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Resistor Color Codes
An important lesson to learn in electronics is how to read the color codes for resistors. If you plan a career in electronics, it will be a big help to memorize the color code, and the technique for calculating the resistor value. This essay will teach you how to read resistor color codes.
Reading the resistor color code is easy with a little practice. Most resistors have four color bands, while some have five bands. The chart below (Figure 1) shows the color codes and their respective values depending on which band they fall on. The resistor value is decoded by reading the colors from left to right. The tricky part is determining which is the left side and which is the right side. Do this by finding the gold or silver band which is always on the right side. Then start reading the resistor colors from the left.
Look at a common 1K resistor. From left to right, a 1K resistor will have brown-black-red-gold (). Looking at Figure 1, this decodes respectively to 1 - 0 - x100 - ±5%.
Take the first and second significant digits together to be 10. Then multiply by the multiplier 100. That gives you 1000 ohms as the resistor value, which is 1K (see sidebar). So what's with the ±5? The tolerance band tells us that the measured resistance can be off by plus or minus 5%. So the actual measured resistor value could be anywhere from 950 ohms to 1050 ohms.
Now look at a 27K resistor. Again from left to right it reads red-violet-orange-gold which translates to 2-7-x1000-±5. So this would be 27 x 1000 = 27000 ohms which is 27K.
The fifth band is rarely used on resistors, but is included in the color code chart so that you have a more complete reference. The fifth color band signifies the rate at which the resistor fails when operating at its full rated wattage. It is read as a percentage failure per 1000 hours.
For practice, try reading color codes from resistors with unknown values. Try to figure out what the resistor value should be from the color code. Then measure the resistor with an ohmmeter or multimeter to see if you got it right. With some practice, you will be able to pick out common resistors from a box of spare parts just by glancing at the color bands.
Resistors come in standard values such as 1K, 2.2K, 4.7K, and so on. Why these values? Consider a range of resistors with a 10% tolerance. The resistors are designed with values such that a 10% variance of one resistor would meet or overlap with the 10% variance of the next resistance value.
To clarify, the 10% tolerance range for a 1K resistor would be 0.9K to 1.1K. So the next resistor value going up the scale would be 1.2K since it's 10% tolerance range would be 1.08K to 1.32K. The lower end of the tolerance range for the 1.2K overlaps a little with the upper end of the range for the 1K.
In the case of a range of resistors with a 5% tolerance, there would have to be more unique values to allow for overlap. Starting with the 1K resistor, the next resistor value up the scale would have to be 1.1K to allow for an overlap. Resistors with a 20% tolerance on the other hand would have bigger jumps between values. Starting with the 1K resistor, the next value up the scale would be 1.5K. In this case, the upper value for a 1K would be 1.2K, while the lower value for a 1.5K would be 1.2K.
The sidebar shows standard resistor values for 5%, 10%, and 20% tolerance ranges. We will leave the calculation of 1% tolerance resistors as an exercise for you.
Additional Resistor Resources
The University of New Brunswick has an interactive resistor color code calculator.
Written by Sean Moniz Copyright 2001-2007 by Montek Electronics