Did I fry the serial communication by powering Vin from the USB port?

Everything seems to work except serial communication.

  • Arduino MEGA 2560 R2
  • Serial communication is broken (tried different USB cables and computers, and things were working fine previously)
  • RX and TX lights stay off no matter what I do. Loopback test predictably doesn't work either.
  • Resetting makes the L led blink twice, and it loads the last program just fine

I had an external 5v regulator feeding 5v to the Vin pin (below recommended voltage but seemed to work fine). This 5v regulator was also powering a few servos, which meant that current could flow from USB -> Vin -> servos, if the external 5v regulator was not powered (my mistake for not putting in protection diodes). The current coming out was probably > 0.7 amp peak.

This page: 10 Ways to Destroy an Arduino — Rugged CircuitsRugged Industrial Arduino Microcontrollers , method #8, says that that drawing too much from Vin when powered via 5v can destroy the onboard regulator by making current flow backwards. Pin #1 on the 1117 regulator is at 0 volts, which I think is wrong (compared it to a redboard).

One chip is getting REALLY hot, the one with 8 pins which I think is an op amp, in the picture below. I had also been sloppy with thermal paste (10% silver) so there are some flakes sitting around, but those didn't cause any problems for a long time and this only happened when I was fooling around with the power supply.

I just want to confirm my mistake before I possibly destroy another board. Thanks in advance.

Pin #1 on onboard regulator is GND - 0V on this pin is good (see datasheet for NCP1117 - #2 is output and #3 is input from left to right as can be seen on photo). The chip which is get really hot is OpAmp - could be broken or only overloaded from some other reason. Regulator has overcurrent protection an peak current can excess 1A. Clean the board from thermal paste at the first carefully to prevent short circuits. Let's continue in investigation...

It's been a year or two, but there were several people reporting that op-amp getting really hot. I suspect that there was a bad or marginal batch of op-amps that got into the manufacture run at one time. There is no way that op-amp should draw enough current to get hot other then just being a defective component.

And how did they solved this, and which Arduino types?

And how did they solved this, and which Arduino types?

Like I said it was a while back and I'm not sure if it was limited to just Uno boards or Mega boards or both. Solved by exchanging under warranty if contacted I assume. I seem to recall one person posted a picture that showed the op-amp case split open.

Why did you connect 5V to Vin and not to 5V?
I would connect to 5V, add a diode from 5V (anode) to Vin (cathode) to avoid reverse driving the regulator, See page 10:

and use a seperate source of 5V for the servos. 1A surges to get servo to move or change direction can play havoc on same 5V going to logic devices.
Connect all GNDs together.

Thanks for the help everyone.

The regulator pins are 0v, 4.73v, 4.25v when plugged to USB, and 0v, 5v, 8.3v when plugged to a 9v supply. I'm guessing that is correct, and the problem is the OpAmp. Here is an old topic about it: Arduino UNO only supplys 3.9?V - General Electronics - Arduino Forum . I got this arduino in that time frame (January 2012), but I think it was my fault for running the current the wrong way through the regulator and op amp and out of Vin.

The arduino still works so I might try getting a programmer for it, and I could also replace the 5v regulator if that seems worth a try.

This is going to be connected to a computer all the time (it's doing CNC stuff), so I decided to use USB to power the arduino and logic, and have a completely separate supply for the 12v and 5v stuff. So far no problems and found I didn't really need all the pins on the MEGA.