I bought 2 Arduino Nano 33 BLE Sense.I connected 7.4V to Vin(Pin15) and negative electrode to GND（Pin14）.The board works.However，when I connect 7.4V to Vin（Pin15） but negative electrode to GND（Pin19），the LED of the board could not blink.When I connected the board to computer by Micro- USB,it became hot!The same happened in the other board.Is there any difference between Pin14 and Pin 19?How can I fix them?Thanks for your reply!
GND pins are electrically the same in this case.
On 4-layered boards, often GND is separated to Digital and Analog - to reduce A/D noise.
My recommendation is to connect both GND to a common power ground connection; thus lowering the resistance of any single ground connection and improving the overall grounding performance.
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Thank you very much！Could you teach me h
ow to fix them？
There is no fix, replace them.
This unfortunately suggests a "broken" board.
Here is the USB power-supply on the board:
The regulator IC outputs 3.3 Volts. This is most likely the failed part and the one getting hot.
BUT the microcontroller too may be damaged.
Generally speaking, these parts are not user replaceable.
If you have a 2-cell battery pack, 3.0 Volts, you can test the microcontroller by wiring 3.0 Volts to the 3.3 V input and battery negative (-) to GND. If you had the blink sketch loaded you can observe the LED. CAUTION: the components on the board may still become hot and capable of inflicting a nasty burn.
If you succeed in getting the LED to blink, a "maker place" or friend with electronic knowledge may be able to repair the nano but skill is required for success.
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