Lathe control, can it be done?

[medic29]

@JBornstein90

From what I am seeing, as the potentiometer changes its resistance, the reference voltage changes as well. This reference voltage is what is read by the rest of the circuitry. The resistance is not the parameter used to adjust motor speed, but is used to adjust the output voltage of the stage. This voltage reference is then fed as an input to the next stage (however it works to adjust the motor speed)

I think, and a strong I THINK, what you are trying to do is actually adjust a voltage that is fed to the rest of the circuit. The potentiometer is able to accomplish this by adjusting an internal resistance, which then adjusts the output voltage.

Below is how my friend explained to me the dilemma about trying to replace these pots with updated electronics.

you can construct a device that mimics the behavior of the non-linear pot using networks of non-linear electrical components such as zener diodes, but these typically have non-linearity tied to (for example) a voltage or a current rather than a resistance. Since the waveforms applied to both sections of the pot are not really dc (even though P3B nominally regulates a DC voltage) there is no easy way to transform the non-linear voltage or current behavior of the mimicing components into something that looks like variable resistance when the thing that you are basing control on (voltage or current) is varying but the thing you really want to control (resistance) is not.

Hopefully that helps.