How to limit a potentiometer?

[bside2234]

I have a B500K pot that I don't want to go below something like 370K. It's just wired as a variable resistor. Is it possible to do this?

[jacobyjd]

I have a B500K pot that I don't want to go below something like 370K. It's just wired as a variable resistor. Is it possible to do this?

Read 'The Secret Life of Pots' at Geofex. That'll tell you everything you ever wanted to know.

[Gus]

What type of pot, linear, log etc.?  B does not always mean linear.

If linear use a fixed 330K or 390K with say a 250K or 100K pot wired as a variable resistor.  330K + 250K =580K max, 330K min, 390K + 100K =490K max, 390K min

Or you could do something like this use a pointer knob and a mechanical limit like a post in the panel etc to stop the rotation.

[bside2234]

I did read it. It mainly just talks about taper. Maybe I just missed it. I'll read it again. Thanks.

[bside2234]

It's a B linear pot. I know about how to raise and lower the max value of a pot. I just need the resistance of the pot to not go BELOW ~390K. The only way to do this is to put in a mechanical stop? What I am doing is I built an auto wah and I just incorporated a "Q" control and the only usable resistance on the pot is between ~390K and 500K. Anything below ~370K just turns the effect off. So I just want the pot to go from ~390K up to max (500K).

[jacobyjd]

Ok--the main principle to take away from it is that you can use parallel resistors--while they do alter the taper some, they also provide a way to limit the max resistance a pot can have. Do the math to see what would give you 370k if you put it in parallel with 500k, then run a resistor close to that value across the lugs of your variable resistor

[jacobyjd]

It's a B linear pot. I know about how to raise and lower the max value of a pot. I just need the resistance of the pot to not go BELOW ~390K. The only way to do this is to put in a mechanical stop? What I am doing is I built an auto wah and I just incorporated a "Q" control and the only usable resistance on the pot is between ~390K and 500K. Anything below ~370K just turns the effect off. So I just want the pot to go from ~390K up to max (500K).

Ah, sorry, I misread your OP. My bad!

Ok, to limit how low the pot goes, you're going to have to do something like this: use a fixed resistor in series with the pot wired as a variable resistor. However, the 500k pot may not be your best bet to use then if you absolutely have to keep the MAX resistance at 500k.

Gus's suggestion is what you want then.

[bside2234]

That's what I was afraid of. Thanks. I'll look into a mechanical stop. Maybe I can just solder a piece of wire to the inside of the case of the pot where I want it to stop the contact or maybe I'll just scrape off the resistive material just so the effect doesn't cut out.

[jacobyjd]

how would any of that be easier than soldering in a series resistor?

[bside2234]

Putting a resistor in series with the pot only increases the value of the pot. It still goes from 0 ohms to fixed resistor + pot value. It doesn't limit how low the pots resistance will go. Am I wrong?

[jacobyjd]

Putting a resistor in series with the pot only increases the value of the pot. It still goes from 0 ohms to fixed resistor + pot value. It doesn't limit how low the pots resistance will go. Am I wrong?

Right...so the suggestion is to use a lower value pot along with the series resistor. Also, have you tried a higher resistance? That may not cause a problem.

If you don't have a 100k or 250k pot on hand, you can always limit that 500k pot down to whatever maximum you want by ALSO adding a parallel resistor to it. This way, you'd have the 370k series resistor to set the minimum and the XXXk parallel resistor on the pot to limit the maximum.

[The French connection]

Do like Gus said

What type of pot, linear, log etc.?  B does not always mean linear.

If linear use a fixed 330K or 390K with say a 250K or 100K pot wired as a variable resistor.  330K + 250K =580K max, 330K min, 390K + 100K =490K max, 390K min

Or you could do something like this use a pointer knob and a mechanical limit like a post in the panel etc to stop the rotation.

If you don't have a 250K pot, put a 500K res (or two 250K) across your 500K pot and you should have what you want...

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

Dan

[bside2234]

I'll try that. I didn't realize using a smaller value pot and series resistor would not only raise the max resistance but also raise the min resistance.

[bside2234]

Thanks guys! Tried it out today and it worked great.

[jacobyjd]

Thanks guys! Tried it out today and it worked great.

Cool man! Finding fixes like this helped me a lot for all sorts of projects, especially with different circuit adaptations or mods. If I can't use a 1k, 10k, 100k, 250k, or 500k pot, then I usually use fixed resistors to allow it...there's really no need for crazy values, and it gets too expensive to keep every value around.