A simple 0-30V adjustable power supply can be made using LM723 voltage regulator.
This LM723 0-30V adjustable power supply circuit diagram provides output voltage between 0 and 30 volts at a maximum current of 1 Amp. Because the standard LM723 circuit minimum output voltage is 2 volts, to the ground of the integrated circuit terminal (pin 7) is applied - 4.7 volts, using Zener diode D4. R8 resistor limits the maximum output current, which in this case is 1 ampere.
With potentiometer P3 can be adjusted maximum output current continuously.
Before first use the adjustable source must be configured as follows:
Connects at output a resistance with value 1k/1watt and rotates potentiometer P3 the least resistance. At this point potentiometer P2 is adjusted so that the cursor to find the output voltage. If the adjustment of P2 does not reach up to 30 volts, can decrease the value of R6. Transistor T3 requires mounting on a corresponding heatsink (2 degrees / watt).