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 ;D
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
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)
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!
Quote from: Govmnt_Lacky on March 05, 2012, 01:21:39 PM
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
> 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.
Quote from: PRR on March 06, 2012, 12:59:10 AM
> 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?
Quote from: Govmnt_Lacky on March 06, 2012, 06:49:13 AM
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.
Quote from: Govmnt_Lacky on March 06, 2012, 06:49:13 AM
Quote from: PRR on March 06, 2012, 12:59:10 AM
> 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}] (http://www.wolframalpha.com/input/?i=Plot%5B3.3*%285*x%2F100%29%2F%283.3%2Bx*5%2F100%29%2C%7Bx%2C0%2C100%7D)
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}] (http://www.wolframalpha.com/input/?i=Plot%5B3.3*%285*%28100-x%29%2F100%29%2F%283.3%2B%28100-x%29*5%2F100%29%2C%7Bx%2C0%2C100%7D)
Thanks merlin and Seljer ;D
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 :(
Quote from: Govmnt_Lacky on March 06, 2012, 09:44:51 AM
Thanks merlin and Seljer ;D
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)
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.
Quote from: Mark Hammer on March 06, 2012, 10:44:17 AM
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. ;)
> 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.
Quote from: PRR on March 06, 2012, 10:16:47 PM
> 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" :)
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.
Quote from: slacker on March 07, 2012, 03:21:32 AM
What circuit is this for?
EH Echoflanger :icon_eek:
We want the blue curve:
(http://i.imgur.com/jwBkY.gif)
Quote from: PRR on March 09, 2012, 01:31:40 AMis
We want the blue curve:
(http://i.imgur.com/jwBkY.gif)
Actually Paul, I think it is curve #5 that would be ideal for a rev log taper.
Am I wrong in thinking this?
Quote from: Govmnt_Lacky on March 06, 2012, 06:49:13 AM
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 .
Quote from: Seljer on March 07, 2012, 02:20:51 AM
Quote from: PRR on March 06, 2012, 10:16:47 PM
> 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 .
Quote from: Govmnt_Lacky on March 09, 2012, 01:28:42 PM
Quote from: PRR on March 09, 2012, 01:31:40 AMis
We want the blue curve:
(http://i.imgur.com/jwBkY.gif)
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
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.
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.
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! ;D
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? :-\
> 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.
Thanks Paul!
I guess I will just have to grab the DMM and work with some tapering resistors ;)
Quote from: Govmnt_Lacky on March 05, 2012, 12:59:29 PM
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 ;D
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