Help with a potentiometer tapering question

[DavenPaget]

> According to RG's "Secret Life of Pots," this CAN be done with tapering resistors

Read it closely. I agree this distinction is not clear in this essay. But he only covers potentiometers in 3-terminal use, not 2-terminal use.

He does too! at the very end there is "reverse log series resistor" 

Nah . It will be a log pot not a reverse log unless you wired it wrongly .

[DavenPaget]

is
We want the blue curve:

Actually Paul, I think it is curve #5 that would be ideal for a rev log taper.

Am I wrong in thinking this?

Correct . Rev log would be curve 5

[Yazoo]

I get very confused about this as well but I think it is plot 5 which matches reverse log taper. There is an earlier graph which shows the 4 different type of pots and reverse log matches plot 5. I have also seen the advice that you can use a log pot wired backwards in place of a reverse log pot but I don't think this is quite right either because the curve in a reverse log pot is like the top left quarter of a circle whereas a log pot is more like the bottom right quarter of a circle. Again, if I understand this correctly, there is much finer control of the last 20% of the resistance because it is spread over about 70% of the pot rotation.

This has always nagged at me so please advise me if I have got this wrong.

[PRR]

Study your circuit. At one end you have max resistance. At the other end you have zero resistance (often with a stopper resistor). Now turn to the middle. What value do you want there?

If it isn't "half" (would be simple Linear), then you "usually" want 10% resistance. My blue curve.

[Govmnt_Lacky]

OK... need some inputs here. This is what I have so far....

5M Linear pot with a 3M resistor strapped across lugs 1 & 3 gets me to 1.95M on the DMM across Lugs 1 & 3. So now I have, in effect, a 2M Linear pot! 

During my research, I found this formula for changing from Linear to Rev log:

X=1.25A (X=the value resistor to strap across Lugs 2 & 3 to make the Rev Log pot. A= the value of the Linear pot you currently have)

So 1.25x2 = 2.5M

So according to this, if I strap a 3M resistor across Lugs 1 & 3 AND I strap an additional 2.5M resistor across Lugs 2 & 3 that should give me ~2M Rev Log pot!

Can anyone confirm this thinking for me? 

[PRR]

> confirm this thinking for me?

Why think??

In a few moments you can wire a pot, a resistor, 4 clip-leads, and an ohmmeter, and TRY it.

Perhaps the _real_ question is: what is the question? Or rather, what answer do you want?

The usual need for a 2-wire reverse-taper pot is something like a timing resistor in a tremolo. "Reverse" means "in terms of human convention, tapered backward from a Volume knob taper".

Using "0"= CCW and "10"= CW, you want something like:
0 = 2Meg
5 = 0.2Meg
10 = 0.02Meg (or zero plus a 20K fixed)

Mount the pot and knob so you know where 0 5 and 10 are.

Clip the meter to center and end terminals. Check for 0=2Meg and 10=zero. (If wrong, use the other end term.)

Now clip various resistors to the pot terminals. Yes you can get 5=200K but then you don't get 10=2Meg.

What you need is a negative resistance. DigiKey has no stock. There are synthesized negative resistances but simple tricks (neons, UJTs, TDs) are bias-critical and small dynamic range. An op-amp can make a not-bad NR but even with 19-cent opamps the complication starts to favor some other approach not needing a reverse-taper.

[Govmnt_Lacky]

Thanks Paul!

I guess I will just have to grab the DMM and work with some tapering resistors 

[fair.child]

Quick and dirty...

I want to convert a 5M Linear pot into a 2M Reverse Log pot.

What would I need, resistor wise, to add to the pot to achieve the proper taper? And how should it be attached? (i.e. Lug 3 to Lug 2, etc.)

Thanks 

try to put 5M into lug 3 and 2 of linear pot, it will result 2.5M linear pot then I will suggest you to put 500k between 2 and 1 pot lugs, it will make 2.5M reverse pot.
This is just my simple guessing. I tried to make it tapering about 20% (based on secret of pots-geofex)

If you have 2M pot in your hand, it could be easier to built 2M reverse pot, because all of you need 400k resistor into lugs 2 and 3 of 2M linear pot.

Hope it will work

Cheers