What we need for this project is very simple. A N-channel MOSFET and a power resistor as a load. To control the voltage we need an OPAMP and a potentiometer. The rest are just other components in order to read and show the values on the display. The potentiometer must be at least 10 turns in order to have good precision. Very important, you need a good heat dissipator because the MOSFET will get hot fast and without it it will burn out in seconds.