Hi all, First I should say I know almost nothing about what I'm talking about.

I'm designing a power supply I need some kind of variable voltage (like a PWM with a filter or a pot) to control an opamp that gives it gain then that controls the voltage regulator(refer to attachment). my power supply is 0 to 30volts so a 1 torn pot is not going to work because if I breath on it and it will move 500mv. So I was trying to find away to use two pots fine and course but I'd like to have the fine pot have control of about 1 volt on all ranges. Or have some kind of rotary encoder.
Any way you can think of to be able to easily have fine control of the hole 30volt range
my main problems are:

I'm try to keep this part of the circuit under $5.
I know nothing about Microcontrolers and Dac's and that sort of thing.
I don't want to have to work with software.
This is my first (big) project so I'd like to keep it semi simple

And if you could you could give me a diagram that would be very helpful.
Thank you.

 

To provide a vernier control simple place a small value pot, connected as a rheostat, in series with the input of a larger value pot. You could also use a ten turn pot.

Your power supply has no current limit so you may blow a transistor if you accidentally short the output.

It might be better if you used an LM317 to build your power supply. It's simple, and has build-in current limiting and over temperature protection. It's only possible disadvantage is that it's minimum output voltage is 1.2V

 

Thank you crutschow,

I probably should have added that thats only part of my circuit I'v also got a current limiter and and the transistor is mounted to a good size heat sink that has a temp switch so if it goes over 60c it will turn on a fan.
I was looking into a lm317 but I really want it to go down to 10mv. But a pot won't do that ether so I was thinking of putting a diode in there to drop the voltage 700mv (refer to attachment) so it can go down to 1mv. And I put in some resistors to make a potentiometer.
If someone could look at the schematic and make sure it will work that would be great

 

Did you simulate your circuit and observe that the output oscillates? Or did you see it and just ignore the small wiggles? If you look at U2's output you will see a much larger oscillation voltage. To correct that I added C3 and R12 to give some lag compensation (high frequency rolloff).

Also there was noticeable overshoot upon turn on so I added R2 and C4 to slow the startup time and prevent the overshoot.

You don't need the diode. The circuit will go down to 0V without it if you place the 1kΩ vernier pot at the top of the 10kΩ main pot, not the bottom.

I changed the zener to a 6.2V device since they have good temperature stability. To still give 30V output I increased the gain of U1.

The revised circuit is attached. I suggest you simulate it and observe its operation at various nodes in the circuit for different input voltages and output loads to become familiar with its operation.

View attachment Voltage controller (1).asc

 

Wow thanks a bunch I din't know how to get rid of the output waves. And and you even fixed the overshoot. I've been playing with it and seems to work good can't thank you enough.