A 15v variable power supply circuit which can be adjusted in the range 1.5 15 volts, can be designed using this circuit diagram.
This 15v variable power supply circuit is made using an 2N3055 power transistor and can provide a maximum current of 500 mA. Stabilization is better than 2%, if the currents do not exceed 350 mA. Voltage is set using a potentiometer, and the presence of overload is announced by an audible indication of overload.
T4 transistor compare the voltage at the cursor P1 with the output. When this voltage is higher with 0.65 V than regulated voltage, T2 opens, leading to suppression of the current Darlington power stage, T3-T5. In this mode, the output voltage of the source is higher than 0.65 V reference voltage on the base of T4, which comes from a 15 V zener diode.