Looking for a circuit to turn off normally on negative 500 volts with logic high TTL. The negative voltage would be normally on with TTL low or with no control signal being applied. A high TTL would turn off the negative voltage (off period will be perhaps 100 microseconds). The negative voltage is applied to plates in a vacuum so very little or no current draw. Turn off/on times of tens of microseconds. It would be ok to use a battery for bias if necessary.
Steve
Sounds like an optocoupler job. N-channel FET that is rated 800V or better with source to -500V. 12V zener from gate to -500V, large resistor from gate to GND. Opto output across that zener. Opto input to TTL via resistor.
If the resistor's draw is a concern or switching times become too slow use a 9V battery and resistor (no zener required in that case). Select the resistor as high as possible while maintaining desired switching speed.
Try not to oversize the FET, take the smallest you can find for 800V breakdown since it will then exhibit the lowest gate capacitance. Of course, you can also do more esoteric stuff, like sending a high frequency across a gate transformer that gets rectified on the other side and becomes the gate control voltage. That would draw no quiescent current at all from your -500V rail.
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