High Voltage Linear Supply inspired by LM723 datasheet..

This is a much easier to read schematic of Fig. 22 in the LM723 datasheet. It illustrates the connections required to get a regulator circuit to "float" on the output voltage. This one is designed for 1000 volts with a 50 ma constant current circuit.

Caveats: The MOSFET will dissipate 50 watts. You will need a heatsink. The triple resistor string is required to have enough spacing that 1000 volts will not arc through the resistors. Half watt resistors are rated at 350 volts. One watt resistors are rated at 500 volts. The 2 voltage supplies must be isolated from each other and the isolation must withstand 1000 volts. All the part numbers are from mouser.com. I didn't use a 723 chip because the requirement for a negative supply voltage ruins the usefulness of the onboard voltage regulator. A negative supply voltage is required in order to adjust the output all the way down to zero volts. The output load is required to dump the leakage of the MOSFET at a high temperature. That figure is on the datasheet for whatever MOSFET you use. The load resistors must be able to survive 1000 volts without arcing. Any common npn transistor will do the current limiting job. Any bipolar opamp will do the regulating job but, high voltage gain at zero frequency is a good thing.

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