Strange electrical behaviour with Ramps 1.4

As this is my first post, hello to everybody!

A few weeks ago I purchased a full P3steel kit controlled with Ramps 1.4. I assembled it, connect it to my laptop and everything was working fine.
One day I had the idea of controlling my 3D printer from a raspberry pi. I opened printrun and connected to Arduino correctly, but as soon as I plugged in the 12v power supply the raspberry reboot and the screen was completely dark. The HDMI driver was fried and I assumed it was because of the cheap HDMI-VGA converter I used.
I abandoned the RasPi idea and continued to print with my laptop without further problem until today that I tried to connect the 3D printer to an old laptop running Ubuntu. Same thing happened, open Printrun, connect to Arduino, but as soon as the 12v power supply was turned on...
At least this time I didn´t fry the laptop, but instead the following Arduino USB component was burnt


I guess I´m having some kind of shotrcircuit that makes overcurrent to kill some components, but I dont get why with my laptop everything works fine, but when I use any other device I have those problems...
Any idea? Where should I look for the fault?

Thank you in advance guys!

At a guess you have a damaged voltage regulator on the mega (normally caused by plugging in endstops wrong and shorting 5v to gnd)

This takes the 12v and turns it into 5v, I suspect its putting out something other than 5v on the 5v lines

This in turn burnt out some of the USB protection on the mega.

Plug in the mega to a dc power supply (and nothing else), check what voltage is on a 5v pin, or the output of the voltage regulator.. anywhere you should see 5v.
Also check the 3.3v lines

Hi,

First of all thanks for the reply.
That would make sense if it was a one time thing and I blew up the Arduino.
The thing is that I can print anything with my computer, but when I connect the printer to and old laptop or a Raspberry Pi I get the problems with USB overcurrent.

I´ve made the test you say and in the 5 volts pin I get 6.3 volts, and in the 3.3v I measured 3.29. Endstops are connected properly.
I don´t really care about fixing the Arduino, I already have another one installed in my printer. The thing I´m concerned about is why I get those current peaks in the USB port when I connect to another device than my "modern" laptop.