Saturday, April 30, 2011

Voltage Division with coarse and fine adjust

In designing the Voltage source of the last post, I originally wanted to have 2 adjustment knobs, one coarse and one fine. I concidered designs using two linear potentiometers but the fine adjustment wasn't as I wanted it to be. I wanted it to always provide a uniform range +- around the coarse setting ( I call this +- range delta V). It didn't provide this, instead it would give a large delta V when the coarse was set at one extreme and zero delta V at the other. Circuits A and B below demonstrate this effect.
I then discovered that by using a standard linear pot for the coarse adjustment and a dual gang linear pot for the fine adjustment my desire could be accomplished as represented in Circuit C.

Equations for Vout/Vin for the three circuits and corresponding plots with R1=5k, R2=100k, R3=20k are shown below. I've introduced variables x and y to represent the fraction of R2 and R3 which are enabled by the pot knob and can take values from 0 to 1. The plots show the extremes of the fine pot (y=0, y=1) for all values of x. The size of delta V can be set primarily with the value of R3. I do like this design but didn't end up using it because I was impatient and didn't have immediate acces to 5k dual gang pot. Maybe next time!


Ivan Eduardo said...

Awesome, thanks! You saved me 30 bucks in some 10 turn pots and having to order them online.

J4e8a16n said...


How did you find the numbers for x and y?
Did you red every step numbers on the multimeter?
Does y=1/x or else?

J4e8a16n said...

On the shema C, you show the output on the resistor between the two linear pots. Does the output comes from the top side of the resistor or the bottom side of the resistor.

city said...

thanks for sharing.

Torsten Dietrich said...

Hi Jonathan,
inspired from your dual-gear solution I found another simple solution connection two standard (single gear) potentiometers in H-shape.

I've put my solution and some calculation results for your proposal on my home page ( [] , see under Coarse/Fine Potentiometers).
Maybe this helps others.
Best regards