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.

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