The Resistor Color Code is used for marking and identifying pertinent data for standard resistors. Figures 2.41 and 2.42 show the color coding scheme per EIA Standard RS-279 and MIL-STD- 1285A respectively.
Figure 2.42. Resistor Color Code per MIL-STD-1285A
In a color coded scheme, each color represents a single digit number, or conversely, a single digit number can be represented by a particular color band as shown in Table 2.3 that is based on MILSTD- 1285A color code.
As shown in Figure 2.42, the first and second bands designate the first and second significant digits respectively, the third represents the multiplier, that is, the number by which the first two digits are multiplied, and the fourth and fifth bands, if they exist, indicate the tolerance and failure rate respectively.
The tolerance is the maximum deviation from the specified nominal value and it is given as a percentage. The failure rate is the percent probability of failure in a 1000-hour time interval. Let A and B represent the first and second significant digits and C represent the multiplier.
Then the resistance value is found from the expression
R = (10 × A + B)×10^C
Example
The value of a resistor is coded with the following colored band code, left to right: Brown, Green, Blue, Gold, Red. What is the value, tolerance, and probability of failure for that resistor?
Solution:
Table 2.3 yields the following data: Brown (1st significant digit) = 1, Green (2nd significant digit) = 5, and Blue (multiplier) = 1,000,000. Therefore, the nominal value of this resistor is 15,000,000 Ohms or 15 MΩ.
The 4th band is Gold indicating a ±5% tolerance meaning that the maximum deviation from the nominal value is 15,000,000 ±5% = 15,000,000 × ±0.05 = ±750,000 Ohms or ±0.75 MΩ. That is, this resistor can have a value anywhere between 14.25 MΩ and 15.75 MΩ. Since the 5th band is Red, there is a 0.1% probability that this resistor will fail after 1000 hours of operation.
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