This simple variable power supply circuit has a low production cost and delivers an output voltage between 1,5 V and 15 V with a 500 mA maximum current. Its stabilization is better than 2% if the current consumption do not exceed 350 mA. The variation of the power supply voltage can be made with a potentiometer and when overloading occurs a buzzer sounds a alarm.
T4 compairs the P1 slider voltage with the output voltage. Then the P1 slider voltage is 0,65 V higher than the adjusted voltage, T2 opens, which stops the T3-T5 Darlington base current.
The 18V, 1A transformer voltage must be filtered with B1 and C1. When the output current is more then 500 mA, the Bz1 starts the alarm (overloading). Bz1 must be a 24 V type, auto-oscillating.