Changing Pot resistance calculations

[Billy]

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

 

[wilkie1]

Thanks, Billy.

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

[Barry]

Thank you Billy very helpful.

[Author]

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