Measure multiple voltages



  • I need help with my build. I had what I thought was a great idea but now I'm having doubts & regrets.

    I want to

    1. monitor the utility power (three phase. L1, L2, L3)
    2. monitor solar inverter power (single phase)
    3. monitor solar battery 48+ V
    4. monitor 12V power supply (DC-DC 48=>12V)
    5. send alarms if any are down & return back up
    6. have six green leds on in a dash when all is clear - extra orange and red led for alarm states

    The 240Vs each have a plug and a wall wart/phone charger plugged in.
    I built a thru-hole pcb with 6 LEDs and voltage dividers for each power supply.
    A Rpi Zero + MCP3008 reads the voltage dividers.
    I connected the grounds (neg wires) on the chargers (4x) and hooked it to the pcb. I have leds light up. The 48v and 12v however don't light the leds (their gnd is not connected). The MCP3008 is not reading (the code could be wrong - just did a quick test).

    As I was thinking about this, I got worried.
    Can I connect all the grounds together (4x wall plug, 48V and 12V) ?
    If not, what would be a good way to accomplish what I'm trying to do? I would like to read at least the solar battery voltage (40-57V) beacause that changes. The others can be on/off-type.

    Any help or advice is appreciated!



  • If all grounds are already connected, then you should get along without potential separation. But this is usually not the case. I would do the analog measurement on the high side. A very simple variant would be to measure the voltage with an attiny13 and transfer the measured value serially via an optocoupler to the Rpi. And that for each circuit separately. Then there are no more ground connections.



  • If the grounds are connected: you mean I could connect them. Is it a bad idea?
    When I really look at my goals, measuring is necessary only for the 48V line.
    All the others I can get by with a on/off-info. Optocouplers for 240v data maybe?
    The 48v and 12v probably have common ground considering the dc-dc converter? I'm not familiar with their design.



  • Never connect the grounds of the DC voltages together.
    Everything that does not have the same ground as the power supply of the RPi must be connected via optocouplers.



  • Check. Gotta redesign.
    I'll get optos on the AC-voltages and power the Rpi from the 48/12V ground source.

    Thanks for confirming my doubts.


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