# Calculating preferred resistor values

Discussion in 'Electronic Basics' started by Rheilly Phoull, Jan 15, 2004.

1. ### Rheilly PhoullGuest

G'Day All

Can anyone help me out with the formula that calculates the various values
used for the preferred ranges, like say 1% and 5% ??

2. ### Costas VlachosGuest

Do you mean the different value ranges like E12, E24, etc.? These are
logarithmically spaced, but unfortunately there is no direct formula to
calculate their exact values.

For the E12 series the approximate values can be computed using the formula
10^(n/12), where n goes from 0 to 11. For the general case, the approximate
values for the range Em are given by 10^(n/m), where n = 0,1,2,...,m-1.

If you apply this to the E12 series, you'll see that the values you get are
pretty close to those of the true E12 series, but deviate somewhat in a way
that is more complex than simply rounding the values.

http://www.logwell.com/tech/components/resistor_values.html

Regards,
Costas
_________________________________________________
Costas Vlachos Email:

3. ### GarethGuest

I wrote a simple Windows program which finds the nearest preferred
resistor value from the E12, E24, E48 or E96 series. It uses a look up
table not a formula.

Email me if you (or anyone else) want a copy.

Gareth.
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4. ### Rheilly PhoullGuest

Yeah, thanks for the thought.
I'm just diddlin' about learning C++ and thought I'd re-invent the wheel for
something meaningful to write.
My first attempt uses an array with the values in but I was looking for a
more elegant method.