[sdiy] Pot linearity towards extremes

[Rutger Vlek]

Hi guys,

Thanks for the explanations. It makes sense, I just never saw the effect being so obvious. Removing the pointer on the knob is indeed a very effective solution, and it was already on my list of ways to overcome this. I just felt I wanted to make sure I understood the problem first, before attempting to work around it. I haven't been able to find the curves for these knobs, but I will do another search. It would be useful to know them.

Indeed it also makes sense that you don't want to risk running into mechanical span limitations before reaching the full electrical span.

Rutger


On 15 dec 2016, at 18:00, Chromatest J. Pantsmaker wrote:

> 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
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> 
> 
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2016.0.7924 / Virus Database: 4739/13594 - Release Date: 12/14/16 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20161216/b8a4e82b/attachment.htm>