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The values for resistors in a typical series, like E12, follow an exponential series (hence the "E"?). 10, 12, 15, 18, 22, etc., everybody knows them by heart (at least E12). But often you want simple ratios in dividers, where a series like 10, 20, 30, 40, etc. would be more convenient. The problem with the exponential series is that, if 15/10 is the wrong ratio, so will 22/15 be, or 33/22. They'll all produce the same result.
Are there resistors with values 10, 20, 30,... ?

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The E12 series isn't quite evenly spaced from an exponential standpoint.

1.0 1.2 1.5 2.2 2.7 3.3 3.9 4.7 5.1 5.6 6.8 8.2

For example, 10/3.3 is much closer to 3 than to 3.3. It is admittedly curious that the 12-per-decade series offers two ways of getting a 3:1 ratio within 1.3% (one is 10/3.3; the other is 8.2/2.7) but the best 2:1 ratios it offers are off by 3% or more (5.6/2.7 is 3.7% off; 6.8/3.3 is 3.03% off). On the other hand, since the E12 series is typically used with 10%-tolerance parts, the sloppiness of the ratios doesn't really matter.

The E24 series (5% parts) offers values of 2.0 and 3.0; the E96 and E192 series offer 2.00, but the closest they offer to 3.0 is 3.01.

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The exponential series ensures that on average, you get minimum error when you choose the nearest preferred value to a calculated value see Preferred Number. If you go to E192 you will get close as supercat says but some manufacturers make special non-preferred values (eg 600 Ohms).

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Yes, but E192 is not very common, and more expensive as well, because being precision, (0.1%?) – Federico Russo Apr 18 '11 at 15:40
@Federico - True, but the only times I have ever needed a very accurate ratio, I needed the precision anyway. – MikeJ-UK Apr 18 '11 at 15:48

Digikey reports this Panasonic 30 ohm part, among others. $0.003 US, 0805.

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