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If I search on internet I see thousands of serial or parallel resistor calculators, where you can fill in the 'input' resistors, and the output resistance is given.

However, I wonder if there is a program/app/website where you can set the output resistance, and it calculates the closest value using e.g. 1, 2, 3 and 4 resistors in any serial/parallel combination.

So. e.g. I want a resistance of 1.5K.

The result would be something like:

  • 1 resistor:1 K ( = 1000R )
  • 2 resistors: 1 K + 510 ( = 1510 R)
  • 3 resistors: 1 K + 470 + 22 (= 1492 R)
  • 4 resistors: 1 K + 470 + 22 + 10 ( = 1503 R)

Probably there are better solutions using parallel combinations, but to find them is quite hard (trial and error).

Michel Keijzers
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    Very often you can just make the value that you want using only 2 resistors from E12 series. 1.5 K is in E12 so too easy, but 3 Kohm: start with first available higher value so 3.3 K ohm, then determine parallel value to get 3 K total, it is 33 K. So: 3.3 K || 33 K = 3 K. Your 3 and 4 resistor examples are not so practical because you'd have to use 1% accurate (or better) resistors. For example 1% of 1 K is 10 ohms, if the 1 K is 1% too high you already have that 10 ohms. – Bimpelrekkie Feb 07 '19 at 14:26
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    Here you have one https://www.qsl.net/in3otd/parallr.html – G36 Feb 07 '19 at 14:29
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    I wrote one for myself. You can specify E6, E12..E48 series. It has series and parallel circuits. voltage divider, but you can even give your own formula. e.g. this: '1.25*(1+A/B)-0.0001*A' calculates the values A,B needed to get a certain voltage (value) out of an LM317 devic – Oldfart Feb 07 '19 at 14:29
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    Or this site http://jansson.us/resistors.html – G36 Feb 07 '19 at 14:32
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    Neat, that's two bookmarks added! – Neil_UK Feb 07 '19 at 14:40
  • @Bimpelrekkie Guess this is indeed the easiest way, and yes 3 or 4 is maybe a bit impracticale, although most of my resistors are 1%. – Michel Keijzers Feb 07 '19 at 14:45
  • @G36 Thanks ... just found out my chinese packages do not really relate to E12, but that site is really useful anyway. – Michel Keijzers Feb 07 '19 at 14:45
  • @Oldfart E6 might be interesting, although my chinese packages are not fully E6 I think, it's like a combination of E6/E12. – Michel Keijzers Feb 07 '19 at 14:46
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    About 35 years ago I started a programming and electronics apprenticeship. One of my first programming tasks was to write a program to calculate the resistance of two series resistors and two parallel resistors using all E24 and E48 to give \$ 10 \Omega \leq R \lt 100 \Omega \$ sort it by value and save it to a text file. I cut and pasted the results into a word document I still use today. – Warren Hill Feb 07 '19 at 14:58

2 Answers2

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Having the need to find a combination of non inductive wirewound power resistors from the stock of your favourite distributor (or some other nitty gritty task) might leave you with little value choice. Combined with limited experience, a resistor calculator becomes handy, like this one:

http://kirr.homeunix.org/electronics/resistor-network-finder/

Leaving my stock empty and only filling "Plus these extra values": Resistor network finder query

I could quickly find the resistor combination: Resistor network finder result

It even can show a graphical representation of the resistor network: Graphical result

Pro Backup
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  • Thank you, and upvoted. Meanwhile I changed to 0805 resistors, and those are so dirtcheap I bought a set of 50 or more values so I kind of never have to calculate, I just use what I need (and otherwise I order even more values). – Michel Keijzers Sep 07 '19 at 17:24
  • the first one I've seen with an actually useful UX, not just a toy. Thanks! – jberryman Jul 30 '23 at 16:19
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Once you have got accustomed to the existing resistor values E24/E48 and to certain extremes even E96 and E192, there is no real need for such application/website. First of all, with a bit of experience, you can guesstimate the value of two resistors in parallel, bu knowing for example that 2 resistors of the same value in parallel result to a combined half the value of each.

In general when you select the resistors you start with the E24 series, and then, depending on the required tolerance of the circuit, you adjust to the series with more values. In most cases you will find that there is no need to put them in parallel or series, because the value you need already exists, or the ones available are within the tolerance of the circuit.

For example when it comes to setting the resistors for the feedback of an adjustable voltage regulator, you start with an arbitrary value for one, and then you calculate the second resistor. If this one is nowhere near an existing value, you change the first resistor and recalculate again the second one, and so on, until you find a combination that works for you. I usually enter the formula in excel, and then play with the values, until I get what I want. Remember that as long at the output of a regulator is stable, the circuit will work even if you supply it with a 1% higher/lower voltage.

Elmesito
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  • Thanks, yes you are right, I just found out my resistors are from 'typical 30 value China packages', which are not related to E12, more like E6. – Michel Keijzers Feb 07 '19 at 14:44