Something happened to my mega2650 as I was experimenting the other day. It no longer turned on when powering from the 2.1mm jack or from Vin, regardless of the voltage applied. A plastic bubble had formed on the 5V regulator, I assumed I had shorted something and fried it. If I applied 5V to the 5V pin everything worked fine. It also works fine when powered from USB. So I replaced the 5V regulator with a NCP1117 version. But I have the same situation. If I apply voltage directly to the 5V regulator input pin I get 5V on the output pin and everything is fine. I tested up to 12V and it is dead on at 5V. I also checked the PNP switching transistor, and found it was working correctly. As far as I can tell, the only thing remaining is that the trace is severed somewhere between just after the Vin pin and the 5V regulator, maybe the regulator input is not soldered well to the trace? But this was one of the better solder joints I made, so I thought. Any other ideas as to the root cause?


  • 2
    How about D1? The M7 diode. Try testing before and after that with power applied to the power jack.
    – Nick Gammon
    Commented Apr 6, 2016 at 6:42
  • I wonder if I should do a forensic nuking of my Mega2560? Connect it to a nice meaty PSU and short the 5V, see exactly what does happen besides the Magic Smoke™ escaping from the 1117...
    – Majenko
    Commented Apr 6, 2016 at 12:22
  • @NickGammon I know of one very popular board designed by an associate of mine that uses a 500mA diode on both the USB and main power inputs leading to the Vin of an 800mA 3.3V regulator. Short out the 3.3V power and the diode feeding the power at the time literally blows itself clean off the board.
    – Majenko
    Commented Apr 6, 2016 at 12:32
  • What I did was power the mega by 12v to Vin. But, I also powered a LED constant current boost driver from the same 12v terminals. I had the usb plugged in for programming. When I switched off the 12v battery, then the usb was now powering the mega and the boost driver via Vin header pin instead of receiving power from it. There was 5v reverse voltage across the 5v reg. Commented Apr 7, 2016 at 4:40

2 Answers 2


You obviously have some form of DMM or volt meter at your disposal, since you know the voltages around the regulator.

Some places you should, therefore, probe for voltages, and what voltages you should expect. With the barrel jack connected and powered up, and assuming a 12V supply, you should expect:

  1. At the rear of the socket: 12V
  2. At the socket side of the M7 diode: 12V
  3. At the header side of the M7 diode: 11.4V
  4. At the right-hand (VIN) pin of the regulator: 11.4V
  5. On the tab of the regulator: 5V

If you find any of those are wrong then the component that comes before it may well be faulty.

  • Ok, I checked and found these: 11.14 at battery; 11.14V at Vin header pin; 11.14V at D1 +; 11.14V at D1 - ; 0V at 5V header pin; 0V at 3.3V header pin; 0V at IC1 (5V reg) at all 3 pins; 0V at IC5B (comparator) at all 6 pins; 0V at T2 (5V switch) at a 3 pins; 0V at IC6 (3.3V regulator) at all 5 pins. If I look close it appears the 3.3V reg also has some visible heat damage but slight. So in this case it seems to me a power supply smoothing capacitor is at fault. Commented Apr 7, 2016 at 2:29
  • I got EagleCAD - wow that is extremely helpful. The trace between the PC1 smoothing cap and the 5V reg. must be open because the cap terminal is at Vin, probably a bad solder job on the 5V reg. Thanks. Commented Apr 7, 2016 at 2:52

Well the answer is ... I fried the rest of the board when I was trying to troubleshoot it further :( I think 12V went to the 328p, it got hot. Oh well, thanks everyone!

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