Pot taper

[Pooh Bear]

Is there a pot that has a taper in the shape of 1/X if X is greater than
zero and less than 1. I guess you'd call it reciprocal taper.

The only non-linear tapers commonly available are so-called log and antilog.
You'd do best to take a look at them. They aren't very accurate though.

Creating any non-linear taper presents fundamental problems. Probably best
to law bend in hardware/software.

Graham

 

[bg]

Is there a pot that has a taper in the shape of 1/X if X is greater than
zero and less than 1. I guess you'd call it reciprocal taper.
bg

 

[Tony Williams]

Is there a pot that has a taper in the shape of 1/X if X is
greater than zero and less than 1. I guess you'd call it
reciprocal taper. bg

I = V/R. So if you apply a voltage across a variable
resistance the current is the required reciprocal.

Easily seen in the non-inverting amplifier.

Rf
+----/\/\---+
| _ |
+---|- \ |
| | >---+--->
Vin---------|---|+_/
\
/RV1 Vout = Vin(1 + Rf/RV1)
\
|
0v-----------+---------------

You can get rid of the "1 +" non-linearity by
multiplying Vin by 2x and injecting an offset.

Rf Rf
2xVin--/\/\---+----/\/\---+
| _ |
+---|- \ |
| | >---+--->
Vin---------|---|+_/
\
/RV1 Vout = Vin(Rf/RV1)
\
|
0v-----------+---------------

Note though that Vout = Infinity when RV1= 0.

Hmmm... I wonder if that circuit could form the basis
of a non-inverting (single rail) integrator.

C C
2xVin---||----+-----||----+
| _ |
+---|- \ |
| | >---+--->
Vin---------|---|+_/
\
/R
\
|
0v-----------+---------------

 

[Joerg]

Hello bg,

Is there a pot that has a taper in the shape of 1/X if X is greater than
zero and less than 1. I guess you'd call it reciprocal taper.
bg

We had a similar discussion in a thread that started 1/24/2006. Martin's
old web site contains some really good info on how to taper pots if you
are using them in a voltage divider configuration. Those graphs can save
a lot of spreadsheet work:
http://www.wavefront.mcmail.com/pot.htm

There is also the old trick of resistor-diode ladders for a piecewise
linear approximation. Essentially diodes kick in one after the other at
various voltage levels as set by a resistor ladder.

A word of caution regarding log and anti-log taper pots. They aren't
like those in grampa's radio anymore. Nowadays they usually have two or
three resistive pastes on the track. This means that you'd see two or
three somewhat linear sections at different 'steepness' but not a true
log pattern. The transitions are usually somewhat soft because of overspray.

Regards, Joerg

 

[Walter Harley]

Is there a pot that has a taper in the shape of 1/X if X is greater than
zero and less than 1. I guess you'd call it reciprocal taper.

No.

Panasonic seems to have the widest variety of tapers, and the best online
documentation (though that is not saying much). E.g., look at
http://industrial.panasonic.com/www-data/pdf/AOK0000/AOK0000PE1.pdf; a few
pages in there's a chart showing all their tapers. But there is not a
reciprocal taper.

In general the way that tapered pots are made is with a few (two or three)
linear sections stuck together. They're not truly smooth, as you can see
from the Panasonic charts. So, it's really more a question of "how close to
reciprocal do you need". Also, although all those tapers and options are
available in principle, in practice it is very hard to purchase them in
quantities less than ten thousand.

As Tony and Pooh suggest, you'll probably have better luck starting with a
linear pot and solving your problem in hardware or software. What's the
problem you're trying to solve?

 

[bg]

Walter Harley wrote in message ...

No.

Panasonic seems to have the widest variety of tapers, and the best online
documentation (though that is not saying much). E.g., look at
http://industrial.panasonic.com/www-data/pdf/AOK0000/AOK0000PE1.pdf; a few
pages in there's a chart showing all their tapers. But there is not a
reciprocal taper.

In general the way that tapered pots are made is with a few (two or three)
linear sections stuck together. They're not truly smooth, as you can see
from the Panasonic charts. So, it's really more a question of "how close to
reciprocal do you need". Also, although all those tapers and options are
available in principle, in practice it is very hard to purchase them in
quantities less than ten thousand.

As Tony and Pooh suggest, you'll probably have better luck starting with a
linear pot and solving your problem in hardware or software. What's the
problem you're trying to solve?

It's a simple RC bridge circuit for measuring caps. The C scale gets
crunched together (C=t/r). I know there are other ways to measure caps but
this method is very simple and there are no restrictions on the frequency.
The accuracy depends only on the bridge arm components.
bg

 

[bg]

Joerg wrote in message ...

Hello bg,



We had a similar discussion in a thread that started 1/24/2006. Martin's
old web site contains some really good info on how to taper pots if you
are using them in a voltage divider configuration. Those graphs can save
a lot of spreadsheet work:
http://www.wavefront.mcmail.com/pot.htm

There is also the old trick of resistor-diode ladders for a piecewise
linear approximation. Essentially diodes kick in one after the other at
various voltage levels as set by a resistor ladder.

A word of caution regarding log and anti-log taper pots. They aren't
like those in grampa's radio anymore. Nowadays they usually have two or
three resistive pastes on the track. This means that you'd see two or
three somewhat linear sections at different 'steepness' but not a true
log pattern. The transitions are usually somewhat soft because of overspray.

Regards, Joerg

I used to have an article from one of the old electronics magazines that
gave a dozen or so ways to modify pots for various tapers. Now that I need
it, I can't find it! Your web link looks really good, Thanks
bg

 

[Joerg]

Hello bg,

I used to have an article from one of the old electronics magazines that
gave a dozen or so ways to modify pots for various tapers. Now that I need
it, I can't find it! Your web link looks really good, Thanks

Well, thank Martin Griffith. He mentioned that it's an older web site he
once created.

I used to have the same problem, not finding articles back. Then I got
into the habit of tearing out interesting pages and filing them in
technology binders. Now it take five minutes tops to locate older stuff.
I do not keep any old magazine copies anymore, takes too much space.

Regards, Joerg