RAMPS 1.4 burned Arduino Mega 2560

Hi!

I´m really new to RepRap & Co. so I have made a mistake:

I have connected my RAMPS 1.4 Board to an external power supply, the I connected my PC (or otherway) and my Arduino Mega 2560 fried, it works but the FTDI chip isn´t working anymore, I can get an serial port connection but can´t upload anything, the TX/RX leds are always on...
The chip also gets extremely hot when connected to the supply afterwards...

What have I done wrong? How can I connect the new Arduino properly?

How many volts did you run into it?

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[scara3dprinter.wordpress.com]

12V

it was 12v dc? not ac? (ac would be bad)

12V DC, im not so new to electronics and arduino to plug it directly to AC ^^

Have you tried pushing the little reset button on the side?

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[scara3dprinter.wordpress.com]

Yes...
i think the usb communication chip of the arduino got burn, why?

If you remove the RAMPS shield
and try to load --- BLINK --- or some example does it work?

You may have to reflash the comm firmware or bootloader

[learn.sparkfun.com]

[www.arduino.cc]

No, als I Said before, I can't get any connection, seems like the arduino usb chip broke...

the Arduino 2560 has a separate programming port for the USB interface. In my case, I fried (Or screwed up the programming for) the USB to serial chip, so I can connect to the Arduino to upload programs, but not to the serial port, which makes it useless for the printer, but I can do other things with it. I had intended to attempt to reflash the USB chip, but I haven't yet done it.

If the chip is getting hot, then that's likely a problem. It may be an issue with the RAMPS board. did he combination ever work, or did it fail from the first connection after the RAMPS board was attached and powered?

The RAMPS board is working, everything worked fine until I plugged in USB, then it burned.
So I can use the power supply with the board without any errors, but when I then connect USB, it fries...

It´s when the ramps is connected to the power supply and THEN the usb gets plugged in, or otherwise, i forgot, but dont want to test it ^^