Heat bed MOSFET (not FUSE) overheating after auto tune command

Hi everybody, long time no hear welcome to my next episode...

I bought a new RAMPS board from the guys at [www.staticboards.es] and connected all the wires one at a time and tested it: heated bed, extruder, stepper motors. Everything worked as expected, just the extruder heated up and would not stop at the requested temperature. So I switched off everything disconnected, connected and double checked every connection and everything was perfect.

I thought to go ahead and print my first test cube and sent the run command. Strangely the extruder would not heat up So I switched off everything and double checked the connections and everything seemed just fine. I uploaded a new version of Marlin and configured with my settings. When now switching on the PSU it would turn on and off immediately so I tried a couple more times and then it made "puff" as if something had popped and I was like wow... and seconds later my computers USB starts to smoke and I quickly pulled the plug.

I took everything apart again and can connect the Arduino board to the computer but can not seem to upload a new firmware. I get this error messages:

avrdude: ser_open(): can't open device "/dev/cu.usbmodem1411": No such file or directory
ioctl("TIOCMGET"): Inappropriate ioctl for device
ioctl("TIOCMGET"): Inappropriate ioctl for device
avrdude: ser_send(): write error: Bad file descriptor

To summarise:
1 - when testing and setting the extruder temperature it heated up but did not stop at the expected temperature
2 - when running the test print, the extruder did not heat up at all
3 - PSU did power on and off immediately
4 - something popped and USB cable became extremely hot
5 - can not connect to the Arduino board

Can it be that my Arduino got damaged by the previous episode? Any recommendations of how to evaluate which mistake / error I am having?

I am so sorry to bother you with this but I really would LOVE to print something finally...
Big thank you! Andreas

Wow Andreas, it's amazing that you can remain so positive with all the problems you have had! If your USB cable got hot something is seriously wrong. I'd start by checking that any USB device will still work in that port - got a USB thumb drive you can test with?

Thank you James, USB on my computer is still working, tried it with a usb drive and by charging my phone.

A "melting" USB-cable is a sign for a bad ground loop.
The common advice is, to use the same wall socket for both PSUs ( PC and printer ).
But it can also happen, when you use thin cables for the heated bed or the power connector. Or you´ve clamped the cables on the insulation instead of bare wire. Bad solder job on the heatbed, too.

Your arduino´s USB chip sure has popped. There are ways to bypass the broken chip, but I´m not sure, if they work with a Ramps connected. ( experts chime in please )

The problem with the hotend temp is independent from the above.
You were able to read the hotend temp, while it heated up? Was the temp climbing constantly, or were there jumps and sags in between?
I suspect, the wiring for the thermistor became hot and unsoldered mid heating. Can you still read the environment temp from the hotend or does it say "0"?
The legs of the thermistor have to be crimped to the extension wire. My china-made ones are only soldered ( and badly soldered, too )
-Olaf

Frying cables and usb interface chips seems to me a switching psu problem. What type of psu is it? I'd buy a couple of extra arduino boards and a different psu. Also get an power extension cord with hub/splitter and powerswitch. Use the switch instead of plugging/unplugging power cords. Between two switching psu ground levels can be a high voltage. It may damage anything you connect to it. Make sure everything is powered off before plugging in or out. Use the switch as said, it's very quick and avoids sparks.

Also, if you have an sd card connector, I'd try to test the printer without being connected to the pc or usb. Load the gcode on the card and start it using the display.
If there is temperature readout but it doesn't stop heating, check the temperature set in the slicer, also run pid tuning before printing and save the settings in the firmware. See reprap wiki for instructions. Also make sure you didn't swap heated bed and hot end connectors on the ramps board.

I'd also test the printer without using the heated bed first.

Thank you Olaf and imqqmi,

- I use thick cables for the heated bed, 2mm
- the power connection uses 3 cables from the PSU per +/- all with 12A (images attached)
- neither for the heated bed or extruder I had to clamp cables

- I was able to read the hotend temp while heating up and it was climbing constantly. I also got constant room temperature when the hotend stopped working.
- with the ardunio USB not working, I can not read any temperature right now as I can not connect to the printer.
- I only soldered the thermistor extensions and isolated them with little plastic tubes (which shrink when applying heat to them), so I will clamp them
- unfortunately I don't have a sd card reader or display so I will need a PC for running the printer


Questions:
- how can I verify / correct a bad solder job on the heated bed
- do you have any recommendation of where to buy additional Arduino boards? I have seen compatible boards from SainSmart and others...

Thank you, Andreas