# Parallel resistors app for you

Discussion in 'General Electronics Chat' started by CVMichael, Sep 27, 2011.

1. ### CVMichael Thread Starter Senior Member

Aug 3, 2007
416
17
A few days ago I needed a resistor of a certain value, but it was not a standard resistor value. So I had to put 2 or more resistors in parallel to get my value.
Doing a google search I found wesites that can calculate resistors in parallel, but I had to guess (kind of) what resistors combined would give the end result I needed. I had to try quite a few variations to get my value.

So... Since I'm a programmer I thought "I can do this better". So I went to my SQL Server and I developed some procedures that would calculate all the permutations between standard value resistors to get the combination of parallel resistors that would add up to the value I need.
I made procedures for 2, 3, and 4 resistors in parallel. I stopped at 4 because the number of permutations is enormous. 169^4=815,730,721 permutations. I actually had to limit the number of resistors to make it faster.

Anyways... After all that, here are my questions:

Is there something (online) that already does this?
When combining 4(or more) resistors in parallel, is there a formula to calculate faster than what I'm doing right now to try all permutations?
If something like this is not available already, would you like to have something like this? I can make a web interface for the procedures I developed and post it online on my website?

And finally, do you have other similar ideas that I would be able to develop? (with your help)

2. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Well, there are several out there already that work similarly for resistors (pairs), but I threw together one in Yab (YetAnotherBasic) a month ago; it's in the Tips & Tricks thread, here:
YAB website: http://www.yabasic.de/ (freeware)
I used YAB as a "lowest basic denominator" kind of thing to just get something out there; the program runs in a DOS box/CMD window, and will also run on Unix under wine. Handy to have if you're offline.

As you probably already know, you calculate resistance in series as capacitance in parallel, and resistance in parallel as capacitance in series - so just by changing the indications of series or parallel, they are pretty much interchangeable.

I just used a modified "brute force" method. Crude, but functional - and actually reasonably quick until you start using the higher E-series resistors.

I'd been motivated by this website:
http://www.qsl.net/in3otd/parallr.html
a series/parallel resistor calculator which more or less works, but it seemed to miss a number of combinations, and I felt the formatting could use a bit of tweaking.

I'm attaching the source as a .txt file; you might find it handy to have the various E-series in a text file that you can just copy from. I didn't bother with the E6 series, but E12 through E192 are down in the DATA statements near the end. It's sort of a "thrown together" thing that still has some GOTOs and other nasties in it, but it works.

As far as combining more than three - you're really getting into overkill, and you may be assuming that caps in a given E-series are very close to their marked ratings; which is wishful thinking. Resistors are much better, but still not exact.

Usually, a resistor pair program will get you very close to the desired value, and then when you test the installed pair, you can "tweak" it some more by going back into a parallel/serial calculator and obtaining the 3rd value. We did this a lot for production lines where certain values needed to be nearly exact.

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Last edited: Sep 27, 2011

Nov 9, 2007
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