulty. The percentage variation between the marked value and the actual value of a resistor is known as the “tolerance” of a resistor. A resistor coded for a 5 percent tolerance is not more than 5 percent higher or lower than the value indicated by the color code.

The resistor color code is made up of a group of colors, numbers, and tolerance values. Each color is represented by a number, and in most cases, by a tolerance value. [Figure 12-49]

When the color code is used with the end-to-center band marking system, the resistor is normally marked with bands of color at one end of the resistor. The body or base color of the resistor has nothing to do with the color code, and in no way indicates a resistance value. To prevent confusion, this body is never the same color as any of the bands indicating resistance value.

Figure 12-45. Unconnected crossover wires.
Figure 12-45. Unconnected crossover wires.
Figure 12-46. Connected wires.
Figure 12-46. Connected wires.
Figure 12-47. Common wire types.
Figure 12-47. Common wire types.

Color Band Decoding

When the end-to-center band marking system is used, either three or four bands mark the resistor.

  1. The first color band (nearest the end of the resistor) indicates the first digit in the numerical resistance value. This band is never gold or silver in color.
  2. The second color band always indicates the second digit of ohmic value. It is never gold or silver in color. [Figure 12-50]
  3. The third color band indicates the number of zeros to be added to the two digits derived from the first and second bands, except in the following two cases: (a) If the third band is gold in color, the first two digits must be multiplied by 10 percent. (b) If the third band is silver in color, the first two digits must be multiplied by 1 percent.
  4. If there is a fourth color band, it is used as a multiplier for percentage of tolerance, as indicated in the color code chart in Figure 12-49. If there is no fourth band, the tolerance is understood to be 20 percent.

Figure 12-50 illustrates the rules for reading the resistance value of a resistor marked with the end-to-center band system. This resistor is marked with three bands of color, which must be read from the end toward the center.

There is no fourth color band; therefore, the tolerance is

Formula: understood to be 20 percent. 20 percent of 250,000 Ω, equals ; 50,000 Ω. ; Since the 20 percent tolerance is plus or minus, ; Maximum resistance ; = 250,000 Ω + 50,000 Ω ; = 300,000 Ω ; Minimum resistance ; = 250,000 Ω − 50,000 Ω ; = 200,000 Ω

The following paragraphs provide a few extra examples of resistor color band decoding. Figure 12-51 contains a resistor with another set of colors. This resistor code should be read as follows:

Figure 12-48. Fixed resistor schematic.
Figure 12-48. Fixed resistor schematic.
Formula: The resistance of this resistor is 86,000 ±10 percent ohms. ; The maximum resistance is 94,600 ohms, and the minimum ; resistance is 77,400 ohms. ; Another example is the resistance of the resistor in Figure 12-52 ; is 960 ±5 percent ohms. The maximum resistance is 1,008

ohms, and the minimum resistance is 912 ohms.