ElCaron:
This switches between off (base high, zener limits gate voltage to 12V) and broken (base low, zener connected to nonconductive BJT, gate voltage not limited, so 24V), doesn't it?
No. Actually, when the BJT is open, the 10k resistor pulls the Gate up to 24V and thus, there is zero volts from the Gate-to-Source. This happens because the open BJT behaves like a very large resistance, which, in series with the 10k resistor behaves like a voltage divider, and because 10k is a very low resistance compared to the effective resistance across BJT [Collector-Emitter], all of the voltage is dropped across the BJT [and the zener, which, actually, will have very little voltage across it, because of the extremely low current].
