It might help if you drew the schematic with and without the fuse.In TomGeorges schematic there is not path for the base current.Look at it this way:The fuse is gone.The input voltage is now below the 16V of the zener so the zener is not conducting.There is no path for the base current.
@TomGeorge,I don't see the current path you are describing. If the base is essentially open circuit, no current is going to flow.Go back to the sketch (crude) in post #10, Remove the fuse and Zener, you will see a current path from emitter -> base -> R2 -> GroundPutting the fuse and zener back in. The fuse essentially shorts out the base emitter junction, not allowing any base current to flow out of the transistor.John
I think the OP should drop the 10k to 5k (roughly) else the PNP will be on the edge of full conduction.In switching applications I always start with a force beta of ~10.John
yes, now can I ask, what is the science behind moving the resistor (R2) from one side of the zener to the other?