Thanks FvM,
That looks good, and we would like to connect the pin 5 (negative supply) of the INA826 to earth ground as we are not interested in the negative going peak. We would then connect live to P1, neutral to P2, and add a 1N4148 diode across R303.
(though looking at the INA826 datasheet it appears that the “V-“ pin can be connected to a voltage 40V below the voltage at “V-“, which is impressive, so we wouldn’t really need the diode across R303.)
Also, I like the 0.1% resistor accuracy, but in our rough-and-ready case, I think we would just use 1% resistors throughout as we will calibrate it…ie look at what output voltage the INA826 gives at 240VAC and then we would know from that whether the mains had gone up or down thereafter, and roughly by how much to the nearest 2V or so.
We are indeed not worried about leakage current to earth, and I think we will make the divider 1MEG/10K instead of 10MEG/1K….since I believe it makes it slightly less noise prone….and we are never going to sell this product, its purely for in-house use.
We will have to remember not to feed the INA826 inputs with the output of one of our 1kVA mains isolation transformers. (we would be unlikely to do this anyway as we need that for other uses)
Thanks Again