[sdiy] Pot linearity towards extremes

[Mattias Rickardsson]

Are you pulling any current through the wiper of the pot?
I guess that would also ruin the linearity.

/mr



Den 16 dec. 2016 10:26 fm skrev "Rutger Vlek" <rutgervlek at gmail.com>:

> 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
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