Safe to connect both VIN and a USB connector?

Hi,

I had an Arduino Nano v3 hooked up to an LCD display and a servo driver from RobotShop. I was powering the whole arrangement using a 12V DC supply that fed into both the Arduino's VIN and the input to the servo driver. This is all on a printed PCB.

Everything has been working just fine, but after a few a days I plugged the Arduino into my PC to modify the code it's running. Both the DC input and the USB input were in place simultaneously. I've done a similar thing before with an Arduino Mego and a 9V supply to no ill effect. This time, however, a capacitor near the Arduino's voltage regulator seems to have burnt out and also a resistor on the LCD display started smoking. The LCD display remains functional but the Arduino does not.

Does anyone know if connecting both USB and an external power supply is known to be problematic? If not, then I may have a separate problem with this circuit.

Thanks!

Were all the GNDs connected together?

Yes, there's a ground plane that links them all together.

It is a problem if the chip isn’t designed for it. However, if you have an official nano, then it in theory should be supported. Quoting from: http://arduino.cc/en/Main/ArduinoBoardNano.

The Arduino Nano can be powered via the Mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source. The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.

MichaelMeissner:
It is a problem if the chip isn’t designed for it. However, if you have an official nano, then it in theory should be supported. Quoting from: http://arduino.cc/en/Main/ArduinoBoardNano.

The Arduino Nano can be powered via the Mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source. The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.

There is a discrepancy between what the product description page says and what the actual schematic shows for the nano 3.0. Until proved otherwise I will assume that the schematic is correct, unless someone has other proof to share?

If you look closely at the nano 3.0 schematic ( http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf ) you will see that the FTDI chip is NOT powered directly from the USB +5vdc (which is named VUSB), but rather powered from the same +5vdc source that the 328p chip is, named +5V. Which means that the FTDI chip is powered up no matter if it’s plugged into a PC or not. So simply if the 328p is powered up so will the FTDI chip be. The older arduino boards using a FTDI chip operated the same, the FTDI chip is powered up even if not plugged into a PC USB.

raacampbell: Does anyone know if connecting both USB and an external power supply is known to be problematic?

No, it isn't.

raacampbell: If not, then I may have a separate problem with this circuit.

Clearly, if something smoked, you have connected something badly wrong.

No way we can tell what that was without clear pictures of the boards in question. Resolution 1024 by 768 maximum, a focused photo of that size (or less) will readily permit diagnosis, a blurry photo of any size is quite useless.

I don’t see anything obviously wrong with my circuit so I replaced the smoked Nano with a new one, plugged in both power sources and hoped for the best. Everything seems to work just fine. Perhaps the original Nano was defective in some way or was an older design. Hopefully things stay fine! Thanks all for your input.

I have the same issue with an arduino nano project wich is powered by a battery. The USB is not working when the nano board is powered by the baterry. When I unplug the Nano board the USB is working fine. I don't think that the nano has smoked. If the FTDI is smoked it shouldn't work in any case.

At the start of this project I have use an arduino Uno board. All is working just fine with the same parts that don't work with the Nano board. I think that the FTDI component should be powered by the USB. It should solve this issue..

Djoe: I have the same issue with an Arduino nano project which is powered by a battery. The USB is not working when the nano board is powered by the battery. When I unplug the Nano board the USB is working fine. I don't think that the nano has smoked. If the FTDI is smoked it shouldn't work in any case.

Not sure what you are describing here. The Nano is essentially a miniature version of the Duemilanove. It is true that if the FT232 chip was damaged it would not download sketches, but when you say "When I unplug the Nano board the USB is working fine", exactly what do you mean?

Paul__B: Not sure what you are describing here. The Nano is essentially a miniature version of the Duemilanove. It is true that if the FT232 chip was damaged it would not download sketches, but when you say "When I unplug the Nano board the USB is working fine", exactly what do you mean?

I have build an board with sensors and power where I plug the Nano. When the Nano is plug and powered by this board the FTDI chip does not work. But when I unplug the Nano board and connect only the USB cable on it, all is working.

Djoe:

Paul__B: Not sure what you are describing here. The Nano is essentially a miniature version of the Duemilanove. It is true that if the FT232 chip was damaged it would not download sketches, but when you say "When I unplug the Nano board the USB is working fine", exactly what do you mean?

I have build an board with sensors and power where I plug the Nano. When the Nano is plug and powered by this board the FTDI chip does not work. But when I unplug the Nano board and connect only the USB cable on it, all is working.

Well possibly you have a wiring problem with the board you plug the nano into, maybe trying to use pins 0 or 1? Can you post a schematic drawing of the board you are plugging the nano into?

Pin 0 and 1 are used in my design. I have try to bend them out of the socket. And the USB is OK now!

Thank’s a lot!!

Djoe: Pin 0 and 1 are used in my design. I have try to bend them out of the socket.

So there's the question - why are you using pins 0 and 1?