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We have a system which discharges a capacitor (20 uF) charged to approx. 3kV DC using a spark gap. The principle is simple, when the voltage exceeds a certain value, it jumps the gap (around 0.6 mm) and the energy is discharged through a load.

We are now considering replacing the spark gap with a semiconductor switch to guarantee reliable switching. The system should now sense when a certain voltage has been reached and then safely discharge the capacitor. Could anyone suggest which components and which peripheral circuitry will be best suited for such an application?

More details:
The capacitor discharge time is of the order of 5 to 10 ms. For 3 kV and 20 µF, that works out to an average discharge current of 12A. I don't have the transient values with me though. The said system is intended for an industrial system but in low production volumes.
We need 2 discharges per second for 5 seconds in a row. This cycle repeats itself every 30 seconds.

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    \$\begingroup\$ What peak current is taken by the discharge path and load? \$\endgroup\$
    – Andy aka
    Commented Mar 23, 2023 at 10:59
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    \$\begingroup\$ what load is it discharged into, ie the load current that the switch must handle? Is this a medical, industrial, or just a hobby system, one off or production. This will affect the quality of design you must achieve. 3kV solid state switches are not trivial things, spark gaps are super robust and simple, replacement will not be easy to do. \$\endgroup\$
    – Neil_UK
    Commented Mar 23, 2023 at 11:00
  • \$\begingroup\$ How often do you need to discharge it? Do you need an indication or remote sensing/report of discharge? \$\endgroup\$
    – Edin Fifić
    Commented Mar 24, 2023 at 7:33
  • \$\begingroup\$ Also, what is the charging current of the capacitor? \$\endgroup\$
    – Edin Fifić
    Commented Mar 24, 2023 at 7:49
  • \$\begingroup\$ One more question: how low and how fast do you need to discharge it (what is the highest voltage acceptable on the discharged capacitor)? \$\endgroup\$
    – Edin Fifić
    Commented Mar 24, 2023 at 9:08

1 Answer 1

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The simplest approach is a resistive voltage divider with a comparator and a high-voltage thyristor.
You would keep a spark-gap of 3.5-4kV in place just to prevent the voltage from going too high if the discharge circuit fails.
The schematic is below. I will modify it as necessary, based on your feedback. Unfortunately, the SCR is not "simulateable" in this schematic.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Note that the discharge current can be extremely high, with no impedance between SCR and capacitor; a series inductor (don't forget a snubber!), resistor or both may be desirable. ESR only has the downside that I1 * ESR = residual voltage on the cap. SCR also needs either minimum holding current larger than I1 to turn off automatically, or I1 needs to turn off momentarily. Interestingly, with enough inductance, this (commutation) happens automatically, but C1 is left with some negative voltage (a clamp diode may be used to address that). \$\endgroup\$
    – Tim Williams
    Commented Mar 24, 2023 at 14:01
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    \$\begingroup\$ You're right, I do need those passive elements. I've only drawn this as a basic sketch to give a basic idea, and I expect to improve it with time as I get more feedback. \$\endgroup\$
    – Edin Fifić
    Commented Mar 24, 2023 at 14:24

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