Altering a pots value?

[JJGross]

Is it possible to alter a pots value by adding resistors or something? I have a digital delay I'd like to button up but I'm short one lousy pot. I have 25k and 100k pots, I need a 50k. Possible to rig this up somehow or should I just order parts & wait a week for delivery.

Cheers,
- JJ

[52 Bill]

It depends upon how the pot is wired. If it is basically a 2-terminal variable resistor, yes. Start with a higher value and add a parallel resistance and hope that the taper is not a problem.

If it is used as a 3-terminal voltage divider, no. There is no way to add resistors to make it work correctly.

Hope this helps,
Bill

[Albert Kreuzer]

Hi JJ,

Geofex has the answer (thanx R.G.):

http://www.geofex.com/Article_Folder...s/potscret.htm

HTH
Albert

[JJGross]

Thanks guys.
It's a voltage divider so I guess I'm SOL on tweaking it.
I might get a pot @ Radio Shack for the week it takes to order something. At least I can test it and only mess with 3 wires when the parts get here. OK, 2 wires, a nut and a washer.

Cheers,
- JJ

[Enzo]

Did you check out Albert's link?

[JJGross]

Did you check out Albert's link?

Yes I did. I'd read that before asking here. There's a lot of info about altering the response of pots by adding tapering resistors, but taking one apart to rebuild the guts to make a 100k into a 50k by shaving the resistive band inside seems like more work than it's worth.

I just bought an over-sized 50k pot at RS to get the digital delay I built into the QC phase and I'll replace the pot next week. RS only has 24mm diameter pots with extra long shafts in the store and I prefer to use 16mm pots with short shafts in stompboxes. The shaft on the pots RS had are almost 2" long - about 1/4" more than the depth of the aluminum enclosure. It actually looks kinda funny, but I won't leave it in there. I have to order more parts anyway to build a couple compressors, 2 more delays, another Tycobrahe Octavia clone, maybe a Rangemaster or Fuzzface clone or two (I just salvaged a PILE of old Germanium transistors & diodes) and some boosts so it's not like I'm paying $8 shipping on one pot.

Cheers,
- JJ

[Enzo]

DO some experiments, slap a 100k resistor across the end legs of a 100k pot and measure the resistance to the wiper at several points. The only difference betwen that and a 50k pot would be the taper. If you want to get fancier, you can use two resistors from the wiper to each end. That will yield a different taper, and they don't have to be the same value, for even more fun. You can alter the taper with external resistors, but they will also change the overall resistance of the pot. I wouldn't shave down a pot either, but I can and do add parallel resistors to get values I want.


Wait, what does he mean if it is a voltage divider it won't work? Unless the pot is only part of a divider it won't matter much at all. The whole point of a divider is some relatively large resistance has the signal across it, and then some point in the middle is tapped for some percent of it. 50k, 100k, 250k, doesn't matter a lot. If it is the feedback element in an op amp, then it does matter, but that isn't a voltage divider then. The only difference will be the taper.

[Steve Conner]

Last time I was in a situation like this (trying to fix a worn-out resonance pot on a TB-303 clone, the day before a gig) I found that connecting two 100k resistors across a 100k pot (one between each end lug and the wiper) made a good first-order approximation to a 50k pot. :P

[JJGross]

I was going to get this finished today so I think I'll run a couple resistors across the legs on this an hit it with a VOM to check it. I'll probably use the 2 resistor method since the pot I'm using is an audio taper and the schematic calls for a linear taper. I'm already working with a scooped taper and don't want to shove the curve much further off linear. Maybe wiring it as a reverse taper will push it toward more of a linear curve (or lack of one).

The pot in question is the Delay control on a BYOC Digital Delay pedal. I bought the board from BYOC and sourced the parts almost entirely from salvage. I wanted to see how much I could get away with in a build like this using as many free parts as possible. Including the enclosure, I think I'll have something like $20 into this thing at the most. In fact most of the components in this came from an old PC power supply that was on the curb on garbage day. Obviously not the pots or the actual delay IC, but almost everything else. It's also been an exercise in desoldering skills to get all those little caps & resistors of the PCB in the power supply in good shape. They're not SMD, they're all pretty standard components, but boy do they pack stuff in tight in those power supplies.

I really appreciate all the help here. Thanks!

Cheers,
- JJ

[52 Bill]

Wait, what does he mean if it is a voltage divider it won't work? Unless the pot is only part of a divider it won't matter much at all. The whole point of a divider is some relatively large resistance has the signal across it, and then some point in the middle is tapped for some percent of it. 50k, 100k, 250k, doesn't matter a lot. If it is the feedback element in an op amp, then it does matter, but that isn't a voltage divider then. The only difference will be the taper.

Enzo is absolutely right, what I meant was that in my experience adding resistors to change the value of a pot can change the apparent taper so much, that in some applications it will not be a satisfactory replacement.

Sorry for the error.
Bill

[Steve A.]

Enzo is absolutely right, what I meant was that in my experience adding resistors to change the value of a pot can change the apparent taper so much, that in some applications it will not be a satisfactory replacement.

Sorry for the error.
Bill

Bill:

The tapering resistors can also change the impedance seen by the preceeding stage so your original caveat is not entirely out of place. But it isn't just the taper that is involved (which would mean finding it very hard to hit a desired setting, or conversely it could be spread widely across the full range of the pot).

Steve Ahola