Simple regulated power supply with overcurrent trip

  
The 15v zener is fed via the 4K7 resistor from the input. With 0.6v across the base - emitter of the PNP transistor, this establishes a voltage of 14.4 across the 3K3, so there must be a current of nearly 4.4mA through the 3K3. This current is available to feed the emitter of the transistor - provided that the diode D1 doesn`t pinch any of it. However with no load on the output, some of this current flows through the PNP to turn on the BFR39 and the 2N6254 which will conduct.
Simple regulated power supply with overcurrent trip - schematic




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