I was planning to use a 317-adj to power the
arduino and an external cct. It worked until I connected the arduino to USB without the 317 being powered externally.
Next time I tried to power it from the 317 it was blown and passed the full ext voltage unregulated.
[EDIT: Arduino was NOT connected to the 317 output but it's input. So I'm wondering what blew it out]
Looking closer at the arduino I see that the 5v regulator has 47uF on the input which in effect is then on the output of the 317. In this case the 317 needs a by pass diode to discharge the cap. Since I had only put 10uF on the 317 (which does not require the diode) I had not used a diode.
This would account for the problem but before blowing any more 317s I wanted to check the rest of that part of the cct. but can't find out what the other power supply chip is labelled DC21MMX .
Most google results seem only to refer back to Arduino.
What is this chip?