Pot taper

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-----------+---------------

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Tony Williams.

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:

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

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

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?

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to

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

overspray.

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

Hello bg,

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