Mega 2560 won't power up from external power supplies...

Hi There,

My Mega 2560 R3 was working fine up until yesterday, using a Wall Wart, a 12 volt car battery, and also from the USB connection (I wasn't using them all simultaneously though). I am using a Ramps 1.4 shield with it, and yesterday I was taking voltage readings from the end stop pins on the Ramps. Suddenly, my board went dead. There was no noise or smoke or anything, so I'm thinking possible ESD, although my meter is a good one and should be somewhat protected against doing that?

I've been troubleshooting the problem, and I have discovered that the 2560 does still work, and I can use the Ramps shield with a car battery to power my five stepper motors, and everything is working fine - if I have the Arduino plugged into a USB port on my computer at the same time. But nothing works if I just plug in a battery, or a wall wart - either through Vin or the power jack.

I've chased power through the Ramps shield and found that it supplies the 12 volts directly to the Arduino Mega 2560 through the pin labled Vin and that the 2560 is supposed to automatically detect the external source and switch over, and I presume, regulate the power to 5 volts on board. I've poured over the schematics (what I could find), but I can't figure out what components handle that job, or if they are peripheral and can be replaced. There is a chip near the power jack that looks like it could be a voltage regulator, but I can't read the numbers on it, and it is not labeled on the board.

I can operate my project using external power for the shield, and a USB connected to a computer if I have no choice, however the Ramps I have also has a SDram port, and I'd like to fix this so that I can have it all stand alone.

My questions to those who might help me are as follows:

1 - Does anybody know how to repair the Arduino based on what I've described, (yes I can do surface mount repairs)?

2 - If it can't be fixed, can I make a small external 5 volt regulator (LM7805 ?) that taps into the 12 volt line I'm using, and connect it directly to the USB 5 volt input - perhaps with a switch to disconnect it when I actually use a computer USB connection?


Just left of the paragraph of text is the auto-selector.

Since Vin is causing the USB’s 5V to be disconnected, it sounds like MOSFET is okay but the on-board 5V regulator has been damaged. The on-board 5V regulator is shown on the top left of the schematic.

Thanks for this James,

The USB's 5v isn't being disconnected. When I connect just a 12volt battery to Ramps, it pipes 12 volts through the Vin pin on the Arduino, but the 2560 will not turn on. If I then hook up a USB cable, the 2560 will come to life. (It will do that even without the 12 volts). The Ramps shield drives stepper motors and it requires at least 12 volts to operate. Normally, I'd just hook a 12volt car battery to the Ramps power input, and both it and the Arduino would work fine. Now, I have to have the 12 volts hooked up to Ramps for it to run, AND, a USB cable to power the 2560. Otherwise, only the fan on the Ramps will turn on, and it's connected directly to the 12 volt supply.

Do you happen to have a component layout for Mega 2560 R3? I have seen a picture with the componets labeled, but it doesn't match mine exactly. The part that looks like a 5 volt regulator looks like maybe "AMS ?17", and I can see 5.0 stamped below that on it. I think it may even be showing signs of a burn in the center, but it's hard to tell. If it is a 5 volt regulator, do you know where I'd find a replacement for something like that?


Hi James,

Thanks again for the schematic, it proved helpful to figure this out. The LM1117 5.0 regulator is indeed burned out, and I have sourced replacements on Ebay. I'm having 2 of them shipped to my country (canada) from China for a total cost of $2.80 including free shipping. Gotta love Ebay when it works like this. (Of course they won't be here for about 3 weeks, but that's fine because I can still use the unit with a computer. Thanks for your help.


=The LM1117 5.0 regulator is indeed burned out, and I have sourced replacements on Ebay.

Keep in mind if you remove the regulator now, you can continue to use an external supply by providing a regulated 5V to the 5V pin. (Or use USB.)

Thanks James,

I'm going to pull it out now, but I'll probably just use my netbook to power the Arduino with a USB cable until the new part arrives.

Thanks for all of your help.


I've got exactly the same Problem with a Ramps1.4 RepRap Prusa i3

Arduino needs 5V on USB to start :frowning:

Also just happend "over night".
Was working perfectly with external PowerSupply and then it just did not react at all, LCDisplay dark, ...

I'm currently using StepDownElectronics with USB-Socket connected to Arduino, ... :-/

Any Chance you could Post a Picture of the component you replaced?
Thanks a Lot


The component that he changed would have been the one encircled in black, just above the barrel jack, in the image attached.