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Given is a ratio for resistors, e.g. two resistors \$\frac{R_1}{R_2}\$ = "some odd value", like 257:42.

How do I find matching resistor pairs (triples, ...) from E series, without having to try them out one by one?

When I search for the title, I merely find tools, which calculate R2 if the ratio as well as R1 are given.

However, I want a way which gives me matching resistor values given only the ratio. I did not find any list/table with all ratios or a calculator tool online.

Finding the magnitude is answered here. Another answer talks about "Sallen Key", which seems to be more complicated than my task.

What am I trying to achieve?
I need a certain gain in my measuring chain. When designing the op-amp circuit, I calculated a certain ratio which is needed for the resistors. Only some standard E series ones are available at the moment.

ocrdu
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Paul Smith
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  • There are online calculators for the very purpose. I used to have a shareware (yes, shareware) desktop application for the very purpose. Very useful. Gladly paid the 5 bucks for it. – winny Sep 05 '22 at 21:11
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    What tolerance resistors are you considering using? Will you be happy with the nominal value ratio being correct (rather than measuring and calibrating the resistors)? – Transistor Sep 05 '22 at 21:12
  • @winny: Could you maybe share a name of such a tool? Transistor:I do not know. My plan was to use the nominal value and test if the op-amp setup meets requirements. – Paul Smith Sep 05 '22 at 21:19
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    [Resistor Value and Ratio Calculator](https://jansson.us/resistors.html) – StainlessSteelRat Sep 05 '22 at 21:52
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    Use 11k and 1.8k, it's accurate to within 0.13% :-) I wrote a small Perl script to do such calculations about 20 years ago; I use it all the time. But there are online calculators, as mentioned above. – Dave Tweed Sep 05 '22 at 22:00
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    [Resistor optimizer](http://jahonen.kapsi.fi/Electronics/ResOptimizer/) – StainlessSteelRat Sep 05 '22 at 22:11
  • @PaulfromC You want to first find a resistor series that will accommodate it. To do that, compute \$\log_{10}\left(\frac{257}{42}\right)\cdot E_{_\text{series}}\$, adjusting the selected \$E_{_\text{series}}\$ until you get a near-integer at the output. In this case, it suggests E192, which gives about 151.0433. Close enough. So if you just want a series that will get you there without complex sum-combos, that's one way. From E192 you might select 612 and 100, for example. For sum combos of lesser E series, see rule-notes at end [here](https://electronics.stackexchange.com/a/440073/38098). – jonk Sep 05 '22 at 23:59
  • Thanks for the tool recommendation, It worked well. What I actually looked for was the spreadsheet shared by Seir. – Paul Smith Sep 06 '22 at 20:58

2 Answers2

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You can create such a table yourself using Excel and a macro function.

I've created one for the E24 series.

enter image description here

Formulas:

  • F3 cell: =E3*POWER(10;$C$2)
  • G3 cell: =F3/$C$3
  • H3 cell: =GetNearestE24Resistor(G3)
  • I3 cell: =100*(H3/G3-1)
  • J3 cell: =IF(ABS(I3)<=$C$4;"TRUE";"")

Drag these formulas to populate all rows.

Macro function:

Function GetNearestE24Resistor(Rnom As Double)
    Dim Reff As Double
    Dim Re24(25) As Double
    
    Re24(0) = 1
    Re24(1) = 1.1
    Re24(2) = 1.2
    Re24(3) = 1.3
    Re24(4) = 1.5
    Re24(5) = 1.6
    Re24(6) = 1.8
    Re24(7) = 2
    Re24(8) = 2.2
    Re24(9) = 2.4
    Re24(10) = 2.7
    Re24(11) = 3
    Re24(12) = 3.3
    Re24(13) = 3.6
    Re24(14) = 3.9
    Re24(15) = 4.3
    Re24(16) = 4.7
    Re24(17) = 5.1
    Re24(18) = 5.6
    Re24(19) = 6.2
    Re24(20) = 6.8
    Re24(21) = 7.5
    Re24(22) = 8.2
    Re24(23) = 9.1
    Re24(24) = 10
    
    Dim F As Double  ' Factor
    
    Dim Rl As Double ' Lower Limit
    Dim Ru As Double ' Upper Limit
    
    F = 1
    
    ' Find range of the resistor Rnom within the E24 series
    Do
        If Rnom < Rl Then
            F = F / 10
        ElseIf Rnom > Ru Then
            F = F * 10
        End If
        
        Rl = Re24(0) * F
        Ru = Re24(24) * F
    Loop Until Rnom >= Rl And Rnom <= Ru
    
    ' Find the two nearest E24 resistors
    Dim i
    For i = 0 To 23
        Rl = Re24(i) * F
        Ru = Re24(i + 1) * F
        
        If Rnom >= Rl And Rnom <= Ru Then
            Exit For
        End If
    Next i
    
    ' Find the nearest E24 resisitor
    Dim Dl As Double
    Dim Du As Double
    
    Dl = Rnom - Rl
    Du = Ru - Rnom
    
    If Dl < Du Then
        GetNearestE24Resistor = Rl
    Else
        GetNearestE24Resistor = Ru
    End If
    
    Exit Function
End Function
Velvel
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I have faced this tricky little problem many times in my career, and eventually ended up with the solution of putting the E24 series (and 48, 96 and 192) into a spreadsheet. When I want to find a ratio, I fill in the 'ratio' cell, and the 'series' x 'ratio' column updates. Then I scan my eye down that column to look for integer or near integer results. The biggest drawback with that method is remembering where I archived it from last use.

You're unlikely to find E24 components in better than 1% tolerance. For many purposes anyway, that's good enough. Don't let the 'perfect' be the enemy of 'meets specification'.

Resistors are dirt cheap. If you aim to use three from the start, choose a 'shim' resistor nominally 10x the value of one of the others, and when your main resistor ratio is close, adjust the parallel shim to bring it closer. Or you could choose a /10 resistor in series, whichever is more convenient.

Neil_UK
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  • Thank you for more illumination on the topic. Though all I needed was the spreadsheet shared above, so one does not have to do the same work, which you people already invested. – Paul Smith Sep 06 '22 at 21:01