Lifted ground in serial communication?

Question

I have a battery, where the negative terminal is being switched by an NMOS circuit. I also have a solar charge controller, which is controlled by serial interface. The serial interface requires common ground. A microcontroller is connected directly to the battery.

Problem: Current flows to the battery via the serial interface ground loop (and causing lots of smoke, too).

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Question: How can I maintain the serial connection, without creating a ground loop? Can I make galvanic isolation with discrete components?

opto-isolators (optocouplers) should do the trick. – 12Lappie Mar 09 '17 at 13:03 — 12Lappie, Mar 09 '17 at 13:03

score 3 · Accepted Answer · answered Mar 09 '17 at 13:00

Unidirectional links are easy to isolate. Bidirecional links, a little harder.

A serial interface (RXD, TXD) is very easy to isolate: two simple optocouplers, plus assorted resistors, and you're done.

If it is I2C, then the bidirectional DATA wire will need some special care. There are specialized I2C isolator chips for this purpose. Here is an example (I did not read it in detail):

http://www.nxp.com/documents/application_note/AN10364.pdf

Lifted ground in serial communication?

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