Why is it easy to find 4.7k resistors, but not 4.8k resistors? Where do common values like 1.2k, 2.7k, 560, and 820 come from and who decides them? As you may know, resistors come in different tolerances, as indicated by the 4th band (gold = 5%, silver = 10%). A 100 ohm resistor with a 10% tolerance is expected to have a value somewhere between 90 and 110 ohms, so it wouldn’t make much sense to buy a 101 Ohm resistor when it’s actual value could be less than a 95 Ohm, 10% resistor. The Electronic Industries Association (EIA) is the primary body that standardizes the values for resistors, and they publish value lists called “E” series. In the 10% series, known as E12, each value is spaced so that there won’t be overlap. The min and max values are listed:
| (min) | value | (max) |
|---|---|---|
| (90) | 100 | (110) |
| (108) | 120 | (132) |
| (135) | 150 | (165) |
| (162) | 180 | (198) |
| (198) | 220 | (242) |
| (243) | 270 | (297) |
The number following the “E” stands for the number of logarithmic steps per decade. Logwell has a table that lists common values from 10% through 1% Here is a neat resistor selection tool from uCHobby that allows you to select only legal values