What is wrong with the MOSFET IRF540N ? Even with the zero gate voltage, why current is still flowing through drain to source?
When you push 900mA through 1K you get a 900V drop.  36% of that is 324V.  The zener is probably breaking down at 100V and passing enough current to drag the 324V down to 114 and change.  That pot will burn up pretty quickly in real life.