Voltage Trigger

If you can sacrifice a tiny current from the capacitor, you could make a voltage divider with two resistors that scales the capacitor voltage down to something more practical to react to and compare that to a voltage reference somewhere around 5 to 15 volts with a comparator. This assumes you have a low voltage supply available for this comparator to be powered by.

For instance, you could connect a 10 meg resistor (rated for

1000 volts) in series with a 127k resistor. When the total voltage reaches 800 volts, the pair will be consuming 79 uA and the drop across the 127k will be about 10 volts.

An integrated voltage reference like the LM4041

Here is a dual comparator that might be used to tell you when the divider output exceeds the reference voltage:

Using the output to fire the SCR is a separate problem to solve, that depends on where the SCR is in the output circuit.

maybe use a PUT instead.

ONSemi has an excellent thyristor document "HBD855/D"

Bye. Jasen

Thanks guys. You've given me enough to keep me busy for a couple of days. Thanks again.

There is also a way to use the LM4041 as the comparator, as well as the reference. Divide the high voltage with two resistors, so that the divider voltage just reaches 1.2 volts when the high voltage reaches your target. Connect that divider output to the reference pin of the LM4041, with the anode tied to zero volts. As the divider voltage rises through 1.2 volts, the cathode current will rise very rapidly fro, somethhing like 40 uA to a much larger current limited to about 10 mA, with a saturation voltage of about 1.5 volts. You may be able to figure out how to trigger your SCR off that current swing.