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Resistor Color Code

When I imagine resistors in general, I always think of the ones that are nice little brown cylinders with colored bands painted on. I always liked those! Not only did they look neat, they also had their own secret code.


Even better, the code contained the colors of the spectrum, which was something else that fascinated me.

The code is also very well designed … among other things, it's mnemonic, easy to remember. It contains the spectrum, and has black at one end and white at the other, so all you need to remember is that brown is 1 (which is easy since you see that all the time), and that the other color is gray, and the whole thing is uniquely determined.

Having said that, I guess I also ought to admit that I didn't write the code down from memory, I had to break out my copy of The Art of Electronics and look it up. On second thought, I should have just found it on line.

How does it work? A resistor normally has three or four bands on it. The first three are digits, two mantissa and one exponent … in other words, if the digits are abc, the number they represent is ab × 10c. (This is a useful example to have in mind if you're ever learning how computers represent floating-point numbers.) The fourth band, if present, indicates the tolerance. So, for example, a resistor with the pattern “brown black red” is 1000 ohms, plus or minus twenty percent.

Another reason I wanted to write all this down is that it makes me sad that one doesn't see resistor codes much any more. It used to be that if you opened up any electronic device—like, say, an Apple II—among other things you'd see a bunch of color-coded resistors. Nowadays, though, all you see is a bunch of tiny indistinguishable blocks.


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@ September (2004)