[sdiy] allen bradley pots

[Theo]

Cermet:
Ferrite powder sintered (sp?) together (@ around 800 to 900 degrees Celsius)
with an non conductive ceramic base material.
The base material will vary upon the manufacturer.

Conductive plastic:
Carbon powder mixed in a polymer resin.

For what its worth, to me Piher does not seem to be the most high-tech of
the pot builders.
The law curves on the Ohmeg site indicate a +/- 4% position/resistance
accuracy, not sure if they really deliver though.
Alps and Bourns can give you better than +/- 30% for sure.

Laser trimming is expensive,
using the cheaper carbon element would not make sense.
Besides a carbon pot would wear off its precision rather quickly.

Cheers,
Theo



From: Grant Richter <grichter at asapnet.net>

> I'm in the process of learning more than I ever wanted to know about
> potentiometers. Job related item.
>
> Can anybody give me a quick data dump on Cermet, the material?
>
> Piher for example tells me that carbon film pots can only be constructed
> with a +/- 30% accuracy on the relation between knob position and absolute
> resistance. In other words, the curve for an antilog potentiometer can
> deviate by up to 30% absolute value, but still meet specs as a relative
> resistance. They tell me the problem is silkscreening 3 different
densities
> of resistance ink to make the curve in three sections.
>
> In order to get better tolerance (for absolute value) I have to move to a
> Cermet element, which is twice the cost. A precision curve can be
generated
> by laser trimming a Cermet element, but they do not laser trim carbon
film.
>
> Anyone familiar with the dirty details of making custom potentiometer
> curves?
>
> Thanks!
>
>