[sdiy] Potentiometer linearity questionable..

[harrybissell at wowway.com]

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