Hello I am trying to measure 240v mains AC as safely as possible. There seems to be 2 ways to do it, which of the two is better and preferred?
1. Voltage divider directly from the mains
I have found an schematic that seems to be form a data sheet of an adc where in the mains is directly voltage divided and connected to the adc
Rhi is 300k Ohm and Rlo is 750 ohm . I dont know why the 3 300kOhm are in series though. I do not know how to pick a appropriate varistor though i assume a 375 peak to peak voltage , would this be correct? Also is that earth or ground (adc ground) where neutral is connected because they sometime interchange the two and it becomes confusing in these situations.
- Voltage divider After a transformer
This approach seems to be the safer one but not a space saving one using a very small transformer like this that can output 5v and voltage divide from that
Is the ground necessary? I only added it because the other circuit added one.
The reason i like number one is because it can be made much more compact than the second, I cannot decide if is it worth the protection of the entire circuit for a compact size. How likely is circuit 1 more to fail than circuit 2?