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I would like to create a safe electrical circuit that measures the voltage from the wall (220vac). And the safest way that i know of so that when something fails there would be a high chance that my electronics could still survive is by using a step down transformer.

The circuit i have came up with looks like this

enter image description here

I have several problems though:

  1. Im having trouble looking for a transformer, since its for voltage sensing only i dont really need highoutput current, but most important of all it must be as small as possible so that i can place it on my pcb. I even opted for smd only to ensure that there will be no arc going onto another pcb on the bottom. But even with that there seems to be a lot type of transformers and dont know which one will work. Here are the three types that i think will likely to work. SMPS, Pulse, Current.

    The SMPS seems to be the most likely i will be using having high isolation and its actually stated that it can handle 220VAC. I am having second thoughts on the other tweo because some of them does not say on the datasheet the max voltage the input can handle.

  2. For the value of the voltage divider, to get the desired output voltage that my differential ADC can handle. getting the ratio of the resistor is simple. But i am not sure what the values should be should i go higher or lower? In the circuit above i used 1k and 0.5k but i can also be 10k and 5k. which would be better?

  3. A gut feeling is telling me that my circuit seems to be only good on paper, and in real world there would be more components,? what should i add to this circuit to make more like a proper circuit that can survive in the real world. Heck how do industrial grade equipment do their single phase voltage sensing.

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    \$\begingroup\$ The SMPS link takes you to a page where there are several different types hence, you need to specifically state which model number. In fact, now that I've seen that page in more detail, it's clear that none of them can handle 220 volt AC unless it's at several tens of kHz i.e. they'll detonate at 50 or 60 Hz. \$\endgroup\$
    – Andy aka
    Commented May 12, 2020 at 13:22
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    \$\begingroup\$ For a 50/60Hz mains transformer, about the lowest power you'll find is 3W, and that'll be something like an inch cube. \$\endgroup\$
    – user16324
    Commented May 12, 2020 at 13:30
  • \$\begingroup\$ If you need any decent amount of accuracy and are planning to use an ADC to look at the voltage then I would consider my answer here. \$\endgroup\$
    – Andy aka
    Commented May 12, 2020 at 13:37
  • \$\begingroup\$ @Andyaka oh so transformers must be operated at their frequency range, Thank you that would be a great reference, is the 24 bit ADC overkill? 5volts at 24bits is 0.000000298 thats a lot of zeros, 16bit is 0.0000763 which i think is still okay, I do not have a frame of reference so i cant tell. Before i get to that i might have a problem looking for a transformer. \$\endgroup\$
    – DrakeJest
    Commented May 12, 2020 at 13:50
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    \$\begingroup\$ DrakeJest, You mean 25mm x 35mm? \$\endgroup\$
    – vu2nan
    Commented May 13, 2020 at 13:34

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