Reverse Log Pots

[littlegreiger]

Does anyone know how to make a reverse log pot from either a log pot or a linear pot?
And does anyone know how to create a log pot from a linear pot?

[stm]

Just take a look here:

http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

Regards,

STM

[petemoore]

I've never done the actual equations///except by ear...
 By adding resistor[s[ to the pot [or to the outside lugs] the effective taper of the pot can be altered.
 I think I'd start with a linear, and taper one side of it with a resistor 1/2 x or 2 x as big as the pot...
  I modded a pots taper for the tone knob of a TS Clone, just 'tagging' resistors on one side and the outside of it till I found the range of tone I felt satisfied with.
 I don't know think you can get the exact tapering through the pots range using a modded pot to imitate a tapered one.

[zachary vex]

one way to figure out what to do for your particular application is to tie trimpots from each end of a linear pot to the wiper.  for example, i had a ernie ball pedal that i converted to a voltage controller, and to make the action smoother (because it had an audio-taper pot) i chose 500k pots to put across each leg to the wiper of the volume pot.  then i adjusted those two "trimmer" pots until the rocker "felt" right to me.  i was going to measure the values and replace them with fixed resistors, when i realized that i liked being able to re-adjust the taper for sitting down v.s. standing up!  

i imagine you are trying to make a reverse-log control for a tempo control of some type... here's another solution:  use a regular log or audio pot and wire it backwards.  then, label the knob with numbers and put a pointer on your enclosure!  8^)  that's how hewlett-packard handled the problem of backwards controls on their oscillators, and lots of radio manufacturers labeled their tuners that way too.

[littlegreiger]

I had trouble understanding the thing on Geofex.  So if someone could explain it better that would help.  By the way I need it to be a 500k reverse log pot in the end.

[cd]

IIRC you have to use a linear pot (NOT a log pot) and stick a resistor of around 20% the full value of the pot (i.e 100k linear pot, use 22k resistor) across the wiper and one leg.  If you attach it to the CW leg, it's anti-log, if you attach it to the CCW leg, it's log.  Or it could be the other way around, I always get pot nomenclature wrong.  Anyway, put TWO resistors (22k each) from the wiper to either end, and you get a log/anti-log pot.

Note that this will only work if you're setting up the pot as a voltage divider.  I think you can only get anti-log response when setting up a linear pot as a rheostat.  Then you have to use 25% of the full value of the pot (i.e 100k pot wired as a rheo use 25k resistor).

[littlegreiger]

I'm using the pot in a MXR Distortion+ that I am building. It needs to be 500k so would I take a 500k linear and put a 100k resistor from CW to the wiper?

[Fret Wire]

We all read that article, and we all basically understand it, but I think we need a paint by numbers example to get started. I know I do. I think I could disarm a Polaris nuke easier than figuring out that formula. Then, as CD pointed out, I forget which apps it will work for.

RG, how about some "Pot Taper Math for Dummies", vol 1.  

[petemoore]

I think it comes down to mess with it and take notes on the results...try of course to follow the directions as you go, but 'till you start to see what's happening when changeing X or Y resistor value or the thing works the way you want it to.
 Seems like if you start with a large value pot and run decreasing resistance tests across the outside lugs with fixed resistors or fixed resistors from wiper to outside lug you're bound to hit something 'close' or what can be expected from a tapering fix substitute.

[petemoore]

That's just a V/R wiring of a pot [wiper and 1 outside lug wired together]
 In that case wiring resistor to the V/R only reduces the minimum gain.[?]right?

[trescat]

Why don't you just purchase one from small bear electronics, he sells them

[cd]

I'm using the pot in a MXR Distortion+ that I am building. It needs to be 500k so would I take a 500k linear and put a 100k resistor from CW to the wiper?

There's no reason to mess around if you can just stick a resistor there and get what you want.  If you have a 500k linear pot and want to change it to anti-log response, put a 125k resistor across two of the lugs.  100k is not quite large enough.  I have a spreadsheet with a graph and all the values somewhere, I'll see if I can post it.

Pots can basically be wired in two ways: (1) voltage divider (input to CW, CCW ground, output taken off wiper) or (2) rheostat (input to CW, output to wiper).

[siaoguitar]

Yea, cd, tell us how to do it, i can't find reverse log pot here too, and i need to build the mxr microamp & phase90.both uses reverse log pots.......thanks man

[cd]

Yea, cd, tell us how to do it, i can't find reverse log pot here too, and i need to build the mxr microamp & phase90.both uses reverse log pots.......thanks man

Did you bother to read my posts above?  That's all there is to it.