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Revision to USB ground loops

David Crocker

 
* The USB cable and mains ground connections form an alternative path for current to flow between the Duet and the negative power supply output. If the connection between the negative terminal of the Duet's VIN terminal block and the negative output from the power supply is weak, then this alternative path will be used. The USB cable will become hot and you risk damaging the USB ports on the PC and the Duet.
* If the ground connections of the mains supplies otof the Duet and the PC are at different potentialspotentials, for example because of a mains fault or because other equipment connected to the mains supply has significant ground current, a large current will flow through the USB cable ground wire.
* If the ground connections of the mains supplies otof the Duet and the PC are at different potentialspotentials, for example because of a mains fault or because other equipment connected to the mains supply has significant ground current, a large current will flow through the USB cable ground wire.
* Other devices powered from the mains may create ground transients. These will flow through the USB cable and may cause the Duet to reset or perform abnormally in other ways.
 
# Connect the Duet via USB to a laptop running off batteries, with the charger not connected.
# Use a USB isolator module such as this one https://www.adafruit.com/product/2107 or this one https://uk.farnell.com/olimex/usb-iso/usb-isolator-1000vdc-for-pc-laptop/dp/1795095.
# Disconnect the power supply negative output from mains ground. '''''ForFor safety reasons, this is not recommended'''.''recommended'''.
# Disconnect the power supply negative output from mains ground. '''''ForFor safety reasons, this is not recommended'''.''recommended'''.

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