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Resistor Color Codes

  Resistor values are marked in a number of different ways based on the manufacturers preference. One common method is to use colored bands to denote the resistor’s value and tolerance. Although there are color band decoder apps, it is worthwhile taking the time to learn the color codes. This is particularly true when you are assembling a circuit board or looking through a junk box trying to find a value for prototyping. With a little information and a helpful mnemonic, color coded resistors are easy to read. The following figure depicts two common marking schemes.

This figure depicts three and four band resistor color codes with the tolerance and tempco bands.

    The difference between the markings shown above is the number of color bands used to indicate the value. In both cases the resistors color code is broken into two groups. The first group conveys the resistors value and the second group the tolerance and temperature coefficient (tempco). The tempco band is very rarely used and, in 3 band resistors, the tolerance band is not always present.

What the Colors Stand for

     Reading the resistors value involves mentally changing each of the colors shown to a number. The following table shows the color to number translation.

This table shows how the color, color name, and numberic value are associated.

    Remembering which color stands for which digit can be challenging unless you use a memory aid. The following mnemonics can help. You can choose one of these or make up your own. The one I was first was taught is a bit bawdy but, because of that, I find it hard to forget. Wikipedia has a list of tame and not so tame mnemonics.

This figure lists several mnemonics that make memorizing the color codes easier.

    The mnemonics tell you the order of the colors. In other words, black is first, brown is second, red is third and so forth. When you first start using color codes, it is not uncommon to use your fingers and a piece of scratch paper to keep track of things. After a while, the color code will be committed to memory and reading color codes will become second nature.

Reading the Value

    Reading a three or four band value follows the same pattern. With the value group of colors on the left, assign a number to each color moving from left to right (remember not to include the tolerance or tempco colors). In the case of a three value band resistor, the first two numbers assigned are the beginning of the resistance value and the last number assigned is the number of 0s that follow the first two digits. In the case of a four value band resistor, the first three numbers assigned are the first three digits of the resistance value and the fourth band is the number of 0s that follow the first three digits. The following figure depicts this graphically.

This figure shows several examples of how color codes are read.

Low Value Resistors

    The last band in the resistors value is technically called a multiplier. Its is used as an exponent of ten to multiply the proceeding color values by. In most cases, it is much easier to think of it as the number of 0’s to be added to the proceeding color values as shown above.

    This works really well until you try and read a low value resistor’s color code. There are three additional colors that can used as the third band in a three band value or the fourth band in a four band resistors. They are gold and silver and pink. Gold means multiply the value from the previous bands by 0.1, silver means multiply the previous bands by 0.01, and pink means multiply the previous bands by 0.001.

    Suppose you had a resistor with the bands: black, brown, gold. The black and brown yield a value of 10. The gold multiplier band means you should multiply 10 by 0.1 which yields a value of 1Ω.

Reading the Tolerance

    The colors associated with the tolerance of a resistor work differently that those for the value. Each color stands for a discrete tolerance value. The following table lists the tolerances for three and four band resistors.

This table lists the tolerance values associated with different colors in three and four band resistors.

What About the Temperature Coefficient? 

    The color code for this is rarely marked on the part. The tempco band was mentioned to avoid confusion if it happens to appear on the part you are reading. If you designed the circuit, particularly one that needs to maintain its accuracy over temperature, you should know if this is something to be concerned about and act accordingly by selecting a resistor whose parameters meet your requirements. If you are building to an established design and parts list you can assume the designer took this into consideration.  For a vast majority of garden variety circuits, you can ignore the tempco.

copyright © 2021 John Miskimins