I've made this simple circuit to read the line voltage (the 1 pin is conected in a uC by a resistor):

Line Voltage read

In the image NEUTRO is Mains Neutral AC, FASE is Mains Line AC and TENSÃO is the signal to the uC (microcontroller).

The circuit worked very wel until a connecting with the PC's serial connector has been made with de eqp. This connection had increased the signal readed (1 pin), making a measure error by 30Vrms in the uC.

When i was probing and investigating what would be the problem, i noticed that in the PC everything is connected with earth! USB shield and GND pin, GND of audio conector, GND signal tracks in the motherboard, shield and GND of the serial conector, everything. I've tested 3 PCs and all of them is the same. Searching in the internet i have found conflicted opinions about the union between Signal GND and Earth.


  • What is better to stop the circuit functioning variation? Make the union once of all?

  • If this represent's users risks why in the PC everithing is Earthed?

  • And finally what kind of problem this connection could cause? Could expose the circuit to line noise for example?

  • Any other thing to concern?

Thank you!

  • \$\begingroup\$ You haven't identified what your mains is, but almost certainly earth and neutral are connected at some point in your PDN. \$\endgroup\$
    – sstobbe
    Jul 5, 2017 at 17:32
  • \$\begingroup\$ Yeah, that it! Sorry. \$\endgroup\$
    – Henrique
    Jul 5, 2017 at 17:37
  • 2
    \$\begingroup\$ "Any other thing to concern?" YES. Your circuit has no galvanic isolation, a fault in your circuit could mean full 230 VAC (or 115) where you least expect it. \$\endgroup\$
    – pipe
    Jul 5, 2017 at 18:15

1 Answer 1


What are you doing with your resistor values? You need matching to obtain minimal common mode errors (such as when neutral becomes connected to a different point or ground): -

enter image description here

Study the above. R1 and R2 and R3 = R4 and R5 and R6. Even C1 = C2 and importantly R7 and R8 are equal.

If you got rid of your R12 (s/c) and made R1 and R9 each 16k4 (twice 8k2) then it would work better. You also need to ensure that your R11 and R14 are rated for the incoming supply voltage maximum peak voltage. That is why you see three resistors in series in the reference design I showed.

  • \$\begingroup\$ Thanks @Andy. I already removed the R12. and i'll test more values of R1 and R4. But where you got this values? \$\endgroup\$
    – Henrique
    Jul 5, 2017 at 17:35
  • 1
    \$\begingroup\$ @Rique I meant 16k4 and that is twice 8k2. I amended my error. \$\endgroup\$
    – Andy aka
    Jul 5, 2017 at 17:40

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