Thursday, October 15, 2015

Color Codes

This is from my favorite web-comic XKCD.

The image above suggesting a mnemonic for resistor color codes got me thinking about their origins. For some reason I had assumed color codes were an RCA design from the 1950s. No idea how that wrong idea got into my head. I was off by at least three decades. Firstly, electronic color code  are used to indicate the ratings of an electronic component, not just resistors. Capacitors, inductors, and others components also use color codes, but resistors are the most common today. More here.

The electronic color code was developed in the early 1920s by the Radio Manufacturers Association. It was later published as EIA-RS-279. They later joined with the Electronic Industries Alliance (EIA) The modern international standard is published as IEC 60062 under the International Electrotechnical Commission.  Blah blah blah. This tutorial covers the backstory in summary.

Color bands were used for a number of reasons. the components are tiny so tiny text would be hard to read. A color band is visible from any angle and text can't provide that readability. It's also a very cheap solution. Cheap effective solutions have staying power. Enough said, explaining the code is all too easy.

Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet = 7
Gray = 8
White = 9

Notice the colors of the rainbow were kept in sequence: Red, Orange, Yellow, Green, Blue, Violet. Brown is the only composite color on the list, and of course Black and White at opposing ends. (You can always use the XKCD mnemonic.) Wikipedia lists several dozen others in several different languages here. Some common ones I've heard are as as follows:

Bad Beer Rots Out Your Guts But Vodka Goes Well
Better Be Ready Or Your Great Big Plan Goes Wrong
Big Boys Race Our Young Girls But Violet Generally Wins
Beetle Bailey Runs Over Your General Before Very Good Witnesses

Most resistors have 3 or 4 color bands on them, and precision resistors have 5. The fourth band indicates tolerance, silver means +/- 10% the value shown by the first 3 bands, gold means +/-5%. Let's keep it simple. With a four band resistor the first 2 bands represent the digits of the resistance in ohms. That 3rd band is a multiplier, the number of zeros after the first two digits. With one exception. If the 3rd band is silver or gold then you multiply that value by 0.1 or 0.01 respectively. So to give an example Red-Red-Brown is 2-2-1, so that's a 220 ohm resistor.  There are also 6 bands resistors. Those are basically 5-band resistors with an additional ring indicating the reliability. I've never had a use for the 6th band.

But there were down sides to the color band system. Color blind people obviously find them a struggle.  Overheating components can shift or destroy the color bands. Even dust accumulation can make it difficult to distinguish red from brown. Also the color of the available light can cause color confusion. All of the above could make text hard to read as well but overall it's held up well for a 90+ year old technology.