Destroy Arduino Uno with Parallel Power via Power Jack (12V) and USB Jack?

I have now two non-working Arduino Uno (one R3 and one R2 - the version without extra I2C pins). :frowning:

All worked fine some hours of testing. I connected the Arduino via Power Jack 12V and parallel per USB. After a dozen of successful uploads the Arduino is working with the last uploaded sketch but tx/rx LED is no more working/lightning up. Also uploading is no more possible. Error message

Binary sketch size: 5,586 bytes (of a 32,256 byte maximum)
avrdude: stk500_getsync(): not in sync: resp=0x00

The Arduino is working after exchanging the ATmega328 . And this is now the second Arduino / MC that I destroyed with this setting; On the Arduino side only two test LEDs with resistors are working and a radar module http://www.produktinfo.conrad.com/datenblaetter/500000-524999/502667-da-01-en-RADAR_MODUL_RAD_MOD.pdf (power via Vin) with open collector and 10K pullup. All is still working. So I do not think some wiring mistakes are the reason. But uploading is not working. Also after restarting Windows and resetting / unplugging the Arduino uploading is still not working.

Do I have to unplug the 12V Power Jack always before uploading a sketch? Could this be the reason for the failure? How can I find out what part of the ATmega328 is destroyed? How can I “reset” the ATmega328–if this is possible without a special programmer hardware?

This page says:

The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically.

i dont know if this will work for u but try ctrl+r and then ctrl+u.

This forces the IDE only to compile and upload. And both I tried already.

JimboZA: This page says:

The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically.

Question would be: Is it a strict "or" and it must be power supply and disconnected USB or vice versa. Or can it be both in parallel without danger for the MC?

Question would be: Is it a strict "or" and it must be power supply and disconnected USB or vice versa. Or can it be both in parallel without danger for the MC?

That's a very good point: I always took it to mean both may be plugged in at the same time and the board chooses one or the other. But now that you point it out, it may mean that one xor the the other may be plugged in, and the board detects which, and that it's not talking about both being present at once.

I've been running my Uno for months on a car charger adapter plugged into my 12V solar power system Every few days I plug my smartphone into the USB port to see how things are going. I'm quite certain that this is harmless to the Uno. I believe that you do have a wiring (or handling) error. You should be able to sort this with a schematic and a voltmeter. Once you find the damaged component you can likely replace it.

Both plugged in worked a while but after uploading a new sketch communication was no more possible. There was no opportunity to upload a new sketch. Something must went wrong while uploading. I do not know what exactly is going on while uploading but it seems that the "uploading mechanism" or something like that is destroyed.

Have you applied a voltmeter to your "12V supply"?

I've seen wallwarts labeled 12V do 18v or more.