I2C protection question [HELP!]

nomorebots

Hello there,
I have question about how to set up design I2C between devices for protection if any is necessary.

Say you have three systems (as in the bigger picture in attachments)
System A is something like a Raspberry Pi. System B is battery powered unit like a multimeter. System C is something that uses much more power, say a washing machine. I want to modify the multimeter to measure appliance and use I2C to connect to Raspberry Pi.

If I want to link System A and System B together. Since System B's ground might not have the same potential as System A, (as the picture below, one is at 100V while the other one is at 0) would something get damaged when I hook them up through I2C?

System B's ground will change according to System C's ground. There is a 1V difference between the two. Do I need any protection on the meter pin that is going to connect to the Raspberry's I2C from system B?

I've found the method with a diode and a resistor (as in the smaller picture) but I am assuming this works during data transfer (when the ground is already equalized)

I've heard mixed answers from friends, some say the above method is good enough. Some said a optical-cuplor is necessary. Some of the I2C isolation IC looks like the method above to me but its pricy. Since PCB space is extremely limited, something that is small and cheap is the best.

Please help!

kwinn

I would definitely use an optoisolator/optocoupler between the RPi and the other systems. The diode and resistor on the Pi end is totally inadequate. On top of that the voltage on system B could kill someone, and the voltages on both B and C could fry the RPi if there was a problem.

If space is a problem the isolation circuit could be part of the cable or B/C.

Mark_T

Sounds like you need to find a multimeter with an opto-isolated USB serial interface.

nomorebots

I see guys, thanks for the feed back. Will look into opto-isolator or possibly use BT to transmit data...