Home Forums General DIY Pedal Discussion Changing Pot resistance calculations

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  • #2574
    Billy
    Moderator

    All debate regarding how taper is affected aside, here’s a couple of handy calculations to change what you have to what you need regarding pot value

     

    To find out the parallel resistance we would need across pot lugs 1 and 3 we would use this formula

     

    Rp = 1 / ( 1/R – 1/Rpot)

     

    where Rp is the parallel resistor required R is the required pot value and Rpot is the actual value you have

     

    As a working example we’ll say we have a 50K and need a 20K pot so following convention brackets first

     

    1/20 – 1/50  = 0.05 – 0.02 = 0.03 then 1 / 0.03 = 33.33 The nearest practical value being 33K for Rp

     

    The second calculation allows you to check how close you get to your desired value

     

    R = R1 x R2 / R1 + R2

     

    R1 is your actual pot value and R2 your parallel resistor value Rp

     

    So R1 x R2 = 50 x 33  = 1650 divided by R1 + R2 = 50 + 33 = 83 so 1650 /  83  =  19.87 almost exactly 20K as required

     

    #2579
    wilkie1
    Moderator

    Thanks, Billy.

    This will be very useful.  I’m saving it for future use.

    #2582
    Barry
    Keymaster

    Thank you Billy very helpful.

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