Identifying resistors
Most axial resistors use a pattern of colored stripes to indicate resistance. Surface-mount ones are marked numerically. Cases are usually brown, blue, or green, though other colors are occasionally found such as dark red or dark gray.

One can use a multimeter or ohmmeter to test the values of a resistor.

Resistor standards

• MIL-R-11
• MIL-R-39008
• MIL-R-39017
• MIL-PRF-26
• MIL-PRF-39007
• BS 1852
• EIA-RS-279

There are other MIL-R- standards.

Resistor Color Code

Four-band axial resistors
Four-band identification is the most commonly used color coding scheme on all resistors. It consists of four colored bands that are painted around the body of the resistor. The scheme is simple: The first two numbers are the first two significant digits of the resistance value, the third is a multiplier, and the fourth is the tolerance of the value. Each color corresponds to a certain number, shown in the chart below. The tolerance for a 4-band resistor will be 2%, 5%, or 10%.

Note: red to violet are the colors of the rainbow where red is low energy and violet is higher energy.

As an example, let us take a resistor which (read left to right) displays the colors yellow, violet, yellow, brown. We take the first two bands as the value, giving us 4, 7. Then the third band, another yellow, gives us the multiplier 10⁴. Our total value is then 47 x 10⁴ Ω, totalling 470,000 Ω or 470 kΩ. Our brown is then a tolerance of ±1%.

Resistors use specific values, which are determined by their tolerance. These values repeat for every exponent; 6.8, 68, 680, and so forth. This is useful because the digits, and hence the first two or three stripes, will always be similar patterns of colors, which make them easier to recognize.

5-band axial resistors
5-band identification is used for higher precision (lower tolerance) resistors (1%, 0.5%, 0.25%, 0.1%), to notate the extra digit. The first three bands represent the significant digits, the fourth is the multiplier, and the fifth is the tolerance. 5-band standard tolerance resistors are sometimes encountered, generally on older or specialized resistors. They can be identified by noting a standard tolerance color in the 4th band. The 5th band in this case is the temperature coefficient.

Preferred values
Resistors are manufactured in values from a few milliohms to about a gigaohm; only a limited range of values from the IEC 60063 preferred number series are commonly available. These series are called E6, E12, E24, E96 and E192. The number tells how many standardized values exist in each decade (e.g. between 10 and 100, or between 100 and 1000). So resistors conforming to the E12 series, can have 12 distinct values between 10 and 100, whereas those confirming to the E24 series would have 24 distinct values. In practice, the discrete component sold as a “resistor” is not a perfect resistance, as defined above. Resistors are often marked with their tolerance (maximum expected variation from the marked resistance). On color coded resistors the color of the rightmost band denotes the tolerance:

silver 10%

gold 5%

red 2%

brown 1%

green 0.5%.

Closer tolerance resistors, called precision resistors, are also available.

Since some manufacturers may sort resistors into tolerance classes, prudent design of circuits should assess the effect of any or all resistors being at the upper limits of the tolerance range.

E12 preferred values: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82

Multiples of 10 of these values are used, eg. 0.47 Ω, 4.7 Ω, 47 Ω, 470 Ω, 4.7 kΩ, 47 kΩ, 470 kΩ, and so forth.

E24 preferred values, includes E12 values and: 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91

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