Switching power supply high voltage constant current source circuit diagram

To develop the instrument, we need a constant current source that can generate 1MA current on a resistance of 0 to 3 megohms. We designed one using UC3845 combined with a 12V battery. The transformer uses a color TV high-voltage package, in which L1 is wound with 24 turns of enameled wire around the core of the original high-voltage package. , L3 uses a coil of the original high-voltage package, and L2 uses the high-voltage part of the high-voltage package. L3 and LM393 form a voltage limiting circuit to limit the output voltage from being too high. Adjusting R10 can adjust the open-circuit output voltage