I said voltage divider on the title, but could be anything.
I have a DC source of 19.7V.
I need an output that varies from 0V to 18V.
My only requirement, besides part count, is that both potentiometers be used: the 1k being the main, and the 5k acting for fine tuning.
And the adjust... not too sensitive. The more linear, the merrier, and taking advantage of the whole excursion of the potentiometer.
In the example below I could not obey the requirement of sensitivity and excursion.
The transistor I'm trying to use has a cutoff around 680mV and saturation around 820mV. Very small window. Hence, my trouble.
The output current can be small. 5mA will work;
This whole system will control the GND pin of a LM7805, and later on, of a 7905;
The 1k potentiometer can be turned by hand and is what I have available. The 5k trimpot needs a screwdriver.
At first I thought a voltage divider could do it, but in my attempts, the voltage hardly approaches 15V.
Still open for this type of design, though (just bear the requirements if you'd like to help me this way)
After giving up, I tried using OpAmp. It worked. But later I will need a negative version of this structure I'm trying to build, but 19.7V-(-19.7V) my opamps won't support.
Maybe I could reduce the supply voltage of this OpAmp, but then... part count.
Third attempt, more promising but difficult, was with a transistor.