The title says it.
There are a zillion websites that cover this. Some have online interactive tools you can use to read one resistor or learn the art of doing so. Google "resistor color code" and maybe add -"free software" to avoid spammy sites.
Here's a basic chart: http://www.azega.com/how-to-read-a-resistor-color-code/
Here's an online tool for practicing, once you think you've got the basics. Comes with sound effects to keep you awake and/or remind you of 1980s style arcade games:
These sites could vanish, but others appear. Quality varies wildly but seek and after perusing several, you'll know what you need to know.
I learned 'em the old-fashioned way: By having a stack of assorted resistors on my desk and having to look up the code each time I needed one. Pretty soon, I could recognize a 470 or 10K by sight. (This is what you want, rather than reading digits individually. Or do you read words one letter at a time?)
Now it is 2010, and color codes are obsolete. Do not spend time learning them.
There is an answer to another question that answers this also: Resistors with ends of the same colour
The basic principle is simple enough.
Each color represents a number. Black: 0, Brown: 1, Red: 2, Orange: 3: Yellow: 4, Green: 5, Blue: 6, Violet: 7, Grey: 8, White: 9.
The normal pattern is.
- 2-3 bands for significant digits of the value (interpreed as an integer, e.g. brown-black-black is 100).
- 1 band for multiplier (10n for normal colors 0.1 for gold, 0.01 for silver)
- wider gap
- 1 band for tolerance (this uses it's own meanings for the colors, the most common on modern resistors are probablly 5%: gold 1%: brown and 0.1% violet)
- Some resistors have an extra band for temperature coefficient, but this is rare.
The reality though is that reliablly reading color codes on modern 1% resistors* in the "¼W" or worse "⅛W" body sizes** is very difficult. The bands are often very thin and perception of the colors can easilly be shifted by lighting and the background color, so it's really easy to misread a color band. Further the wider gap is at best only marginally wider and sometimes there doesn't seem to be any distinguishable difference at all in the gaps, so it's easy to end up reading the code backwards.
You can mitigate misreads to some extent by knowing or having a table of likely resistor values. If the value you read isn't in a table of standard resistor values ( http://www.logwell.com/tech/components/resistor_values.html ), then you almost certainly misread it. This doesn't help you with misreading the multiplier though.
* 5% resistors are easier for several reasons, first they usually have the 4-band code giving more space per band. Secondly the last band is gold which basically eliminates the chance of reading the code backwards. Thirdly the background is usually a relatively neutral tan color rather than the blue/green backgrounds typical on metal film resistors.
** Note, body size and power rating are not nessacerally the same, it's common to see 0.6W resistors in the "¼W" body size and 0.4W resistors in the "⅛W" body size
protected by stevenvh Oct 13 '12 at 11:37
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