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?