Turn off normally on negative high voltage with TTL high

[Steve]

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

 

[Joerg]

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.

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.