# High voltage limiter with discrete components

I'm designing high-voltage enclosure circuitry that engages a lock when either of two HV supplies reach 50V or greater. Both are capable of 6kV output. Below is what I have so far, where R7 represents the lock:

The two similar blocks form comparators against a 5V reference, and Q3 and Q6 form an AND gate. Q9 drives the output.

I've had issues with op-amps and logic ICs burning up with previous designs, so all components are discrete.

This design works well in simulation so far, but I have only yet cranked up the sources to 60V. In reality, a 100:1 resistive divider (10M:100k) will be used to yield a 0-60V range, or thereabouts (200:1 for 0-30V, perhaps).

The issue is that, at 50V, the voltage at the comparator input is too low to overcome Q1's or Q4's $V_{be}$. I thought about adding a common-emitter amp with a gain of 10 at the inputs that just saturate a bit above this 50V mark, but that would also require another inverting amp following it.

Is there a simple way to go about this that I'm not thinking of? Or is a change in configuration needed?

• I'd probably try to figure out why the op amps / logic ICs burned out before and then correct the issue, the circuit would be a lot simpler without having to homebrew it with transistors. Failing that, could you decrease the divider so the input voltage goes up, and then shunt the excess voltage via a zener+resistor to ground? Not posting as an answer because I'm not sure I understand the issue enough – Flyingfirepig Mar 29 '18 at 19:17
• @Flyingfirepig The burning-out issue is due to the DUT's gnarly EMI/EMC tossing around the housekeeping supplies like packing peanuts. The discrete BJTs have proven to be much more robust, so that's the game plan. I thought about the Zener, too, but if I want a 5V signal from 50V on the supply I'd be dealing with up to 600V from 6kV. I figured I'm overlooking something, but that may be the way to go I suppose. – calcium3000 Mar 29 '18 at 19:25