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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

enter image description here

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

enter image description here

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?

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    \$\begingroup\$ 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. \$\endgroup\$
    – Justme
    May 15 '20 at 20:26
  • \$\begingroup\$ @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. \$\endgroup\$
    – Russell McMahon
    May 15 '20 at 23:51
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    \$\begingroup\$ 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. \$\endgroup\$
    – Russell McMahon
    May 15 '20 at 23:55
  • \$\begingroup\$ @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. \$\endgroup\$
    – DrakeJest
    May 16 '20 at 5:19
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Don't use this circuit:

enter image description here

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.

So forget about this circuit.

Do use this circuit:

enter image description here

The transformer takes care of the mains isolation so the outputs of the circuit are safe to touch.

The ground connection isn't strictly needed, it is your choice to connect that ground to mains ground, a local ground (like a metal case) or just leave it unconnected.

Use a good quality transformer and reliability (chance that the circuit fails) should not be an issue.

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    \$\begingroup\$ Digital multimeter doesn't have a transformer. How do they measure high voltage or current ? \$\endgroup\$
    – Sadat Rafi
    May 15 '20 at 21:39
  • \$\begingroup\$ @SadatRafi great question, how do multi-meters do it? \$\endgroup\$
    – DrakeJest
    May 16 '20 at 5:23
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    \$\begingroup\$ @DrakeJest Multitesters (DMMs etc) use something like circuit one. HOWEVER they meet all the requirements to do it as safely as their certification requires. A DMM has ALL interior components isolated from user contact, with insulation suited to the specified application. They work just as well and just as safely with the leads reversed. They contain a label that advises to ALWAYS unplug the test leads before charging the battery - this is to maintain safety isolation. For mains use meters must have appropriate CAT ratings - AT LEAST CAT2 rated and ideally CAT 3. \$\endgroup\$
    – Russell McMahon
    May 16 '20 at 7:08
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    \$\begingroup\$ @DrakeJest Fluke Multimeter safety guidlines see chart of page 2. || MUCH more \$\endgroup\$
    – Russell McMahon
    May 16 '20 at 7:09

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