Author Topic: safe connection: CR over USB + USB Debugger  (Read 148 times)

str

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safe connection: CR over USB + USB Debugger
« on: February 27, 2020, 05:02:40 AM »
Hello everyone.
I'm sorry for a potentially noob question. I've read the safety precautions but still not sure if the setup I want to use is "illegal"

I want to use CR in the testbench which would program the MCU over JTAG and then measure sleep current. I want to transfer data over USB, noise is not a problem as I measure average over a long period of time. DUT is powered by 3V battery, there's no power supplied by the debugger (only data+ground), however I have to connect debugger's GND to DUT's "-" terminal, otherwise debugger would have no reference. Here is a quick sketch of the setup. As far as I understood from the manual, this setup is NOT safe.

Quote
It is NOT OK...to measure a DUT that is mains/earth powered/connected, while USB is also connected. This is passing current through the CR, from the input terminal, to the USB earth/ground, and will completely damage the CR

But looking at the CurrentRanger schematic, I can't understand why (where the short circuit would be).
If it IS indeed dangerous, would USB isolator help? Should I use 2 of them on both USB connections (to CR and Programmer)?
Thanks a lot.




Felix

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Re: safe connection: CR over USB + USB Debugger
« Reply #1 on: February 27, 2020, 10:27:44 AM »
The reason I wrote the guide and tried to be very conservative in assumptions is because I don't know all the possible ways to connect the CR in all kinds of setups and test scenarios.
It all comes down to avoiding running current/ground loops through the CR, and this usually means keeping it away from 2 different voltage potentials (oximoron!) connected at 2 different CR inputs/outputs.
To stay safe, use USB isolators for debugger and another one for USB logging.
I am considering carrying the USB isolators that I tested, maybe of interest to some since they are very affordable and should be convenient to bundle along with the CR.