First, did you try using a different USB port on the PC?-Yes I tried different usb ports and also another pc.The only things the LMV358 is connected to are the "L" led and the MOSFET. It is unlikely that the "L" LED is to blame so we are left with the LMV358 and the MOSFET. If the MOSFET is buggered, it might have taken the 3V3 regulator with it tooSo, here's what I'd do. First I'd make some basic checks. I made the following measurements on a working UNO R3.1. Remove the ATmega328 to eliminate it.-Atmega removed.2. Connect the external supply (12V) but do not plug the USB cable in. Connect digital pin 13 to ground. Pin 1 of the LMV358 should be at ~5V. Pin 3 should be at about 6V and pin 2 at ~3.3V. Pins 5,6 and 7 should be at 0V and the "L" LED should be off.-Ground connected with 13 pin.Results:pin 1= 4.96Vpin 2= 3.27Vpin 3= 5.46Vpin 4= 1mVpin 5= 0Vpin 6= 95mVpin 7= 95mVpin 8= 4.96V"L" Led= off3. Connect digital pin 13 to 5V. Pin 5 should be at ~5V, and pins 6 and 7 should be at ~4.5V and the yellow "L" LED should go on.-13pin connected at 5V.Results:pin 5= 4.96Vpin 6= 4.5Vpin 7= 4.5V"L" led= onIf the results of steps 2 and 3 are okay and the LMV358 does not get hot, then it is at least behaving as expected and is likely okay.-At both steps the LMV358 gets really hot as always.Both the gate and the source of the MOSFET should be at 5V. The drain should not have any appreciable voltage on it. Mine measure ~0.5V.-That's exactly what I measure at mine too.The MOSFET is going to be tough to test in circuit, but with no power applied to the Uno, there should be no dead shorts between any of the pins when measured in either direction with a decent DVM.-Mosfet seems fine with the DVM, as when you measure a good transistor or diode, so it has not dead shorts at any of the pins.If the LMV358 measurements and MOSFET check out, the problem is likely the ATmega328.Anyway, make these checks and let us know the results.
2. Connect the external supply (12V) but do not plug the USB cable in. Connect digital pin 13 to ground. Pin 1 of the LMV358 should be at ~5V. Pin 3 should be at about 6V and pin 2 at ~3.3V. Pins 5,6 and 7 should be at 0V and the "L" LED should be off.-Ground connected with 13 pin.Results:pin 1= 4.96Vpin 2= 3.27Vpin 3= 5.46Vpin 4= 1mVpin 5= 0Vpin 6= 95mVpin 7= 95mVpin 8= 4.96mV"L" Led= off
lmv358 is U5A on the schematic.http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdfThat is the same IC and function as on a Mega IC7. http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdfThere seems to be a rash of those unlucky users. Here is another with the same problemhttp://arduino.cc/forum/index.php/topic,135704.0.html
Certainly possible a 'bad batch' of the op amps got into a arduino production run? I had never heard of this symptom reported here before in the years I've been here.
QuoteCertainly possible a 'bad batch' of the op amps got into a arduino production run? I had never heard of this symptom reported here before in the years I've been here.If it were a bad batch of lmv358's, I think you would be correct, it would be more prevalent, which it isn't. It appears to be limited to two conditions in common:1) usb and Vin poweredThat is not and has never been a problem with the arduino auto-voltage selector circuit. It was designed to be powered from either or both sources attached, that is not the root cause of the problem, there has to be some component failure problem either from manufacture or user caused.2) Vin > 11vAgain that is well within the board's operating limits and specifications and has never been a problem in the past.It's an interesting symptom/failure and so far we have not yet determined if there is a common cause to these two reported 'hot op amp' symptom reports or not. A common symptom does not necessarily mean a common cause, further troubleshooting would need to be performed to determine if that is the case or not. Lefty
1)Power the board from the USB cable only, jumper digital pin 13 to ground. The pins on the LMV328 should read as follows;Pin 1 ~ 0VPin 2 ~ 3.3V Pin 3 ~ 0V Pin 4 ~ 0V Pin 5 ~ 0V Pin 6 ~ 0V Pin 7 ~ 0V Pin 8 ~ 5V 2) Does the 16U2 get hot at all?