Q: Replacement of 100k potentiometer with mosfet In this circuit i want to replace 100k potentiometer. As in this type of circuit the output voltage is set by rotating the knob manually so i just want to make it automatically by giving pwm signal to gate of MOSFET and MOSFET used as a variable resistor in place of 100k potentiometer. So i try to connect MOSFET in place of potentiometer it works but when i connect load it doesn't work so can someone please help me to find way to set it's voltage without rotating the knob..


A: You could replace the 100K resistor with a special high voltage digital pot such as an AD5293 (not particularly cheap).
The maximum output voltage would be 30V or so (abs. max is 35V), so you would have to add some kind of external overvoltage protection or shunt the digital pot with a resistor to limit the output voltage (which would make it nonlinear, but that could be fixed in software). You could also adjust the 1K resistor to limit the maximum.
The minimum output voltage is the reference voltage of about 1.25V.
If you need a higher settable output voltage you could muck around with resistor networks, with various trade-offs.
