Maxtemp reached, burning hot thermocouple wires while heater cold...

Hello every one

I'm building my first Reprap, a Prusa, intended to reach high temperatures. Therefore I eqquiped it with a thermocouple sensor able to handle such temperatures, and my control card is a Megatronics 3.0, in order to use the thermocouple efficiently.

As I was starting to test is, everything seemed fine, it heated up to 200°C with no problem, very quick response... but out of nowhere, last time I tried to plug it, I get the error message on Repetier :

16:04:40.979 : Error:0
16:04:40.980 : : Extruder switched off. MAXTEMP triggered !
16:04:41.029 : Errorrinter stopped due to errors. Fix the error and use M999 to restart. (Temperature is reset. Set it after restarting)
16:19:24.109 : Resend: 40
16:21:28.155 : Error:0
16:21:28.180 : : Extruder switched off. MAXTEMP triggered !
16:21:28.240 : Errorrinter stopped due to errors. Fix the error and use M999 to restart. (Temperature is reset. Set it after restarting)

This now happens every time I connect the power supply. At first I thought it was my heating cartrige that started on its own, but it happens to be cold. The "funny" part is that the thermocouple's wires (all the way to (or most likely, from?) the control card) get really hot really fast. The hotend of the printer is still cold, and with the power supply off I can read a 25°C from the thermocouple, so I guess it is still working...

Does anyone have an idea? Short circuit? And by the way sorry for any English mistakes, not my native language ^^

Thanks

Quentin

Edited 1 time(s). Last edit at 07/30/2014 11:03AM by QuentinB.

The only similar case I could find was this one :

[forums.reprap.org]

Same heating case in the wires, but I don't thing my thermocouple is burned or something like that. However, one user talks about contact between the thermocouple head and aluminium parts, and my heater head is a E3D, could this be the explanation? I don't understant why it would happen when I turn the power supply on, as the thermocouple is powered by the card via usb with no problem...

Edited 1 time(s). Last edit at 07/30/2014 11:32AM by QuentinB.

Ok don't bother, it was indeed a contact between the head of the thermocouple and the aluminium nozzle that caused the entire problem. A small piece of Kapton around the head and no more MAXTEMP, hot wires or huge temperature readings.

I hope it will help someone in the future ^^