Analog potentiometer with an adjustable value range?

[Sonic Wizard]

Anyone know how to make an analog circuit to adjust a pot value range? I don't mean adjust in the normal sense - that's exactly what pots are for, right? I mean a way to have one value of pot, say 100K linear, translate to a variety of different resistance ranges. For example, if I had a 100K linear pot, is there a circuit I can build so that when I turn the pot, it can actually control a 500K resistance range? I know digital potentiometers are the first answer. Is there an analog solution?

[frequencycentral]

There's a floating voltage controlled resistor schematic on the LM13700 data sheet that might do the job.

http://cache.national.com/ds/LM/LM13700.pdf

[Zben3129]

If you only want to reduce the value of a pot, rather than increase it, then you can use 2 trimmers wired as variable resistors across lugs 1 and 2, and 2 and 3, to adjust the range.

For example, If you had a 1m lin pot, you could get any value linear pot all the way from 1 ohm up to 667k ohms using 1m trimmers. Also, you could create many different tapers (audio, rev audio, custom tapers) using the same exact circuit from 1 ohms to about 360k ohms

To get the full value of the pot you could use a dpdt switch to disconnect the trimmers.



Zach

[ashcat_lt]

There's a floating voltage controlled resistor schematic on the LM13700 data sheet that might do the job.

http://cache.national.com/ds/LM/LM13700.pdf

This is interesting, thought I'm a little confused by it.  In any case, I'm fairly certain it creates a variable resistor.  Wouldn't we need two working in opposite directions to create a potentiometer?

[frequencycentral]

There's a floating voltage controlled resistor schematic on the LM13700 data sheet that might do the job.

http://cache.national.com/ds/LM/LM13700.pdf

This is interesting, thought I'm a little confused by it.  In any case, I'm fairly certain it creates a variable resistor.  Wouldn't we need two working in opposite directions to create a potentiometer?

I've never used that circuit, just am aware of it. I guess for certain applications where a pot is used as a variable resistor it would work as it is, but you are right, to get a potentiometer you would need to use two, inverting the control voltage to the second. On the same data sheet are a couple of single ended examples too - which again should suit certain applications whre pots are used.

[Sonic Wizard]

If you only want to reduce the value of a pot, rather than increase it, then you can use 2 trimmers wired as variable resistors across lugs 1 and 2, and 2 and 3, to adjust the range.

For example, If you had a 1m lin pot, you could get any value linear pot all the way from 1 ohm up to 667k ohms using 1m trimmers. Also, you could create many different tapers (audio, rev audio, custom tapers) using the same exact circuit from 1 ohms to about 360k ohms

To get the full value of the pot you could use a dpdt switch to disconnect the trimmers.

This seems very promising! Thanks for the hint.

How can I calculate what those trim values would be? Also, how can I tell what taper I'm going to get?

[Zben3129]

http://www.diystompboxes.com/analogalchemy/emh/emh.html

theres 2 calculators there for linear and tapered. To tell what the taper percent is try looking at some graphs. Some examples...linear taper is 50%, audio is 90%, reverse audio is 10%.



Zach

[Sonic Wizard]

Very nice.

I checked out R.G. Keen's article "The Secret Life of Pots" (http://www.geofex.com/article_folders/potsecrets/potscret.htm) and it seems there is a catch:

"Unfortunately there's a gotcha in there. It's true that the voltage division ratio of this rig is arbitrarily close to that of a log taper pot. However, neither the load seen by whatever drives Vin or the source resistance as seen by the input of whatever is connected to Vout is close to what would exist for a real log pot of value R. In fact, the load on Vin varies from 1/(1+1/b)*R up to R. That means that if we're trying to do a log taper with b = 1/4, the load on Vin will be as much as 0.2* R. This may be OK, but you have to keep it in mind."

If I'm not changing the taper, in other words, if b=0, this seems like it wouldn't be an issue.

[Zben3129]

I think what he is saying is that you won't get exactly a log pot. However, you will get something close, definitely usable.

Zach

[frequencycentral]

http://www.diystompboxes.com/smfforum/index.php?topic=70732.0

[earthtonesaudio]

There's also a circuit called a "resistance multiplier" using an op-amp.  But I think it's single-ended (can only attach one end of the "resistor" to your circuit).

[slacker]

What are you actually trying to do, that might make it easier to give an answer.
A lot of the time especially when the pot is being used as a voltage divider the actual value of the pot isn't that important so a 100k might do fine in place of a 500k.

[Sonic Wizard]

I'm trying to use a standard pot value in place of different pots in several effects, as both voltage dividers and series resistors.  I want to be able to use the same pots with different effects circuits. I would need to find a standard value and taper, and then be able to convert up or down in resistance and alter the taper if necessary. I thought a large value like 1M might be good, so I could convert down, but it looks like I could also convert up with a resistor across the outside lugs.

[Zben3129]

I'm trying to use a standard pot value in place of different pots in several effects, as both voltage dividers and series resistors.  I want to be able to use the same pots with different effects circuits. I would need to find a standard value and taper, and then be able to convert up or down in resistance and alter the taper if necessary. I thought a large value like 1M might be good, so I could convert down, but it looks like I could also convert up with a resistor across the outside lugs.

This can only reduce, as you are adding a resistor in parallel.


Zach

[earthtonesaudio]

I'm trying to use a standard pot value in place of different pots in several effects, as both voltage dividers and series resistors.  I want to be able to use the same pots with different effects circuits. I would need to find a standard value and taper, and then be able to convert up or down in resistance and alter the taper if necessary. I thought a large value like 1M might be good, so I could convert down, but it looks like I could also convert up with a resistor across the outside lugs.

If this is your goal, it's probably simpler (in parts count, not in amount of work required) to just adjust the other components so that the 10k or 100k pot (or whatever you're using) makes the circuit respond the same as it would with a 500k or 1M pot.  Impedance scaling, I think it's called.  Think of it like a simple RC filter.  You can make R big and C small, or make C big and R small, and get the same cutoff frequency.  You can do this with all the potentiometers in a circuit, but you basically have to alter a bunch of other values.

Oh, and this will only work if the input/output impedances of the active devices are respectively high/low enough to drive the components you use.

[The French connection]

This thread is really good!

http://www.diystompboxes.com/smfforum/index.php?topic=70732.0