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I recently purchased an Arduino Uno. I've been having a ton of fun fiddling with it -- that is until I pushed +12V to the Vin without any kind of voltage step-down. The smoke was violently released from the Atmega chip and the chip socket itself even melted somewhat.

I purchased a replacement ATMega328P-PU for the unit which was pre-loaded with the Arduino firmware, however when I energize the board from the USB port the status LED stays a solid orange rather than the fast-3-blinks that indicates the firmware is ready. I'm not sure if this is a case of "the chip really wasn't loaded with the Uno bootblock" or if there's another component on my Arduino board that is fried.

I have access to a multimeter and a 10MHz oscilloscope, can anyone give me some ideas on how I can diagnose where the fault may be?

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    \$\begingroup\$ Arduino Uno R3 Schematic. arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf And if you put 12v to the VIN pin/socket, it shouldn't have blown anything. The on board regulator can take 12v just fine, maybe run a little hot, but not let out the smoke. Are you sure it was 12v? Or The VIN pin? No wire shorting things out? \$\endgroup\$ – Passerby Jul 25 '13 at 18:16
  • \$\begingroup\$ I attached a 4x1.5V AA battery holder to Vin and the ground pin next to the Vin pin and it definitely let out the smoke. \$\endgroup\$ – Peter Grace Jul 25 '13 at 19:18
  • \$\begingroup\$ That's only six volts (maybe 6.8v with brand new alkalines) \$\endgroup\$ – Chris Stratton Jul 26 '13 at 2:53
  • \$\begingroup\$ Whoops, I meant 8 cells. 8 cell AA battery holder from Radio Shack, and the voltmeter actually shows 16 volts, which is surprising. So much for 12V. \$\endgroup\$ – Peter Grace Jul 26 '13 at 15:32
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Don't even think about plugging in the new chip until you verify all the power supplies are correct. You haven't shown a schematic of this board, but it is quite possible that overvoltage took out more than just the processor chip. If the power supplies aren't right, the next chip could get fried by the bad power supply.

Use the scope to look at the voltage on every pin without the chip installed. Not only make sure the voltage is correct, but make sure that the power voltages are reasonably clean, not oscillating wildly for example. A voltmeter will only show the average, so it's of little use for debugging stuff like this.

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    \$\begingroup\$ I made a new user mistake and put the ATMega chip in the opposite way. After following your suggestion, everything appeared fine so I turned the chip around and voila, it now works. \$\endgroup\$ – Peter Grace Jul 25 '13 at 19:28

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