I've created aluminum fixtures for VFD tubes, which means that many small wires running voltages up to 70 V are inside these metal tubes. I can connect these metal fixtures to GND, but maybe a better idea (since the fixtures are individually isolated) is to connect them to the voltage comparator of the AVR8 processor, so the software can detect when one of the wires inside the fixtures is leaking power to the fixture (i.e. shorted) and then shut down the 70 V power module through the enable signal of the module.

I've found some questions/answers/ideas based on a voltage divider, possibly including a Zener diode as well. But I don't need linearity, just a signal when there is a positive voltage of any kind on the fixture.

So maybe this could be as simple as a single transistor with a pull-up/-down? E.g. the pull-up to 5 V is activated when the base receives an input higher than 0.7 V?

And is there a way (using diodes?) to keep the individual fixtures isolated from each other, instead of connecting all fixtures together and then connect the whole to the comparator?

Here is my reasoning for this setup:
Since there are about a dozen metal fixtures, all isolated from the rest of the work and from each other, a single broken (shorted) wire within a fixture (as long as it's not GND) can be detected and signaled this way (a positive voltage), without doing any damage to the rest of the board. If I connect the fixture to GND as suggested, a short results in a current that may very likely damage the VFD driver chips before blowing the fuse.
Making any (more) sense?

@bryan: any hints on what such a separate circuit should look like? E.g. any dedicated IC's available for such a feature?

  • \$\begingroup\$ I must point out that you’re building a safety circuit with the microcontroller “in the loop” - code failures could leave high voltages exposed on the chassis. Consider a different approach where a simple circuit detects chassis voltages, latches in the off position and alerts the micro to the failure. \$\endgroup\$
    – Bryan
    Sep 11, 2022 at 14:53
  • \$\begingroup\$ Surely it would be easier to notice the extremely low load resistance on said 70V lines, than getting software involved? \$\endgroup\$ Sep 11, 2022 at 23:08
  • \$\begingroup\$ Instead of an active circuit to detect a probable failure of weak wires, design a wiring with permanent isolation. Keep things simple. Tim's hint is also valuable, for example ground the fixtures and use a fuse. Look at designs for lamps with metal parts driven by mains. and how they are made safe. \$\endgroup\$ Sep 12, 2022 at 6:07


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