3-30V power supply 3 A circuit diagram

Connect a voltmeter to “GND” points and “OUT +” and set “RV1” until the voltage
desired output is reached.
Apply some heat-conductive paste to the lower surface of the transistor and mount it on the
radiator.
If you need 3 to 8 volts then R2 is 5.6K
If you need more than 8 volts then the R2 is 2.2K