Mega 2560 VIN no longer working after applying 5v to barrel jack

Hey folks! I have a Mega 2560 Rev3 board that I've already used successfully for one project, however I recently made the mistake of connecting my 5v 6A wall power supply to the barrel jack (7.5-12v) instead of to the VIN and ground pins as I normally do. I only realized I had done so because the LCD I had connected at the time was suddenly dim and underpowered. I immediately unplugged the power and re-connected via the VIN and Ground pins to find that my steady 5.10v source was now being output as only 3.75v on all 5v pins (still 3v+ on the 3v pin). I disconnected my 5v supply and attached a USB cable to find it working correctly—the full 5v being output on all 5v pins. I then connected a 9v battery to the barrel jack which also worked correctly giving 5v on all 5v pins.

I've read through all the forums seeing all sorts of warnings about reversing polarity, using too high a voltage on the barrel jack, and learning about how the voltage regulator works to reduce voltage down to 5v, but the only thing I found about using too small a voltage on the barrel jack was the following from another forum post Arduino Forum

You'll see that the output of the barrel jack is switched by a mosfet that is controlled by a level detector
that compares HALF of the Vin input to 3.3V, thus anything LESS than 6.6V will NOT switch it on , which
means if you plugged 5V into the barrel jack nothing would happen.

Something definitely happened on my board which has affected only the VIN pin's ability to receive and use 5v.
More about my situation:

  • I have been careful to always only connect one power source at a time
  • the board was purchased new just 1 week ago
  • the 5v supply that I accidentally connected to the barrel jack (having its own barrel plug) had the correct polarity (center+)
  • I have since tested the voltage output using 7.5-12v input on the barrel jack, and the voltage regulator seems to be doing a great job outputting a max 5.10v at 9-12v in and 4.9v at 7.5v in.
  • USB 5v input still working correctly as well.
  • All tests after the mishap were done on my Mega with no external connections (all sensors and modules removed) to reduce current draw.
  • The total current draw through my board with all sensors and modules that were connected at the time of the incident was just under 1A as measured at the barrel jack input powered with a 9v battery. I didn't get a chance to measure current through a 5v input such as the USB.
  • My understanding of electronics is limited to very basic analog connections and Ohm's law, but I'm learning fast. As I mentioned before, I am completely new to using an Arduino.

Any ideas as to what may have happened and how/if I can rectify it are very appreciated. What components may have been affected by an undersupply of voltage on that barrel jack?

Thanks in advance.

I think your board is fine. A 9V on the barrel works, 12V on the barrel works. 5V on USB works.

Your 5V/6A should be connected to the 5V pin, not to Vin.

If that's alright to connect on the 5V pin, then great! I had only been connecting on the Vin thus far, but I suppose that isn't really necessary.

Thanks for confirming :slight_smile:

!! You have to be careful; simultanuous providing 5V on Vin and 5V on USB might cause damage to the PC. !!

The min spec for Vin is something like 7V; this is so the voltage regulator on the board can regulate it down to 5V.

The barrel has a diode (for reverse polarity protection) which will bring the input voltage for the voltage regulator down by (possibly) 0.7V (not sure); there is no diode for that purpose on Vin.

So no surprise that 5V on Vin gives a lower voltage out as the regulator simplycan't make 5V from something that's 5V or lower.

Oh, wow! That's good to know. I had never considered that the Vin required a higher voltage similar to the barrel jack. I wonder how I've been able to use 5V on Vin prior to now with my Uno. Any idea if the Uno and Mega are different in that regard—Vin minimum voltage? Regardless, I will connect my supply to 5V and be sure to only use one power source at a time.

Thank you so much!

You can check the schematics on the respective product pages in the store.

From memory, Uno and Mega use the same circuit. The circuit of the Nano is different; it uses a diode to protect the USB from receiving 5V from the board's 5V.

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