[sdiy] Pot linearity towards extremes

Chromatest J. Pantsmaker chromatest at azburners.org
Thu Dec 15 18:00:55 CET 2016


after all, isn't that the point of the 16 LEDs?

On Wed, Dec 14, 2016 at 5:35 PM, Richie Burnett <
rburnett at richieburnett.co.uk> wrote:

> 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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