[sdiy] allen bradley pots

[Grant Richter]

Thank you!

> From: "Theo" <t.hogers at home.nl>
> Reply-To: owner-synth-diy at dropmix.xs4all.nl
> Date: Sun, 27 Jan 2002 01:27:40 +0100
> To: <grichter at asapnet.net>, <owner-synth-diy at dropmix.xs4all.nl>, "synth DIY"
> <synth-diy at dropmix.xs4all.nl>
> Subject: Re: [sdiy] allen bradley pots
> 
> 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!
>> 
>> 
> 
>