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Thermocouple burnt

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Hi! I encountered very interesting issue.

This how it all began: I started a rather long print and after an hour left it printing. When I returned, the print was just a half complete, but the controller reported it to be finished. I found out that small gear has fallen off the feeder stepper and that caused no extrusion. On the print table there where several drops of overheated PLA, but the PEEK itself seems to remain functional except darker color and small amount of leakage.

Here the interesting began: I powered the machine and immediately smoke appeared from the printhead. Origin of the smoke was one of the thermocouple wires – it was completely burnt. Voltage between amplifier ground and thermocouple turned out to be 19V - proper evidence of heater short circuit to the block. My thoughts are that after extrusion loss some overheat was present which resulted of insulation damage thus resulting in short circuit to the block.

Does anybody encountered something similar? Is MOSFET conducting ground or +?


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That's bad :(

Heating the nozzle without extruding for a long time can destroy both the PEEK and the teflon parts. If you have a leaking hotend now, it probably got damaged...

I know that the Ultimaker firmware / hardware (combination) can go into an "uncontrolled heating" state. I only observed this when I disconnected the temperature sensor from the electronics while the machine was heating.

About your short-circuit:

I don't think it's the thermocouple's fault. I don't know if there's supposed to be any electrical insulation between the sensor and it's case.

I think the problem is the heater cartridge: It's definitely not supposed to put 19V to the heater block! Maybe you got a bad part (reprap stuff quality is usually not too good...)


The MosFET sits in the negative path of the heater (it's the only way how this can work..), so yes, one pole of the heater does carry 19V. But that voltage must be isolated against the case - your part must have been faulty.

Think about dual extrusion with two heaters that both put 19V to the heater blocks. No go...


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There's nothing to add to Johnny's excellent reply other than here is the schematic if you need it:


The circuit diagram, and board layout are here:


There is a zip file at the top. It contains the "brd" file which is the layout. Also the "sch" file which is the schematic.

Both files can be opened by eagle software which is free:




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Thanks. Jonny, gr5!

Hour of tinkering yesterday revealed, that thanks god, nothing unfixable happend: indeed cartridge heater is not so good, and I had to add some insulation. Thermocouple was burnt only near the amplifier terminal, resulting in 1cm shorter wire. And amplifier itself still works properly. Printhead either wasn't damaged.

So I fell in panic too early)))


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