Hotend runaway - mk3s  

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paul.s66
(@paul-s66)
New Member

How can this overheat like this?

After a layer or two my hotend has started overheating and triggering thermal runaway. I've done a PID tune and it's rock solid. I don't see how it can report the temperature correctly back to Octoprint (see image) yet apparently carry on heating. If the thermistor wires were bad the temperature reported should be bad. If the heater wires were bad it shouldn't continue heating. So how is it heating too much like this? And how do I fix it?

 

Posted : 02/08/2020 12:50 pm
Karl Herbert
(@karl-herbert)
Famed Member
Posted by: @paul-s66

How can this overheat like this?

After a layer or two my hotend has started overheating and triggering thermal runaway. I've done a PID tune and it's rock solid. I don't see how it can report the temperature correctly back to Octoprint (see image) yet apparently carry on heating. If the thermistor wires were bad the temperature reported should be bad. If the heater wires were bad it shouldn't continue heating. So how is it heating too much like this? And how do I fix it?

 

This can happen if the thermistor cannot transmit a constant signal due to a wire that is susceptible to interference. This is often a cable break within the insulation, not visible to the naked eye.
I would heat up the nozzle and try to move the wires back and forth. In case of a loose contact the temperature jump will be shown immediately in octoprint or on the lcd display.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 02/08/2020 3:37 pm
zoltan
(@zoltan)
Member Moderator

@paul-s66

thermistors work in invert mode - their resistnace is decreasing with increasing temperature, which in your case could indicate a thermistor shortcut for very short time, because the termistor table (would be longer story) of MK3 is limited to 300°C so the shortcut will show you those 300°C .

The heater can not provide such energy, that the temperature would suddently have this increase , unles you have nuclear heater 😉.

I wonder you did not get thermal runaway - but this is probale for the reason, it did not last for time period the thermal runaway is measured.

Fixing :  try to find the cause of the shortcut

This post was modified 2 months ago 3 times by zoltan
even an old man can learn new things 🙂
Standard I3 mk3s, FW 3.9.0, no closed box, sketchup , fusion 360, PrusaSlicer, Windows 10
PRUSA MINI FW 4.1.0, technical background...
Posted : 02/08/2020 3:57 pm
Karl Herbert
(@karl-herbert)
Famed Member

Conclusion, however: I would try to change the thermistor wires.
I had this problem with the temperature changes once and found the damage in the area of the Einsy board. Because of the many back and forth movements, the wires were broken or short-circuited. After changing the wires the fault was gone. I replaced the wires with heat resistant silicone wires from Lappkabel.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 02/08/2020 4:56 pm
zoltan
(@zoltan)
Member Moderator

@karl-herbert

👍 agreed

even an old man can learn new things 🙂
Standard I3 mk3s, FW 3.9.0, no closed box, sketchup , fusion 360, PrusaSlicer, Windows 10
PRUSA MINI FW 4.1.0, technical background...
Posted : 02/08/2020 5:06 pm
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