Check out this cool "Graphical Pot Taper / 3-Resistor Chain Bias" Calc I made

[stecykmi]

This excel spreadsheet allows you to A) design custom values/tapers for pots by adjusting 3 resistors between the 3 different pot terminals and B) calculate the bias points between 3 resistors in series

the taper calculator is on page 1/3 of the document. it works by calculating the equivalent circuit of all 270deg of rotation that a typical pot has (although it works the same for any type pot, just stretched or compressed) and displays the information in graphs. It also has several of the graphs as "normalized" meaning the range of numbers is shrunk down to a value between 0 to 1. This is useful if you're less concerned with the actual resistance and more the feel of the taper. Just note that when placing resistors between lugs 1&2 and 2&3, you change the _overall_ resistance (resistance between 1&3) as the wiper rotates. this will mess up DC applications such as creating CV's for analog synths.

the bias point calculator is on page 2/3. It has two independent calculators. one that calculates the two bias points from known resistors (as well as supply voltage) and one that calculates the resistors from known bias points (assume that the centre "floating" resistor is known). The first one is a fairly simple application of Ohm's law, but the second one is a little trickier as it involves solving for two equations and two unknowns. The calculator use some interesting matrix operations to compactly solve it, which i'm quite proud of (although the method came from google, haha). This calculator could be very useful for DIY synth people, which involves creating a lot of bias points for CV's.

Lemme know if you guys think this thing is useful, or if you find any bugs or limitations. I've tested it, but limited to simple/expected values and found no flaws yet. Also, is there a more convenient way to share an excel sheet? does google docs do that?

[teknoman2]

Nice one mate...

[brand0nized]

Hey thanks for posting this on my thread question!

I'm a little confused on how I should use this. I'm trying to make a 50K pot do 10K. Should I be entering resistor values until I get Max Resistance 10K with Min Resistance 0?
I'm also confused because when I enter 0 for all resistors, the max and min is the same.

[stecykmi]

no problem, i agree that it is a bit intimidating to use (that's one of the things i'd like to work on). if you want to keep the same taper, you should just use a resistor between lugs 1 & 3. if you want to change the taper, then play with R (1-2) and R (2-3).

So, you should enter the resistance of the pot (in this case 50000), leave R(1-2) and R(2-3) empty, and just play with R(1-3). In the results section, you should ignore the top 2 results (they don't calculate R (1-3)). They were calculated separately because using an R(1-2) and R(1-3) makes the total resistance change over the rotation of the pot, hence the Max/Min resistance.

For me, I got:
Pot = 50000
R1-2 = 0
R2-3 = 0
R1-3 = 12000

Which gets you Max/Min resistance 9677.4 ohms, which is very close to 10k and uses a fairly common resistor value.

This graphs and such displayed are more useful for analyzing the taper. if you want to get a better feel of how this works, enter the data above, then start adding different values for R(1-2) and R(2-3) and see how this affects the "Change in Resistance" chart (which should probably actually be labeled "Taper" since that's what it is). Very large values (say 10x larger than the pot) have very little effect on the taper, while small values (say 10x _smaller_) have very drastic effects on the taper. The "Change in Total Resistance Over Pot Rotation" graph shows how the resistance between lugs 1 & 3 changes as the pot rotates.