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Topic: Problem with lmv358 and usb at UNO (Read 2 times) previous topic - next topic

BillO

#5
Dec 14, 2012, 03:14 pm Last Edit: Dec 14, 2012, 03:22 pm by BillO Reason: 1
The only thing I can think of then is that the LMV385 has some internal issue.  It seems to b functioning as is required in the Arduino, but is getting hot.  Either it is consuming a lot of current, or something attached to it is consuming a lot of current.

As far as external things go... Since the LED goes off with digital pin 13 grounded, then the problem does not lie on that output.  You say the FET measures "okay" in circuit, but that does not mean it is behaving properly when power is applied, and it is the only other external device that could be causing the problem.

So, given that the LMV358 appears to be working as expected, I'd say replace the FET as a first attempt.  Before you do that check 2 more things, again with the 328 out of the board:

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 ~ 0V

  • Pin 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?


Edit:  BTW, your measurements of the NCP1117 are what you should see.  It appears to be working fine.
Facts just don't care if you ignore them.

SurferTim

lmv358 is U5A on the schematic.
http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
That is the same IC and function as on a Mega IC7.
http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf

There seems to be a rash of those unlucky users. Here is another with the same problem
http://arduino.cc/forum/index.php/topic,135704.0.html

retrolefty


lmv358 is U5A on the schematic.
http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
That is the same IC and function as on a Mega IC7.
http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf

There seems to be a rash of those unlucky users. Here is another with the same problem
http://arduino.cc/forum/index.php/topic,135704.0.html



I was going to offer that this is the second report of a hot op amp on an arduino board recently. 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.

Lefty

SurferTim

Quote
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.

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 powered
2) Vin > 11v

retrolefty


Quote
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.

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 powered

That 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 > 11v

Again 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



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