@Cefiar
I can't try the wire connection at the moment since I'm lacking a soldering iron, but I don't think it will do much. Let me emphasize that the parasitic voltage only appears when heaters are active, and in particular I believe it's due to a fraction of the current coming out of mosfet S terminals taking an unintended route through the ground traces. I've done some more measurements with the following results:
- Turning on the bed heating will cause significant current to flow across the two GND pins next to Vin, in the direction from the RAMPS-FD *into* the Due. I measure 5mV across the header connection.
- Similarly, current will also flow into the Due on the GND pin next to AREF (got 5mV here too).
- There is no measurable voltage/current across the two GND pins next to pins 52 and 53, as well as on the SPI GND pin.
That left a mystery, namely, where is this current going out of the Due. Having exhausted all other explanations, the truth must then be that it's not coming out. At least not back into the RAMPS-FD.
And, behold, disconnected the USB and the mysterious currents are gone. The thermistor-ground-to-due-GND voltage is also gone.
Just to be sure I'm not interpreting the results wrong, I tried just touching the USB plug and the socket, without pushing it in. And things went crazy again, currents and voltages all over the grounds.
EDIT: On my PSU, I connect DC negative to mains earth, as per instructions for my printer (RepRapPro). Presumably on the PC, USB ground (shield) also ends up on mains earth. Therefore the USB ground is just one possible path from the board to the PSU's negative terminal, and some current over the USB is actually expected. I just have no idea these problems are properly solved.
I can't try the wire connection at the moment since I'm lacking a soldering iron, but I don't think it will do much. Let me emphasize that the parasitic voltage only appears when heaters are active, and in particular I believe it's due to a fraction of the current coming out of mosfet S terminals taking an unintended route through the ground traces. I've done some more measurements with the following results:
- Turning on the bed heating will cause significant current to flow across the two GND pins next to Vin, in the direction from the RAMPS-FD *into* the Due. I measure 5mV across the header connection.
- Similarly, current will also flow into the Due on the GND pin next to AREF (got 5mV here too).
- There is no measurable voltage/current across the two GND pins next to pins 52 and 53, as well as on the SPI GND pin.
That left a mystery, namely, where is this current going out of the Due. Having exhausted all other explanations, the truth must then be that it's not coming out. At least not back into the RAMPS-FD.
And, behold, disconnected the USB and the mysterious currents are gone. The thermistor-ground-to-due-GND voltage is also gone.
Just to be sure I'm not interpreting the results wrong, I tried just touching the USB plug and the socket, without pushing it in. And things went crazy again, currents and voltages all over the grounds.
EDIT: On my PSU, I connect DC negative to mains earth, as per instructions for my printer (RepRapPro). Presumably on the PC, USB ground (shield) also ends up on mains earth. Therefore the USB ground is just one possible path from the board to the PSU's negative terminal, and some current over the USB is actually expected. I just have no idea these problems are properly solved.