Linearize Potentiometer Input

sethen320

Member
Join Date
Nov 2006
Location
Texas
Posts
21
I have a 5K potentiometer with a 1.2K resistor in series which I am using to convert 24VDC to ~4-20mA (close enough). I did not anticipate the pot being non-linear (logarithmic). I have been racking my brain trying to come up with an equation to at least somewhat linearize the input. Any ideas? Unfortunately switching out the pot is not an option at this point.

Thanks.
 
I was in a similar situation, in my case it was a linear potentiometer that was feeding 0-10V into a PLC analog input. It was supposed to produce a linear signal, but being a cheap pot it wasn't perfect, plus the A/D conversion adds some nonlinearity.

My solution was to calibrate the pot by recording the response across the full scale range, then plotting it in Excel and programming the trendline regression equation into my PLC to interpolate. It was a 5-inch potentiometer so I recorded the analog reading every 1/4 inch for a total of 20 readings. I then had a polynomial equation that would produce a distance [inch] output given an analog input value. (Floating point math was required.) Works great, in fact we've built 8 of these units and calibrate each pot individually since they all have a slightly different response.
 
Just expanding on what kolyur said. Here is a crude method. Set up about 20 rungs with limit or >= and <= compares to output a fixed value. That would give you step increments of 5% change. You would have to move the pot in 5 degree steps to determine the values to compare.
 
Got it.

Thanks guys. I ended up with a similar solution to kolyur, except with the help of an engineer friend. I had already plotted it in excel and was trying to generate the inverse log and devise a formula comparing the two to do it.

My friend did exactly what kolyur said in about 5 minutes and it was most definitely good enough. I only needed to apply an offset to the slope.
 

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