[sdiy] Potentiometer linearity questionable..

[jpdesroc at oricom.ca]

>Any chance that your circuit is loading the pot and degrading the
>linearity further ?
No I'm measuring the pot linearity
unconnected with an electronic ohmmeter.
I plotted the linearity curve on a semilog
graph paper vs 'good' linearity.
I'm thinking of using a lookup table based
microcontroler compensation on future VCO designs..not sure.
I'm already doing so with my ADSR modules with
precise 2msec. to 10sec. marks with success
using a PIC16F88 ADC. I inserted the 'typical'
non-linearity of Alpha 100kB pots.
It gives precise slope times vs pot markings.

J-Pierre
---- Message original ----
De: harrybissell at wowway.com
A: jpdesroc at oricom.ca, synth-diy at dropmix.xs4all.nl
Objet: Re: [sdiy] Potentiometer linearity questionable..
Date: Mon, 12 May 2008 10:02:40 -0500

>Typically the end resistance of pots and where the
>element ~really~ begins are not well specified or
>controlled.  I'd check carefully with the manufacturer
>to see how they specify linearity. Its likely that it is specified
>for full rotation and the small error at the end 
>factors out (see "specsmanship")
>
>If the pots are just aligned with panel markings
>it is an annoyance.  If the pots were used with some vernier
>dial it would be more critical, otoh they would usually be 10 turn
>pots and VERY expensive, so they could take care of it better.
>
>Probably, Moog (or other manufacturers) know what pots they will use
>and make
>the front panel graphics match the actual (typical) pots.
>
>Any chance that your circuit is loading the pot and degrading the
>linearity further ?
>
>H^) harry
>
>
>
>On Mon, 12 May 2008 10:15:39 -0400, jpdesroc wrote
>> Hi list,
>> 
>> I'm in the process of designing
>> the artworks for some of my DIY synth
>> VCO front panels. I found that the supposely
>> '10% linearity' of some pots like
>> Vishay/Spectrol cermet 149 are not that
>> linear in the 0-3 region (0-10 markings).
>> 0 to 10 graded lines around pots
>> on my front panels won't just alligned
>> with some pot real values. I measured 20-30%
>> off values in the 0-3 range which is very disapointing.
>> Some pots have 300deg mechanical rotation
>> but loose around 5deg at each ends
>> which make around 290 of real cursor travel.
>> This does not allign with 0-10 marks.
>> When I look some Moog modules like
>> the 921A I wonder if the 'frequency' pot
>> was accurate around its mechanical
>> rotation ends..(?) Moog utilised
>> Allen-Bradley pots at that time.
>> Where they more 'linearity reliable'?
>> I thought Spectrol type pots were
>> just fine on that..
>> What do you think?
>> J-P
>> 
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>
>
>Harry Bissell & Nora Abdullah 4eva
>