Two-ground issue

I've been working on a project which I need to rectify, filter and later lower the input filtered voltage (311V) to 12V, as shown in the picture making use of a flyback converter.

The problem arises at the moment I need to monitor the rectifier+filter voltage which might swing between 240V - 311V and since that would demand taking the input voltage, passing it through a resistor divider and give me a range of 2V-3V whenever the voltage faced the 240V - 311V, I would need the use the same ground provided by the rectifier+filter.

The following figure depicts the current input situation.

Now, the idea I've come up with to counter the need of isolation devices and whatnot is as follows:

By doing that in my understanding, both grounds would face a large impedance and that would serve me well since the comparator op-amp should need to use the +12V output GND 2.

My question is: Is the solution I have proposed a well-suited one given the condition I don't want to use isolation devices? By simply shorting both grounds (GND 1 and GND 2), would that solve the issue too? I don't know how much trouble return current paths I would face if I shorted both grounds, however it seems like the fastest solution.

• Questions to ask yourself: is GND1 appropriate to use as a low voltage earth - if the DC voltage is derived from AC mains then GND1 is presumably AC neutral and low voltage circuitry should not be exposed to it. And why are you opposed to having an isolated supply? Flyback topology using a coupled inductor or a transformer is well suited. If you’re worried about having multiple ground paths then using isolated supplies would seem intuitive.
– Frog
Commented Mar 5, 2022 at 22:32
• There are safety regulations and implications in doing this, but without knowing a lot more about the application and intended use it's hard to comment. Commented Mar 5, 2022 at 22:32
• Is the mains input rectified with a full bridge rectifier? If yes, think hard what voltages the "ground" of the rectified output has compared to e.g. neutral or earth. Commented Mar 5, 2022 at 22:49
• Lots of good comments. (Your proposed opamp will be sinking current towards two different grounds, just looking at the schematic quickly.) I recall that the HPCL-7800 can be used for applications similar to this. But that's old memory with no idea about today's availability or cost. Also, what's what the first schematic showing 100 V to 350 V but your text saying 240 V to 311 V? Quite a difference.
– jonk
Commented Mar 5, 2022 at 23:09
• @Frog I don't want to expose my low-voltage circuit to the AC neutral, which indeed that's where that GND1 comes from. I wish I'd go with a flyback topology, however the point is that a flyback would demand me a bit more money for the supporting circuitry that comes with it, which I'm kinda running out of. So the idea would be to come up with a simpler method like the proposed circuit in the 2nd figure, but I don't know how appropriate that would be. Commented Mar 6, 2022 at 2:15