Help with a potentiometer tapering question

[Govmnt_Lacky]

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 

[Seljer]

1/R = 1/R1 + 1/R2

1/2 = 1/5 + 1/X
1/X = 1/2 - 1/5
1/X = 5/10-2/10 = 3/10

x = 10/3 = you need 3.33 megaohms so the total resistance is 2 megaohms when hooked up in parallel with 5 megaohms

Refer to the excellent http://www.geofex.com/article_folders/potsecrets/potscret.htm for how to hook it up

[Govmnt_Lacky]

The pot will be used as a variable resistor (NOT a voltage divider) and I have read the geofx link.

I am still unsure about HOW to connect the tapering resistor. It would either be from Lug1 to Lug2 -OR- Lug2 to Lug3   

Also, would a 3M resistor still work? It would not be in parallel with the ENTIRE resistance (i.e. from Lug1 to Lug3)

[Seljer]

yes, 3Meg would probably be close enough

If its used as a variable resistor you hook it up to the two lugs of the potentiometer that are being used in the first place.
If you get it wrong you only get the sweep of the pot the wrong way round (i.e. 1 will be 10 and 10 will be 1), it will however still be reverse log!

[seedlings]

The pot will be used as a variable resistor (NOT a voltage divider) and I have read the geofx link.

I am still unsure about HOW to connect the tapering resistor. It would either be from Lug1 to Lug2 -OR- Lug2 to Lug3    

Also, would a 3M resistor still work? It would not be in parallel with the ENTIRE resistance (i.e. from Lug1 to Lug3)

Use a 3M3 resistor.  Between which pins?  Decide which two lugs (center lug and one or the other outer lug) give you the proper rotation for your application.  It will give you considerably more change in the first or last bit of rotation, so if you want 'reverse log' then you may end up having to turn the knob backwards to get the desired % change in the proper range of turn.

2M = (1/3.3M + 1/5M)^-1

CHAD

[PRR]

> convert a 5M Linear pot into a 2M Reverse Log pot..... used as a variable resistor (NOT a voltage divider)

No. Can't be done by loading.

Prove it to yourself. Pot, resistor, clip-leads, ohmmeter.

You want "0" to be 2Meg, "5" to be 200K, and "10" to be zero.

While you can trim "0"=2Meg and "10"=zero, you can't get 200K in the middle.

Voltage divider, yes. Variable-resistor, no.

Get a 2Meg Audio pot and number the knob backward. This is electrically identical, just counter-customary to the hand.

Odds are the 2Meg value can be transposed to 1Meg or 5Meg by adjusting other components.

[Govmnt_Lacky]

> convert a 5M Linear pot into a 2M Reverse Log pot..... used as a variable resistor (NOT a voltage divider)

No. Can't be done by loading.

Voltage divider, yes. Variable-resistor, no.

According to RG's "Secret Life of Pots," this CAN be done with tapering resistors. I believe that he does mention that it can't be done with a voltage divider but, he does say that it CAN be done with a variable resistor set-up. It wouldn't be a "perfect" 2M Rev Log pot but it would be pretty close.

Am I wrong on this?

[merlinb]

According to RG's "Secret Life of Pots," this CAN be done with tapering resistors. I believe that he does mention that it can't be done with a voltage divider but, he does say that it CAN be done with a variable resistor set-up. It wouldn't be a "perfect" 2M Rev Log pot but it would be pretty close.

Am I wrong on this?

You can turn it from linear to log/antilog with a tapering resistor about 1/5 the resistance of the pot, or 1M in this case. What you can't easily do is to make it nicely tapered AND get a total resistance of 2M that you want.

[Seljer]

> convert a 5M Linear pot into a 2M Reverse Log pot..... used as a variable resistor (NOT a voltage divider)

No. Can't be done by loading.

Voltage divider, yes. Variable-resistor, no.

According to RG's "Secret Life of Pots," this CAN be done with tapering resistors. I believe that he does mention that it can't be done with a voltage divider but, he does say that it CAN be done with a variable resistor set-up. It wouldn't be a "perfect" 2M Rev Log pot but it would be pretty close.

Am I wrong on this?

Nope, just do the math

For the variable resistor type setup you hookup your potentiometer and the resistor in parellel. You have two resistances, on your fixed tapering resistor, the other the resistance between two lugs of the potentiometer which goes linearly from 0 to 5Meg

R1 = 3.3Meg
R2 = x*5Meg/100
where x goes from 0% to 100%

R = R1 || R2 = R1*R2/(R1+R2) = 3.3*(5*x/100)/(3.3+x*5/100)

]http://www.wolframalpha.com/input/?i=Plot[3.3*%285*x%2F100%29%2F%283.3%2Bx*5%2F100%29%2C{x%2C0%2C100}]
The taper isn't perfect, more like semi-antilog but its still probably better than linear. To get a better curve you need a higher ratio between the resistance of the potentiometer and the added resistor, which means a lower value of the tapering resistor, which leads to lower total combined resistance, which might not be feasable for the circuit you're using it in.

If you hook it up between the wrong two lugs of pot, you get the same thing but flipped horizontally, its still the same taper, just going in the other direction
]http://www.wolframalpha.com/input/?i=Plot[3.3*%285*%28100-x%29%2F100%29%2F%283.3%2B%28100-x%29*5%2F100%29%2C{x%2C0%2C100}]

[Govmnt_Lacky]

Thanks merlin and Seljer 

I guess I will have to experiment with the tapering resistor. All of your inputs have given me a starting point as far as where to connect the resistor and what values to use.

As far as the 2MC that I need... it does not need to be perfect. I just need something as close to a 2M Rev Log pot as possible seeing as I cannot find a single place to procure one. Most companies do not produce A-taper or C-taper pots above 1M 

[Seljer]

Thanks merlin and Seljer 

I guess I will have to experiment with the tapering resistor. All of your inputs have given me a starting point as far as where to connect the resistor and what values to use.

As far as the 2MC that I need... it does not need to be perfect. I just need something as close to a 2M Rev Log pot as possible seeing as I cannot find a single place to procure one. Most companies do not produce A-taper or C-taper pots above 1M 

The method mentioned earlier with buying a regular log pot and wiring it in backwards works if you can live having the control sweep in the wrong way (this bugged the hell out of me for the rate control on the Phase 90 I made enough to switch it)

[Mark Hammer]

I'm going to suggest a more empirical approach, in addition to what has been suggested so far.

The reason why tapers are important is because they can either impede or assist the user in having the most dialability for that range of values of most interest and use to them.

"Standard" tapers are approximations of some ideal thought to be as close to universally applicable to a broad range of contexts as possible.  So, log taper is for those contexts where one is interested in, and needs, resistance values to ease in for a bit before changing rapidly, and reverse log is for those contexts where large resistances are of some limited use but one is more interested in distinctions between small resistance values so you want the small-resistance range to be spread more broadly.

So, whatever the function of the 5M pot in question, I suggest to Mr. Lacky that the ideal approach is to leave the 5M pot in place, adjust it slowly, identifying the portion of its range, and accompanying resistance values, that are of most interest/use.  If, let's say, there is really only something interesting going on between 1.4M and zero ohms, and a lot of really interesting stuff happens between, say 75k and 0 ohms, then you set the taper, via parallel resistors so that it does that, and maximizes how much of the pot rotation that 75k-and-lower range occupies. 

Standard tapers be damned.  What matters is that you get the sounds you want.

[Govmnt_Lacky]

So, whatever the function of the 5M pot in question, I suggest to Mr. Lacky that the ideal approach is to leave the 5M pot in place, adjust it slowly, identifying the portion of its range, and accompanying resistance values, that are of most interest/use.  If, let's say, there is really only something interesting going on between 1.4M and zero ohms, and a lot of really interesting stuff happens between, say 75k and 0 ohms, then you set the taper, via parallel resistors so that it does that, and maximizes how much of the pot rotation that 75k-and-lower range occupies. 

Standard tapers be damned.  What matters is that you get the sounds you want.

I agree completely Mark!

I was thinking along these lines as well. I was going to play with the taper resistance values to get what I wanted. I really just needed to know what lugs to put it on and a jump-off point for the resistance value. The +/- will be to my ear. 

[PRR]

> 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.

[Seljer]

> 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" 

[slacker]

What circuit is this for? You might be able to tweak other values so that you can use a pot value that you can make or buy. Or, you might be able to rewire it so that you don't need a reverse taper pot or make the pot value not matter.

[Govmnt_Lacky]

What circuit is this for?

EH Echoflanger 

[PRR]

We want the blue curve:

[Govmnt_Lacky]

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?

[DavenPaget]

According to RG's "Secret Life of Pots," this CAN be done with tapering resistors. I believe that he does mention that it can't be done with a voltage divider but, he does say that it CAN be done with a variable resistor set-up. It wouldn't be a "perfect" 2M Rev Log pot but it would be pretty close.

Am I wrong on this?

Very wrong . It's the other way round .