Changing pot value calculations

[Mcknib]

Here in case anybody want's it is a coupla calculations for altering and checking pot values e.g if you need a 20K but only have a 50K these will tell you what resistance you need in paralell and let you check how close you got to your desired value, I can't remember if I already posted it so just in case

Pot value Calculations

 

[BuddytheReow]

So, the resistor goes between pins 1 and 3?

 

[Mcknib]

So, the resistor goes between pins 1 and 3?

Yeah that's correct lugs 1 and 3 parallel with the pot resistive strip

It's saved me a lot of time trialing different values if it wasn't a simple half the pot value job ie 500K to 250K stick a 500K in parallel, remembering calculations isn't my strong point

 

[Chuck D. Bones]

This works well in some circuits, not so well in others. Don't try to decrease the resistance by more than 2.5x and you'll probably be ok. It will alter the taper, but by how much and whether it matters is circuit dependent.

 

[Bricksnbeatles]

You can also see exactly what the taper will look like using a graphing calculator, including the free Google one that you get just by googling the relevant equation

 

[Chuck D. Bones]

We also have to take into account the surrounding circuitry because the pot's impedance curve changes when we add parallel resistors and that can alter the effective taper.

Graphing an equation will work for linear taper pots, but log, anti-log and W pot tapers are rough approximations to an actual log or reverse-log curve. I have not seen equations for the real A, C and W pot tapers. In LTSpice, I use the .step param list directive to do a piecewise linear approximation based on the pot datasheet curves.