# Voltage Divider or Step-down transformer for voltage sensing 240V AC

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.

1. 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.

My Thoughts

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?

• Why do you want to measure mains to begin with? You have so many questions regarding grounding that indicate it may not be safe for you to measure it. If in the first circuit you accidentally swap neutral and live, you short live to earth. The three resistors are in series, because a single resistor can't handle the mains voltage all by itself. And the circuit will be live. The second circuit is much safer, you can use a mains transformer bought from a store. May 15, 2020 at 20:26
• @Drakejest - Despite Andy closing this question as a duplicate it is not a duplicate. I suggest that you ask the moderators to reopen the question and/or rewrite it to focus on the new material and/or write a new question with the new material in it and a link to the old question. May 15, 2020 at 23:51
• Option 1 is lethally dangerous. It can easily kill you and/or your equipment. It has its place in well controlled conditions but chances are the conditions you have may not be controlled enough. | As Justme touches on - if you reverse phase and neutral (which can happen unexpectedly for various reasons in real life) and something will die if the output is grounded. If the output is not grounded the AINxN is at mains voltage - and something will probably die. It may be you. May 15, 2020 at 23:55
• @RussellMcMahon I always was thought circuit one was very dangerous, but seeing it in a datasheet application makes me second thought if maybe its not that bad, apparently i was right it is really bad ! As you said it place belongs to a very controlled conditions which i doubt i can provide. May 16, 2020 at 5:19

Don't use this circuit:

Why not?

• It connects Neutral to ground, never do that! Although called "Neutral" there can still be a (dangerous) voltage on Neutral. Best case: the ground-fault-protector will trip and that will shut your mains off.

• If you "solve" what I mention above by not connecting ground then your complete circuit could become mains live meaning you get an electric shock when you touch it and everything that is connected to it.

• You cannot rely on the Live and Neutral to always be connected like that, in many countries Live and Neutral connections depend on how you plug in the mains socket. The UK style socket is the exception though. Then still, the socket could be wired in the wrong way.