I have fried two blue Pro Micro clones now. They are 5V/16MHz models which came with SJ1 open so I naively shorted out the SJ1 jumper with a solder bridge to match what the schematic notes state - that SJ1 should be closed for 5V and open for 3.3V. They worked fine on USB power but when I connected +12VDC to RAW while leaving USB connected, the USB host computer shut down the USB port and the Pro Micro boards no longer work: when connecting just USB, the green power LED turns on for a few seconds then fades out (since the polyfuse is in overcurrent mode and shuts down). One reads 4.4 ohms from VCC to GND and the other 6.6 ohms so it seems that the Atmel chip is blown.
This doesn’t make sense to me. There is a protection diode between RAW and UVCC which prevents over-voltage going out the USB. Why are the boards damaged now? Even with SJ1 bridged, the worst case is feeding back the 5V from the regulator output through the polyfuse into the USB power pin which shouldn’t damage either the USB port nor the Pro Micro which is still isolated from RAW by the diode.
I’ve got a few more on order and don’t want to fry them as well. What have I missed?