In the ever-shifting world of electronics, one thing that hasn’t changed over the years is the resistor color code. It is used to identify resistance value and tolerances for cylindrical resistors with axial leads. The International Electronics Commission formally defines the color code in its IEC 60062 standard.
The resistor color code is expressed in bands of various colors imposed sequentially across the circumference of the resistor. There are three-band versions, versions with four and five bands, and a less common version with six bands. Although originally devised for carbon resistors almost a century ago, the code is still alive and well and is used for both carbon resistors as well as other types of resistors whose packaging takes the same cylindrical form.
Three-band code
The picture below represents a resistor that’s value is indicated by the three-band version of the color code.
For all versions of the code, the first band to be read is the one closest to an axial lead. In the three-band version, the first band signifies the first digit in the resistor’s value, and in the case of the example, the band is brown. Moving right, the next band is black, and it signifies the second digit. The legend in the bottom half of the picture describes brown as 1, and black as 0. Notice that the legend describes the first and second bands as exactly the same. Concatenating 1 and 0 yields 10. The third band, the multiplier, is red in the example, signifying 100. Multiplying 10 by 100 yields 1,000, representing the resistor’s value in ohms. The tolerance of a three-band resistor is +/- 20 percent, which means the resistor just described can be relied on to be between 800 and 1,200 ohms.
Four-band and five-band code
The fourth band of a resistor specified by a four-band resistor code is located to the right of the third band, separated by a space, and it specifies the tolerance of the resistor. Otherwise, it is read the same as a three-band resistor. The five-band resistor code is used to specify resistors built to a tighter tolerance. In this case, it is the first three bands that are digits. Then, the fourth band is the multiplier, and the fifth represents the tolerance.
The first three bands of the example resistor (above) are green, purple, and black. That’s 5, 6, and 0. Concatenated together, that yields 560. The fourth band is red, for a multiplier of 100. That indicates a resistance of 56,000 ohms. The last band is gold. Looking at the legend on the bottom of the picture, the gold band shows a tolerance of +/- 5 percent, indicating that this particular resistor can be relied on to be between 53,200 and 58,800 ohms.
Surface-mounted resistors
Of course, most modern resistors are not built into cylindrical packages with axial leads. Surface-mounted devices (SMD) are more widely used today, and these devices most often employ a very simple three- or four-digit code.
The basic three-digit code employs two base numbers and an exponent of ten. For example, 462 would be 46 times 10E2, or 46 times 100 for 4600 ohms. The four-digit code has three base digits and an exponent. Here, 4611 would be 461 times 10E1, or 461 times 10 for 4610.
The CR0603-JW-300ELF is an example of a surface-mounted resistor SIP package. A resistor is revealed on the datasheet with the markings of 100. The first two numbers yield 10. The third number, the exponent, is 0. The computation is 10 times 10E0, or 10 times 1 for 10 ohms.
Of course, it’s not always that simple. The EIA96 series of 1 percent tolerant SMD resistors use a system with two digits and a letter. The two digits are the code for one of only 96 possible three-digit base numbers, and the letter represents the multiplier. You’ll need a lookup table, but whatever code you encounter, you’ll also need a magnifying glass, because these codes have to fit on the tiny resistors they describe.