The Analysis of Changing the Taper in a Potentiometer

[opacheco]

Hi,

I found this old doc about the taper of potentiometers namely "Bending the Law Resistance - ELECTRONICS WORLD WIRELESS WORLD August '93" but this doc doesn't have any math treatment and any Transfer Function research showing the effect of variables (pots, fixed resistor values...) for each topology and rotation versus response.

I would like to know if someone have an idea how we can calculate these or where I can find a discuss of this theme?


Thanks a lot,
Opacheco.

 

[joaquins]

I've seen an excel sheet about this, for figs b, c and d. I could trace it for you if you can't, but I don't remember where it was. It was well done, showing 10% steps with the response, actual response in percentage, source impedance and all in a graph for cross reference. You could run your own if you know ohms law, just messing a bit around with it will get you there.

JS

 

[opacheco]

I've seen an excel sheet about this, for figs b, c and d. I could trace it for you if you can't, but I don't remember where it was. It was well done, showing 10% steps with the response, actual response in percentage, source impedance and all in a graph for cross reference. You could run your own if you know ohms law, just messing a bit around with it will get you there.

JS

JS,
Thanks for your comments but sadly I can't figurate how to do that!!....Will be fine if you show me how.

Do exist a serious electronic book treating with these kind of Pot Curves response??

Opacheco

 

[JohnRoberts]

This too has been discussed here before.

In general this is "stupid math trick" (attempt at comedy). Something that works better on paper than in practice.

Pots are designed to be used ratio-metrically (potentio-metrically?). So pot ratios are pretty accurate, but pot bulk resistance can be +/-20%.

For very gentle taper modification for onsey-twosy DIY have fun, for severe taper modification and/or repeatable production you need to lower your expectations and try to find a pot with the correct taper.

JR

 

[joaquins]

Here is a version in spanish, I think you'll be fine with that, I also have the english version but there is not much more than numbers so it should be fine in any case... Clear the jpg from the extension I added or the attachment wouldn't let me upload it, just a .xls

JS

 

[opacheco]

Here is a version in spanish, I think you'll be fine with that, I also have the english version but there is not much more than numbers so it should be fine in any case... Clear the jpg from the extension I added or the attachment wouldn't let me upload it, just a .xls

JS

JS,

Thanks for your help!....But How about the another ones configuration?, Must be exist a study about too but the question is WHERE??

Opacheco.

 

[joaquins]

Here is a version in spanish, I think you'll be fine with that, I also have the english version but there is not much more than numbers so it should be fine in any case... Clear the jpg from the extension I added or the attachment wouldn't let me upload it, just a .xls

JS

JS,

Thanks for your help!....But How about the another ones configuration?, Must be exist a study about too but the question is WHERE??

Opacheco.

For the 1a there is not much to it, just starting from the Rf value to the top.
For the 1e is the same as 1d but using the other end of the pot as output and RL=RT/2. The curve will be the same.
Also, for this last one and all the next others you need centered tapped pots which are quite tricky to get this days so probably not very applicable, would be nice to get them for some cases but as I said, I hardly believe you can go to the local store and get them.
Still, 1f equals to 1b half of the range back and forth, 1i equals 1c in the same way, 1j equals the linear as the ones before.
1g  makes two linear ranges broken in the middle and 1h looks pretty much the same as 1i but taking the output from other nodes so working backwards.

I don't think there is too much from this to play around, as JR said don't loose much time into it, the 3 configurations you can make with the xls would probably help a bit when fighting against some small taper need to make the control a bit more natural, you can play around with it but won't change your life. I've used in to control the gain of an amplifier which needed anti log taper and it did worked more naturally than linear in some usable range but at the ends started to behave as crazy (max gain proximity was a dark territory) Other than that I haven't used much. It's also useful to have in consideration while making pan controls for example where matching the center point of the rotation makes a big difference, and in this case you usually need a little displacement, not as much as you'd like in a high range gain control.

JS

 

[mutterd]

I did some empirical testing of this kind of thing recently when i was trying to change the curve of a mixing fader to have more resolution between 0 and -20db…

what i found out was that, if you take a liner pot, say 10k for simplicity, and load the wiper with a 1k resistor to ground you get pretty darn close to a log taper… this seems to be the case for 1b and 1c…

so, if RL is 10% of RT…

of course JR is right - the spread of pot values across my 12 channels made it impossible to count on, but if it were just on a case by case basis i believe there to be interesting things to do with this sort of thing…

If some one did find the math, i would be interested in the spread sheet as well…


T.

 

[Kingston]

Here is the de facto DIY article on the topic with some relevant equations and practical musings: http://www.geofex.com/article_folders/potsecrets/potscret.htm

aka The secret life of pots aka The stupid math trick.

 

[leswatts]

That article brings back memories.

Once upon a time I designed custom taper pots, mostly for high volume applications like car stuff.

I would bend and contort the current density vector field with all sorts of width variation and conductive structures.
At first, I would design them with Teledeltos paper and silver ink (a resistive coated paper once used on fax machines)
plotting equipotentials with a high impedance voltmeter, then transferring the design to silkscreens.

Later I wrote some finite element code that would calculate the fields.
I also created laser trimmer code that could trim to a particular curve.

One example was a slide pot for an AC control that was the compliment to the temperature vs blend door position...a strange
nonlinear function due to the fluid flow characteristics.

Nowdays there's little need, since encoders/processors can easily make any control law.

But you can still have fun with potentiometers with a bit of silver and carbon ink and a carbide hand scriber.

Les