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Hotend Stops Heating, causes Thermal Runaway Error  

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printingNinja
(@printingninja)
New Member
Hotend Stops Heating, causes Thermal Runaway Error

I've been trying to debug this for months, so hoping you all could give me some ideas:

So context:

  • Randomly during a print, the hotend will stop working, causing the temperature to drop and cause a thermal runaway error on the printer
    • Also, as a separate issue, the printer randomly jerks some motors and stops throwing a POWER PANIC error, so for context I have unplugged the Power Panic cable during the rest of this debug
  • When this happens, I usually power cycle the whole printer from the PSU side, and magically the printer is able to start heating up again. However, successive power cycles and failures causes the failures to happen sooner and sooner.
  • I initially thought it was room temperature issues, as I placed my printer in the garage which gets chilly during December-April, but recently it's been pretty hot outside, so the garage temperature is around 70F.
  • I thought it was a heating cartridge issue, but there are times when it can maintain temperature very nicely and heat up to target temperatures without any issues.
  • I then started watching the LED connected to the heating cartridge circuit during operation, and noticed that the LED would stop lighting when the hotend would stop maintaining temperature, meaning to me that the error is coming from the board and not the heating cartridge.
    • when applying a multimeter to either pin of the heater cartridge connector, both show 24V, meaning that the 5A fuse wasn't blown
      • this also shows that the transistor that turns on power to this circuit was also being turned off.
      • the transistor is controlled by two signals: a hotend PWM signal, and an nAC_FAULT signal. those two signals go through an AND gate and the output is tied to the transistor.
      • I doubt the nAC_FAULT signal is causing the problem because if that were the case we would also see issues on the heating bed (the nAC_FAULT also feeds into another AND gate that has a heating bed PWM signal as the other signal), which for the most part keeps heating when the hotend stops (you can tell because the LED on the Einsy board and on the heating plate are still lighting up)
      • So knowing that, the fault seems to be on the microcontroller, but I've already updated to the latest firmware.
      • Moreover, I thought it could be a temperature issue, but the LCD display seems to be getting accurate measurements of the temperature, and using a hairdryer causes the temperature to change. 
      • So, basically, I'm not quite sure what is causing the hotend PWM signal to stop turning on.

Can anyone help me on this?

Posted : 11/05/2021 4:53 am
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(@)
Illustrious Member
RE: Hotend Stops Heating, causes Thermal Runaway Error

A failing power supply.

Posted : 11/05/2021 4:59 am
printingNinja
(@printingninja)
New Member
Topic starter answered:
RE: Hotend Stops Heating, causes Thermal Runaway Error

@tim-2

I thought that too, but when I check all of the outputs of the power supply into the EINSY board, all read 24V, which is expected. And those don't give out when the failure occurs. When I use the multimeter after the LED turns off, the voltage from GND to each pin of the heater cartridge connector reads 24V, meaning it's still getting power supplied

Posted : 11/05/2021 5:09 am
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(@)
Illustrious Member
RE: Hotend Stops Heating, causes Thermal Runaway Error

If line or the the supply drops for a few milliseconds, the Einsy goes into lockdown. And your meter most likely won't catch that sort of problem (maybe a Fluke 87 in fast capture). 

If it isn't the power supply, start looking for intermittent cabling to the extruder. They break all the time on well used printers. Thermistors also go intermittent. One second they are working, next they are dead, next they are back (the composite cracks, the cracks expand and contract with heat and even vibration will rattle them). A thermistor failure might explain the thermal runaway shut down, as will the heater element - if the Einsy demands power to heat, and the thermistor doesn't register a temp change in the right direction, it will flag a thermal fault. 

Then again - if both sides of the heater plug are at 24 volts... the circuit is open collector... it means the plug is bad (not connecting to the Einsy)  but the on board LED is also off... so the Einsy FET's are shutting down (over current or over temp) or failing; or that the Einsy has detected a fault and has shut down the driver because the heater / thermistor pair aren't responding as expected.

Or, the power supply dropped out and set this all in motion. 

 

Posted : 11/05/2021 6:03 am
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