Using the example "Blink" sketch only ... With Arduino Uno pins unconnected to any sensors (or anything else) ... And using a "Lab" bench switchmode power supply (up to 40A continuous) set to 11.6 V ... External power supply plugged into the Jack ...
A. The sketch starts perfectly when plugged into USB, and runs as expected. B. The sketch does not start when external power supply is plugged in - but does run perfectly when the reset button is pressed after the external power has been applied.
[u]Attempted Solutions[/u] (for when external power is applied):
Resistor between RX and TX (does not reliably solve the problem) Resistor between RX and ground (does not reliably solve the problem) Diode between Reset and 5V (does not reliably solve the problem).
10uF capacitor between Reset and Ground ... Works faultlessly !!! XD
Being an electrical noob, it took me hours to find this solution,which I adapted after reading the tutorial on ArduinoISP. From my (very neolithic) understanding, the capacitor probably holds the reset pin in reset mode, until the power is stable enough for the CPU to boot up calmly (I will gladly be corrected if I am wrong).
I post this so that other people can find it in the search (it did not appear in search) ... and so that electrically minded folk can set me straight if I have made a mistake.
Regards from Down Under.