[sdiy] Potentiometer linearity questionable..

[Neil Johnson]

Hi,

> 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).

According to the Vishay datasheet for the 148/149 series the taper tolerance is 20% at 50% rotation, with 5% linearity.  The 10% resistance tolerance applies to the entire track, not where the wiper is.

> 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.

Well, refering back to the Spectrol 148 datasheet that is better what you can expect (270 degree +/- 10 degree).  Pretty normal for pots of that price.

> This does not align with 0-10 marks.

All depends on where you put your marks.  And even then there's the 20% taper tolerance, so you're never going to get such high precision with these pots.

If you want better specs try the Spectrol 357-2-0 series for example.

> 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..

For the price they're pretty good.  But if you want better you need to spend more :)

> What do you think?

I think you're expecting a lot more from such pots.

Neil


--
http://www.njohnson.co.uk

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