Programs calculate 1% and ratio-resistor pairs
Computer programs determine the best values for an application.
Carl Rutschow, Upland, CA; Edited by Brad Thompson and Fran Granville -- EDN, December 16, 2005
If you perform analog-circuit design, you'll occasionally need to use a resistor with a nonstandard value to produce a particular gain, ratio, or attenuation factor. You can create resistors of unusual values by connecting two standard-value resistors of 1% tolerance in parallel. Because it is impossible to readily predict which resistor pairs will fall closest to the desired value, a computer program can help by calculating all combinations of standard 1% resistors to determine the best values for your application.
The Visual Basic, compiled, executable file Rratio2.exe checks all standard 1%-resistor values in a given range for a desired ratio, attenuation factor, or noninverting operational-amplifier gain (Figure 1). Click here to download the program. You select the calculation mode via the program's window buttons. As an option, you can choose whether the program displays all possible values or only the values closest to the target value.
Using standard 1%-resistor values, a second program, RPar2a.exe, click here, checks and displays all appropriate combinations that generate a desired parallel resistor's value. The program generally calculates several parallel combination values that fall well within 0.1% of the desired value. By comparison, a single 1% resistor's nominal value may differ by as much as 1.45% from the desired value. Note that, for both programs, the calculated resistance values depend on the paired resistors' tolerances.
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A spreadsheet formula that I have found useful to calculate the nearest 1% standard resistor value is: =ROUND(10^(ROUND((LOG10(R)-INT(LOG10(R)))*96,0)/96),2)*10^(INT(LOG10(R))) .
Albert Zakowski - 2006-1-6 09:15:00 PDT -
I tried Rratio2.exe (Dec 16 Page 66) and it has one problem that I believe should be corrected. It does not include available 1% values that are also available as 5% values.
For example for a ratio of .8, it does not give you 1.2M for R1 and 300K for R2 which is a perfect choice at 0% delta.
Richard Perry - 2006-13-1 06:23:00 PST -
The figure has two errors: 1) The Ratio equation should be R2/R1, not 1+ R2/R1. 2) The amplifer has the + and - terminals reversed. --Carl Rutschow (Author)
Carl Rutschow - 2006-10-1 13:12:00 PST -
We have fixed the problem with the ZIP files. You can now download the real program files using the links in the article. We apologize for the inconvenience and thank you for your patience.
Matthew Miller, Executive Editor, Online - 2006-5-1 10:36:00 PST -
I tried to send an executable to someone and of course had a problem at the receiving end. So I zipped it and STILL had a problem! Then I changed the extension of the "internal" file from .exe to .txt
and STILL had a problem! Apparently some security-conscious servers actually look inside the enclosed package of zipped file(s) and see an executable. Kindly tell me how you folks at EDN finally got the executable to get through your server. I am facing the same problem. Thanks!
Jim Tonne - 2005-24-12 08:08:00 PST
