No1 son has a Fitbit ,like most of his friends.
He handed me the USB charge cable last night saying that "it didn't work" - easy to see why, he had pulled the USB plug off the small circuit board inside.
Not repairable but I thought I could at least hook it up for one last use, unfortunately the PCB is "potted" in a hot glue like material (but tougher) and I managed to pull a resistor off the board so no dice. I'd connect the wire ends to the bench supply but am not totally confident that the Fitbit is designed to get the full 5V from the USB (I think it is but it is an expensive mistake to make for the sake of waiting for Amazon to deliver a replacement).
Anyway the reason for the post is I got curious as to what the circuit does - I guess it is just over voltage/over current protection (probably by crowbaring the supply) but I can't figure out what the two devices are
Pics of the board
R1 is missing. Power goes via the via to one side of the polyfuse and then via R1 to the output - I presume that it is for current sense.
I can't quite figure out what the transistors are, one marked XN (2SJ626 ??) and one marked ND4 (??)
Anyone got any ideas?