[sdiy] multi or single turn trimmers?

[Andre Majorel]

On 2007-06-28 17:10 -0400, ceme at cox.net wrote:

> For a VCO, is it better to use '25-turn' or 'single-turn'
> trimmers?  And in general, is one "better" for certain
> applications over the other?

>From _Troubleshooting analog circuits_ by Bob Pease :

  The major problem area for variable resistors is their
  resolution, or "settability". Some variable resistors claim to
  have an "infinite" resolution ; but, if you apply 2 V across a
  variable resistor's ends and try to trim the wiper voltage to
  any or every millivolt in between, you may find that there are
  some voltage levels you can't achieve. So much for "infinite
  resolution". As a rule of thumb a good pot can usually be set to
  a resolution of 0.1%, or every 2 mV in the previous case. Thus
  counting on a settability of 0.2% is conservative.

  Good settability includes not only being able to set the wiper
  to any desired position but also having it stay there. But, I
  still see people advertising multi-turn pots with the claimed
  advantage of superior settability. The next time you need a pot
  with superior settability, evaluate a multi-turn pot and a
  single-turn pot. Set each one to the desired value, tap the pots
  with a pencil, and tell me which one stays put. I normally
  expect a multi-turn pot, whether it has a linear or circular
  layout, to be a factor of 2 to 4 worse than a single-turn pot
  because the mechanical layout of a single-turn pot is more
  stable and balanced. Does anyone know of an example in which the
  multi-turn pot is better ? A full year after this statement was
  originally published, nobody has tried to contradict me,
  although people who sell multi-turn pots still brag in the
  vaguest possible terms about "infinite resolution"--bleah !

Not sure whether this applies to presets or exactly what is "a
factor of 2 to 4 worse" in multi-turns but there you go.

-- 
André Majorel <URL:http://www.teaser.fr/~amajorel/>
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