1. TekScopes@groups.io

Re: Risetime calculator (in tekwiki)

Leo Bodnar
 

Sorry to keep beating beating the dead horse but I wanted to find out what is important in this circuit and what is not.

It turns out any shunted (or even shorted) linear pot has quadratic and linear components of its partial resistance.

Quadratic component is always there, which is good.

What is not that good - is lenear component that contaminates the output.

But (and this is where the trick is) if the shunting resistance is equal to the nominal pot value then linear component is cancelled out and the result is purely quadratic function of wiper position (if the track is linear taper.)

http://leobodnar.com/files/eq1.jpg

Cheers
Leo

On Sun, Jan 19, 2020 at 08:27 PM, Albert Otten wrote:
This originates I think from the idea that around a maximum or minimum a
function is quadratic plus higher order terms.
If you short together both ends of a linear pot then the resistance to the
wiper is maximum midway and decreases quadratically (in this case exact,
without higher order terms) when moving away from midway. In our instrument
consider a wiper as a wiper of a 10 k pot, blocked midway in one direction.
I must confess that I only recognized the trick after further investigation.

Albert
Join TekScopes@groups.io to automatically receive all group messages.