[sdiy] Pot linearity towards extremes

[Richie Burnett]

Why not fit a control knob that doesn't have any discernible pointer on it, 
then it doesn't need to line up with the LEDs ?

Or use a 16-way switch and loose the ADC and the logic etc.  The ADC output 
code will chatter near it's decision thresholds anyway unless you somehow 
implement a small deadband.

-Richie,

-----Original Message----- 
From: Steve Lenham
Sent: Wednesday, December 14, 2016 9:45 PM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Pot linearity towards extremes

On 14/12/2016 16:03, Rutger Vlek wrote:

> I'm working on what's basically a continuous pot turned into a
> 16-position switch. It's a circuit that uses a simple free-running
> ADC to convert a 0 to 5V output from a potentiometer digital outputs
> that control switching of a 16-way CMOS switch and a 16-way LED
> driver. The led driver lights a LED in a ring of 16 around the pot to
> indicate the selected position. However, in the prototype the LED
> being lit doesn't line up with the position of the pot. The pot is a
> 10K linear one, but from my measurements it seems it is non-linear
> towards the extremes. The angle of physical rotation doesn't line up
> so well with the angle over which the resistance changes linearly. Is
> that a common property of pots? I hadn't seen it so clearly before.
> Any ideas on how to easily overcome it without needing both a gain
> and offset trimmer in the circuit?

Any pot with less than 360 degrees of mechanical rotation will have at
least a small deadzone at the two extremes of travel where the wiper is
fixed at 0% or 100%. Manufacturers cannot risk the scenario where the
mechanical rotation is slightly _less_ than the electrical rotation - in
your application, that would mean that you could not quite reach 0V or
5V - so they err on the side of caution and make the mechanical rotation
slightly larger. The deadzones are where the wiper is running over the
minimally-resistive metallised end contacts rather than the resistive track.

How big the deadzones are will vary from manufacturer to manufacturer,
introducing yet another way in which crap pots can be crapper than good
ones.

Cheers,

Steve L.
Benden Sound Technology
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