I asked a previous question about running circuitry off a high voltage supply with unknown ground potential. The question is here: Generating a 10 volt rail from a High Voltage Supply with unknown Ground Potential
Based on the excellent answers, I realized I may have been making it a bit hard on myself. I have a new question:
Let's assume we have a high voltage source as a large capacitor bank at 200 volts. I have a 12 volt battery I want to use to run an NE555 and a mosfet that is switching the 200 volt circuit.
But let's make it more complicated by assuming I can't guarantee that the 200 volts across the cap is +200 and 0. It might be +100 and -100 (or even +10 and -190), whereas my battery might be +12 and 0 relatively speaking. I need to guarantee that I have 12 volts between the gate and source of mosfet so as to not damage it. The question is: If I simply connect the 12 volt battery right into the circuit, will its low potential automatically match up (Float?) to the low potential on the high voltage circuit, thus guaranteeing a 12 volt difference to between the Gate and Source of the mosfet?
And for a more advanced system: What if the 12 volts isn't even supplied by a battery? What if it's just another capacitor bank?
And for a really complicated system: What if I use the 12 volt and GRND rails from a Switched Mode Power Supply like a homebrew ATX power supply? I tried something like that a long time ago. But I was using rectified power from the 120v 60Hz Mains and filtering it into a capacitor. Then I attempted to share the ground rail with an ATX power supply and I ended up destroying the power supply (smokes and sparks!). I never did find out why, but it might have had something to do with the fact that my SMPS and main circuit voltage were both sharing the mains... thoughts?