High voltage voltage controlled linear variable resistor

[coinmaster]

I have some high voltage (300v+) analog circuits that I want to control digitally which requires the use of a voltage controlled linear resistor that can withstand high voltages. I don't expect the current levels to be that high.
I originally settled on LDR optocouplers but it turns out they can't handle big voltages so that leaves me with transistor or diode optocouplers.
As far as I understand it, photodiodes are light controlled zeners and phototransistors are light controlled transistors.
Which one should I choose for a high voltage linear variable resistor meant for a voltage divider?

 

[rbelli1]

If you put multiple LDR optocouplers in series you would be able to get an arbitrarily high output voltage. You will just need to make sure your LED currents are closely matched. 300V should not be a problem as long as the isolation voltage of the part is high enough.

BoB

 

[jim hardy]

might this help ?
Make a current source...
LDR only has to handle the few volts necessary to bias the FET, FET takes the rest. Voltage across LDR is small compared to 300.
apparent resistance = 300V/current

http://users.ece.gatech.edu/mleach/ece3050/notes/ISources/isources.pdf

see also http://www.vishay.com/docs/70596/70596.pdf