Talk to me about bridging pots with resistors...........

[frequencycentral]

It's a neat trick - take a 100K pot, bridge the two outer lugs with a 100K resistor - hey presto a 50K pot.

Now, if you take a 100K pot and bridge it with say a 1K resistor it would be a 1K pot or thereabouts ( 0.9901K actually).

So, theoretically, one can take a 100K pot, bridge it with a resistor of your choice and create a 'whatever' K pot.

So really all I need to buy is a load of 100K pots and bridge them to suit my needs?

So what's the downside? At what point does it affect the taper?

Educate me!

[John Lyons]

I would think it alters the taper somehow.
I use this trick as well but usually only to halve the pot value.
I've often wondered about the downside.
My brain isn't that big though.

john

[frequencycentral]

The reason I ask is because in my last build I had a 100K volume pot, and was thinking of changing it out for a lower value to attenuate the volume and lower the serial resistance when in a high gain/low volume situation. I ended up experimenting wth bridging the pot with different values. It seemed to work fine. I ended up just using the 100K pot because once I added a tonestack that attenuated the signal sufficiently. I'm just curious as to the limits and negatives of such an approach.

[calpolyengineer]

It does affect the taper, there is an excellent article at geofex about it called "The Secret Life of Pots."

The only serious problem with buying a bunch of 100k pots is that you can't make them bigger. If you needed a pot bigger than 100k, you would have to buy it. Of course you could always go buy a bunch of 1M, but it would affect the taper even more to get them to the lower values.

-Joe

[frequencycentral]

It does affect the taper, there is an excellent article at geofex about it called "The Secret Life of Pots."

The only serious problem with buying a bunch of 100k pots is that you can't make them bigger. If you needed a pot bigger than 100k, you would have to buy it. Of course you could always go buy a bunch of 1M, but it would affect the taper even more to get them to the lower values.

-Joe

Yeah, I find I uses a lot more 100K and lower pots than 1M. Also, IM pots tend to be a little more expensive than 100K. That's why I mentioned 100K. I use a bunch of them..............

[calpolyengineer]

Yeah, I absolutely agree. I was just trying to point out some negatives

-Joe

[frequencycentral]

I read 'The secret life of pots' but couldn't find any reference to bridging the outer lugs - mostly about bridging from the wiper to an outer lug to change the taper.

[petemoore]

  1...Goes right across the smaller resistor, the bigger one has small influence over value...
  I just this minute deleted a whole page .5 of typing.
  Basically that's the one side, the other side is once one resistor gets smaller than the pot...[go back to 1].
  iow, sometimes it is easier to sacrifice the R range of the pot position by using a smaller pot [Ex: 100k Dist+ Gainpot], other times simply bending the R sweep curve a bit closer to what you wish is the ticket [like you have or even across wiper/outside lug].
  Many ways around, surefire is with DMM, a marked potknob centered in a matched, marked panel dial [1-10] so that settings and readings can be recorded, I don't know if you can find a resistor calculator online for this, notice how the R value start moving faster than stock pot sometimes, figure how that figures in circuit.
  The smaller the resistor in proportion to the pot value, the less it's easy to get a smooth or usable sweep of R value.
  I've seen it written in schematics as such 100k and 100k pot paralleled...sure is easy enough to try with/without the fixed R tagged ont he pot.

[zyxwyvu]

I read 'The secret life of pots' but couldn't find any reference to bridging the outer lugs - mostly about bridging from the wiper to an outer lug to change the taper.

Given a potentiometer with 2 lugs connected (P), and a parallel resistor across it (R), the resistance will be 1/(1/R+1/P). This gives a reverse log taper to the resulting combination. Here's a graphic I just drew up to illustrate this:

[dschwartz]

IME, it does affect the taper, use a DMM and measure the taper, you´ll see that it gets wierd, only max and min are ok, but in the middle i get strange readings..

since that i change the value of pots with 2 resistors..that works well...

[John Lyons]

I've read the secret life of pots a few times.
The concepts are there but more examples are needed.


There are (at least) a couple ways to alter pot values with respect to taper.
One resistor across a pot, one resistor across the wiper and each outer lug.
Depending on the fixed resistor's value you can get a different response.

Thanks for the graphs zyxwyvu
I don't have any simulation software.
Would you be interested in making some graphs with different fixed resistances for the other
configurations? It would be great to get this in the wiki.

Here are some schematics for other configurations.




Sometimes I use make the resistance higher in the log voltage divider to get a "super audio" taper level control
It's at unity when half way up, then after half it ramps up a good bit to boost an amp etc.

The rev. audio configuration is great for Fuzz Face and speed controls or tremolos and oscillators.

john

[Dai H.]

wow nice thanks!

[frank_p]

Putting equations on the forum...
That is the reason I posted this:
http://www.diystompboxes.com/smfforum/index.php?topic=70259.msg564775#msg564775
Otherwise it can be fastidious...
Well...

[zyxwyvu]

Thanks for the graphs zyxwyvu
I don't have any simulation software.
Would you be interested in making some graphs with different fixed resistances for the other
configurations? It would be great to get this in the wiki.

That's a good idea. I'll see if I can draw up some more of those. I'll start with the configurations posted. Any other requests?

[John Lyons]

Great, thanks Josh!


john

[zyxwyvu]

OK here's my first shot at a graphic for tapers. Let me know what you guys think.

[frequencycentral]

Josh, that's some great work! Help me understand what the percentages mean with some R values as examples please!

[slacker]

The percentages figure for R is what percentage the resistor (R) is of the pot (P) so for a 100k pot
10% = 10k
20% = 20k etc

The percentage figures for Max are the value of the combined Pot + resistor combo when the pot is set to maximum resistance. This is worked out using formula for resistors in parallel, so for a 100k pot and 100k resistor the maximum resistance is 50k.

The Max values would be useful for the B and C examples because they change in the same way, and the resistance changes as you move the wiper. If you're using these with a circuit with a high output impedance then you need to make sure the combined value is high enough not to load the circuit too much.

[zyxwyvu]

Josh, that's some great work! Help me understand what the percentages mean with some R values as examples please!

Edit: seems slacker has also posted an excellent explanation.

Also, to slacker: I was considering adding the load resistance of A,B, and C, but I couldn't figure out where to put it! It depends on the pot position, so another graph would be needed.


Sure, here are some examples. Keep in mind that the x axis is pot angle, and the y axis is voltage out (A,B,C) or resistance (D,E). Also, the pot is assumed to be linear taper.

First example:

Take Setup B (the letters correspond to the image John posted, as well as the schematics in the bottom right corner).
Your potentiometer (P) is 100k, and your parallel resistor (R) is 10k, or 10% of P.
To see what the taper looks like, Look at graph B, and the red line, which is labeled as 10%.
Note that these graphs are the voltage out.

Another example, for the variable resistances (where the graph is of the resistance):

Take Setup D, with resistor connected to the outer lugs of a potentiometer.
Your potentiometer (P) is 50k, and your resistor (R) is 10k (20% of 50k)
Then look at graph D, and the yellow line (labeled 20%).
This one is a bit odd, in that as you turn the potentiometer, the resistance goes up to a maximum (30% of P, which is 15k), and then goes back down.

Let me know if you have any other questions. Tips for improving it are also welcome - it's a lot of information in one place, and can definitely be hard to decipher.

[ironman28]

here's an article with some graphs. A little hard to read but useful.
http://www.elby-designs.com/documents/tailoringpotentionometers.pdf