So I was connecting up a circuit and I mis-read a data sheet. I connected the V+ on a breakout board to the 5V on the power header of the Netduino Plus 2. It turns out that the V+ did not require 5V power but was actually supplying 12VDC.
There was a strange noise from the computer PSU that was supplying the breakout board but no smoke, I had it connected for maybe 2 seconds max. But as soon as I made this connection with the breakout everything went dark on the Netduino board.
I did not realize what happened until I started to re-read all the data sheets more carefully and discovered the 12V connection error on my part. I thought I double checked all my connections and they should have been fine, but I guess not.
Now I broke out the multi-meter and started to assess the damage. Here is what I have found (referring to schematic part numbers):
- When plugging in the USB connector the white D2 power LED will light up. This indicates to me that the U3 comparator is still working to drive Q4 to turn on the +5.0V power feed through.
- There is no 5V power on the power header so CTRL_OF_PWR_HEADERS is not being enabled, which is driven by PB2/BOOT1 off the STM32 (which makes sense since the 3.3V power is no longer functioning as per below:)
- There is also no 3.3V power on the power header
- I have a 2 line LCD display connected and of course it does not power up since there is no power available on the headers
- I checked the output of U2, the 5V to 3.3V buck regulator and there is no voltage coming out of it, but there is 5V going into it. Looking at the data sheet it says there is a max input of 5.5VDC so I would bet that this got taken out for sure.
- Looking through the schematic it looks like all the 3.3V is connected to the output of U2, since it appears to be hooped I connected 3.3V directly to the power header. Due to the way the Q5A FET is connected it allows power to back feed past it into the 3.3V bus on the board. Using this power connection the LCD (which is 3.3V) would light up and the last program I flashed indeed does write to the LCD as expected!!!!
I am not sure if anyone else has had such an unfortunate event but it looks like if I replace the U2 5 to 3.3V regulator there might be a chance that the Netduino will work again? It looks like there might be some way to salvage this board although I will be ordering another.
The only other part to get stressed was the BOB-12009 logic level converter from SparkFun, but the data sheet for the FET they use on the board says it is good for 50V Drain to Source so I am thinking it should be alright (but those are cheap so not a big deal)
I did not try use the external 3.3VDC power and connect the USB plug at the same time. As far as I can see since the 3.3V regulator is dead this should not be a problem since the output of it is no longer functioning. I was using another board to power it so if the 3.3V output internally failed as a short circuit to ground it would have taken the external 3.3V power supply down (not rated for much current). Since this did not happen I am suspecting that the 3.3V regulator failed open so there should be isolation between the 3.3V and 5V busses.
So I have two questions:
- Based on the above should replacing the 3.3V regulator potentially solve the error?
- Would it be relatively safe to connect the external 3.3V supply and 5V USB connection to see if I can still program it? I am not sure if there would be any ground loop issues or other unforeseen things that could cause further damage?
I was expecting the worst and was totally surprised to see the text show up on the LCD with the 3.3V directly connected, if all this requires to fix is replacing the regulator IC I would be very happy.
