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I'm currently handling a project that requires a transformer to step down 230VAC to 6VAC, from there, further scale-down to a voltage that is in the readable range of the microcontroller (as not to damage it due to high voltage input.) The microcontroller will process the read value and display the voltage measured from the AC wall supply, making the whole system function as a voltmeter.

I am thinking of a way to convert the AC sine voltage from the transformer's output to pulsating DC and with the help of 7805 voltage regulator and some capacitors, I'll be able to get a constant 5VDC as the supply for the microcontroller.

The question is: Where will be the pin for the microcontroller/components to connect to as voltage reference/GND? I was thinking that the 'Earth' from the AC wall supply could be used as the common GND but after some practical testing via oscilloscope, the readings are not as expected.

I need some clarification on this.

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    \$\begingroup\$ Would you mind sharing some more details on your oscilloscope testing, such as how you connected the circuit and probe, and the results you got/expected? \$\endgroup\$
    – nanofarad
    Sep 14, 2020 at 14:12
  • \$\begingroup\$ @nanofarad I built a circuit like this: From 230VAC, live and neutral is connected to the primary winding of the transformer, earth is connected to the gold-plated metal casing of the transformer. At the secondary winding, one pin and the other are connected to the top and bottom terminals of a full-wave bridge rectifier respectively. Left terminal of the rectifier is connected to earth (metal casing). And when the right terminal of the rectifier is probed, the oscilloscope shows the full-wave rectified voltage waveform (which looks good!). \$\endgroup\$
    – Shawn Khoo
    Sep 14, 2020 at 14:17
  • \$\begingroup\$ Maybe your question needs some clarification; can you see the two different things: a) supply for the uC: that's the AC sine voltage, rectified and smoothered by diodes, 7805, and the caps; b) the rectified AC voltage that will be feeded to uC's ADC? The two things a) and b) are independent from each other. You could power the uC from a bunch of AA cells. \$\endgroup\$
    – mguima
    Sep 14, 2020 at 14:18
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    \$\begingroup\$ Show a schematic, please; these things are harder to understand when described with only words. The chances of someone help you are greater if you add a schematic of your circuit. \$\endgroup\$
    – mguima
    Sep 14, 2020 at 14:20
  • \$\begingroup\$ @nanofarad But after implementing a voltage divider at the right terminal of the rectifier (in order to further scale-down to a voltage that the microcontroller can handle (reference of the voltage divider is connected to earth)), both the right terminal of the rectifier and the output of the voltage divider gives an output of nearly 0V! \$\endgroup\$
    – Shawn Khoo
    Sep 14, 2020 at 14:21

1 Answer 1

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Common ground or circuit ground is the point you choose as a reference in your circuit, usually designated 0 V.

Earth ground, the "earth" on the AC socket is a physical connection to our planet. You don't need or want to connect the two.

Earth ground can, for instance, be used to "ground" a metallic enclosure that holds, but isn't connected to, your circuit; should, by accident, a high voltage make its way onto your enclosure, this earth grounding prevents you yourself becoming "earth" for a current that could kill you, much like a lightning rod protects a building.

Unfortunately, the same schematic symbol is often used for all kinds of ground.

TL;DR: you don't need to, and usually don't want to, connect common ground to earth ground.

Also, your microcontroller can't measure AC, but that is another matter.

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    \$\begingroup\$ Thanks for the useful explanation! I get it now. I know microcontroller could not measure AC, I'm programming it to function as a peak detector. But that part should be fine for me. Thanks for clearing up my doubts! \$\endgroup\$
    – Shawn Khoo
    Sep 14, 2020 at 14:35

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