Why the LM358 working in this case and what the definition of "ground"

Question

I asked this question previously Why this ZCD not working? and today I saw my friend using this AC voltage sensor module https://mikroelectron.com/Product/ZMPT101B-Single-Phase-AC-Voltage-Sensor-Module/ and the ZCD is working. The LM358 able to from Zero cross detection with the module because the output voltage is in the range of 0-5V, the AC voltage module level shift the input AC voltage to positive value so no phase reversal encounter when using LM358 that power by a 5V(without negative) supply. In this scenario, I'm able to link what I learn in my previous post to the concept of using AC voltage measurement module does not violate common mode input voltage range of LM358 that why it work.

In my previous post, the circuit below not working(getting phase reversal waveform). For this connection my R1 is connected to the 12V output of transformer and R2 connect to 0V display at the transformer

For my other friend, I saw this circuit design working as a zero cross detection without producing any phase reversal using the same transformer as above. However, in this design, the R7 is connected to the 12V output at transformer but R9 and R10 is connected to the ground of a 5V power adapter, R8 is connected to 0V on the transformer.

I have questions below

a)For this connection, is the LM358 input voltage still reading negative voltage?

b)Is the "0V" at the transformer output same as the 0V(ground) at a 5Vdc power adapter?

Does "the "0V" at the transformer output " mean the node where the negative probe of the voltmeter is connected in the second circuit? — devnull, May 27 '22 at 19:57
yes the 0V at transformer output refer to negative probe of the voltmeter. — chuackt, May 27 '22 at 20:09
It goes negative, but not enough to cause phase reversal, similar to the use of a Schottky diode. — Spehro Pefhany, May 27 '22 at 20:40

devnull · Accepted Answer · 2022-05-27T21:07:07.143

a)For this connection, is the LM358 input voltage still reading negative voltage?

Yes. The lowest voltage they see considering all 3 GND connections shown is the negative diode voltage drop divided by the 12k/4k7 voltage dividers:

\$\frac{\left(-0.6\times4.7\right)}{\left(12+4.7\right)} = -168mV\$

b)Is the "0V" at the transformer output same as the 0V(ground) at a 5Vdc power adapter? the 0V at transformer output refer to negative probe of the voltmeter.

No, they are not connected. Depending on the polarity of the AC voltage D1 or D3 (as identified in your circuit) will be conducting. Note how V1 here "floats" (due to the large resistors) regarding the DC voltage reference (the GND points) until the diodes conduct, forcing the relative potential with the diode voltage drops:

From the TI datasheet:

but this note solves the problem:

Thank you very much for the crystal clear explanation . – chuackt May 27 '22 at 21:02 — chuackt, May 27 '22 at 21:02

Why the LM358 working in this case and what the definition of "ground"

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