Choosing Standard Resistor Values

A common problem faced by electrical engineers and others is that most passive components (resistors and capacitors) are only available in certain values.

Note: if you're willing to pay enough money, you can get any value you want, in any tolerance. The rest of us have to settle for the standard values.

These values were selected to fall uniformly on a logarithmic scale with the spacing determined by the part tolerance. For example, here are the standard values for 5% tolerance resistors in the 1000-ohm decade (a decade is a 10:1 span of part values). There are 24 values in each decade.

**5% E-24 Standard Decade**
1K0	1K1	1K2	1K3	1K5	1K6
1K8	2K0	2K2	2K4	2K7	3K0
3K3	3K6	3K9	4K3	4K7	5K1
5K6	6K2	6K8	7K5	8K2	9K1

My program accepts ideal input values and outputs values that are the nearest standard value based on the tolerance specified, using the standard value series for that tolerance value. As you can see, if you want the values spaced closer together, then you must buy a closer (more precise) tolerance part.

**Component Value Series**
Tol	# Values/Decade	Name
1%	96	E-96
2%	48	E-48
5%	24	E-24
10%	12	E-12
20%	6	E-6

The program runs from the MS-DOS command line, and accepts a space-separated list of values according to the following form: 1.5, 1R5, 1k5, 1.5k, 1500, 1.5e5, 1.5m, 1.5meg, 1500000. The less common formats are described in the table.

**Input Formats**
Input	Value
1R5	1.5
1k5	1.5k / 1500
1e6	1meg
1m(eg)	1meg

The program can also calculate the combination of paralleled 5% values that come closest to the input value. An interesting detail of this program is that it does not use a lookup table to find the standard value.

You get help from the program by invoking it with the -h (ala unix) option. Do this as soon as you get it onto your machine. It has more tricks than described here.

Mac users, sorry...only PC spoken here.

Download

The algorithm for finding the nearest standard value was written by Jerry K. Brown and published as a Design Idea in Electronic Design, May 10, 1974. Jerry's original code was written in BASIC. I rewrote it in C and wrapped a front end around it. Much later, someone redid the same algorithm for Excel, although the idea of typing that much code into one spreadsheet cell is at the very minimum daunting, and worst case it is downright scarry. My source code is available on request.

Last modified 04/14/2011. 18:35:44