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Help to find why NUCLEO boards are getting burnt

Associate III

First of all the board that we use is "NUCLEO-L552ZE". The upper section of the board is the ST-Link which we use to program and debug the chip. The ST-Link also forwards one of the UARTs (LPUART1) over USB. So we conveniently use this feature to test the software.

However on the final product there is no ST-Link available to us (it's custom designed board). We're going to use ST programmer v3 (which is a ST-Link) for the same purpose as above.

For simulating what we have on final board I changed the JP6 on the Nucleo board and set it to 5V_EXT. I connected the 5V (current limited to 500 mA on power supply and SB1 is disconnected) to the 6th pin of CN11 and GND to 8th pin of CN11. I couldn't send and receive anything to UART (over USB). So then I decided to connect the USB cable directly to the ST-Link on the Nucleo board to check the UART on that. Although I hadn't changed anything on the board (specially JP6 jumper and the 5V_EXT pin) the board appeared to get burnt. According to the schematic the current shouldn't damage anything (unless I overlook something).

Here are two thermal pictures of the NUCLEO-L552ZE board: 

with ST-Link:


with 5V_EXT:



After this experiment I tried to do the same with another board. This time the board was "NUCLEO-F401RE". However the board got burnt again right after I connected the rx and tx cable of ST programmer v3 to UART.

Here is a thermal picture of NUCLEO-F401RE:


 Has anybody an idea of what's causing damage to the boards?

Chief II

Show schematics what you connect where, your description isnt clear. But quick result is you dont connect GND as first. And JP6 on the Nucleo board and set it to 5V_EXT change nothing, Nucleo is powered.