[sdiy] allen bradley pots

[Grant Richter]

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!