[Solved] Prusa i3 Mk 3 Heat Bed Won't Shut Down  

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sglt1
(@sglt1)
Active Member

First post and honestly first issue. I have an i3 Mk3 and today after a print the Heat Bed continued to heat and actually went to maximum. I noticed a Error Message and promptly turnoff the power supply which was also very hot. No smoke or meltdown. What could have caused the heat bed not to shut off after a completed print. Best Steve Thomas USA

Posted : 20/04/2020 12:36 am
thedevilsjester
(@thedevilsjester)
Eminent Member

Sounds like the thermistor is broken, damaged, or disconnected from the heated bed.  The thermistor detects the temperature and tells the heater to stop.  Look under the heated bed, there should be a taped wire with a sensor on it.  Thats your bed thermistor.

 

Posted : 20/04/2020 6:15 pm
Neophyl
(@neophyl)
Noble Member

Or for some reason you don’t have the heater shutdown command in your end gcode. If you post the .3mf project you used where the bed didn’t shut down we can take a look.  Or post the actual gcode file as it can be checked for the bed off gcode. 

Posted : 20/04/2020 7:10 pm
sglt1
(@sglt1)
Active Member

@thedevilsjester

Thank you for your quick reply. The 'Thermistor' is connected and I checked conductivity and all is well. I allowed the Power Supply  to fully cool and restarted, reprinted and all is functioning normally, thus far. I will keep an eye on it. Best, Steve

Posted : 20/04/2020 7:14 pm
sglt1
(@sglt1)
Active Member

@neophyl

Thank you. Unfortunately, I delete G-codes as a matter of routine when a print has failed. I still have the STL file and CAD file, so I will repeat the conversion, generate another G-Code and try the same print. I will post here either way, success, failure, or another 'over heat'. Best, Steve Thomas

Posted : 20/04/2020 7:18 pm
thedevilsjester
(@thedevilsjester)
Eminent Member
Posted by: @neophyl

Or for some reason you don’t have the heater shutdown command in your end gcode. If you post the .3mf project you used where the bed didn’t shut down we can take a look.  Or post the actual gcode file as it can be checked for the bed off gcode. 

While that might result in the bed not shutting off, I don't think that would result in:

the Heat Bed continued to heat and actually went to maximum

 

Posted by: @stephen-t6

@thedevilsjester

Thank you for your quick reply. The 'Thermistor' is connected and I checked conductivity and all is well. I allowed the Power Supply  to fully cool and restarted, reprinted and all is functioning normally, thus far. I will keep an eye on it. Best, Steve

There are two main scenarios that can happen with a "bad" thermistor. 

  1. If the thermistor is not wired tightly to the heated bed, or has a short in it (from, say, the cable hitting the wall behind the printer when the bed goes all the way back!), it will cause a temperature error to occur because it detects (or fails to detect) that the ambient temperature is below the minimum.  This can show itself as a temperature error.  You can see this on the LCD sometimes by looking at the bed temp.  If the number on the left is 0, this is your problem.
  2. If the thermistor gets dislodged or is not properly attached to the bed, it will still detect the ambient temperature, but not get enough of the temperature from the bed to know what temperature its at and the bed will continue to heat up, thinking its not hot enough yet.

If the situation happens again, try and determine what might be the cause, and potentially throw a camera up to watch the printer so you can see the moment of failure.

This post was modified 6 months ago by thedevilsjester
Posted : 20/04/2020 7:46 pm
sglt1
(@sglt1)
Active Member

Again, my thanks for the responses. As a matter of elimination, I suspect that my issue was in the G-Code itself. I have been printing today with no issues, including the 'reprint' of the trouble-maker part. I exported that CAD file to STL to Prusa PE to G-code as I usually do and all is fine. I am going to consider this a one time problem (new to me) and post the issue as "solved" until it happens again. I appreciate all the helpful responses and I am grateful for the 'new' insight into Error: Bed Heat Maximum. Best, Steve Thomas, USA

Posted : 21/04/2020 12:31 am
towlerg
(@towlerg)
Honorable Member

You really need to get to the bottom of this, sounds like potential fire. Under no circumstances (I hope) could faulty GCode cause the bed to overheat in the dramatic way you describe.

Posted : 21/04/2020 2:59 pm
sglt1
(@sglt1)
Active Member

Honestly, I do not know what I would check next. As far as the printer itself, the connections are good, the voltage is good, and everything is working as before. My power supply is outside my Ikea printer enclosure  (thank you to whomever put together that little tip) & I have an additional computer fan for the power supply. I don't know enough about G-Code to look at where that issue might have originated even if I still could still send you that code. Trust me when I say, I am monitoring every print until I feel comfortable it was a glitch and not a reoccurring problem. However, if the "Thermistor" is your best-estimate of the issue, I would certainly replace it, but again, if it was not the issue, then the odds of this happening again are still the same. I will monitor for the time being. Best, Steve Thomas USA

Posted : 21/04/2020 3:54 pm
towlerg
(@towlerg)
Honorable Member

If I were you I'd definitely change the bed thermister. As you posted that the power supply got hot, then I guess it's safe to assume it was drawing a higher than usual current, so perhaps some way to limit current (like replacing the fuse with something that will actually do something) would be favorite.

Posted : 21/04/2020 5:52 pm
sglt1
(@sglt1)
Active Member

@towlerg

You're making me laugh with "Fuse" comment. Yeah really, that would be a great place to start. I don't remember what value the OEM fuse is, guessing 15 Amp. I also suppose Power Supply "Heat" is relative to what I normally feel when I place my hand on it. I had also thought that there would be an OEM Auto Shut Off response once the "Max Bed Heat Error" pops up, but possibly that is what the fuse is for. LOL. I'll check out the 'thermistor' part in Support. I know replacing it would eliminate one thing to check if it were to happen again. Best, Steve

Posted : 21/04/2020 10:12 pm
towlerg
(@towlerg)
Honorable Member

@stephen-t6

"OEM Auto Shut Off response once the "Max Bed Heat Error" not sure what you mean, isn't the detection of  "Max Bed Heat Error" the OEM Auto Shut Off response. Should have asked this before but (I assume the error you got was "Max Bed Heat Error") how long after the error message appeared did you turn the power off?

Posted : 22/04/2020 1:43 pm
Neophyl
(@neophyl)
Noble Member

Most firmware has a Thermal Runaway protection response.  When it detects that the temp is increasing in an uncontrolled manner its supposed to shut off the heaters as a safety response.  Likewise if the heaters are on but the temperature reading is not increasing that should also trigger a safety response from the firmware.
If the firmware thinks it was told to go to a certain temperature and its reading that temperature though then it wont trigger that as its acting like its been instructed.  

Posted : 22/04/2020 2:04 pm
thedevilsjester
(@thedevilsjester)
Eminent Member

@neophyl

I wouldn't rely on that behavior as a safe guard.  For example I heated up my hot-end to disassemble it (because it was literally a ball of PETG) and accidentally disconnected the thermistor.  When I did, the hot end proceeded to skyrocket in temperature.  Things began to get extremely hot and the filament began to smoke.  I am sure it would have shut off _eventually_, but there is a time frame between it triggering, and when (if) it decides that it's a problem.

Unless he is modifying the GCode himself, or using some back alley home made slicer, there is no reason that the GCode should have errant temperature instructions.

Posted : 22/04/2020 3:07 pm
sglt1
(@sglt1)
Active Member

@towlerg

I shut power down the moment I saw the Error Message. No more than a few minutes after pulling the poor print pressing the "X" switch to reset the printer. I was obviously away from the printer long enough for it to reach Max Bed Temp from the ABS print settings.

I only use Prusa PE as my Slicer program and use what ever G-Code is generated. I have modified settings only in the Prusa PE and I have never changed a heat setting. I only use Prusa Parts to upgrade or change out any part. I use Hatch Box Filaments (PLA, ABS) for the most part. I do not think this was a G-Code related event. 

Posted : 22/04/2020 3:31 pm
vintagepc
(@vintagepc)
Noble Member

I'm surprised nobody has mentioned the likely culprit of a shorted mosfet. That is the primary explanation if the bed is continuing to heat uncontrolled even when the printer thinks it should have been shut off.

Posted : 22/04/2020 3:38 pm
With_maltodextrin
(@with_maltodextrin)
Trusted Member
Posted by: @vintagepc

I'm surprised nobody has mentioned the likely culprit of a shorted mosfet. That is the primary explanation if the bed is continuing to heat uncontrolled even when the printer thinks it should have been shut off.

Agreed - my first thought was that the heatbed mosfet is borked and that even if it "went back to working OK" after this incident, it's still a huge concern.

Posted : 22/04/2020 4:53 pm
sglt1
(@sglt1)
Active Member

Follow-up on Max Temp Heat Bed Error. The issue appears to have been a slight abrasion on the Thermistor cable to the Heat Bed. This cable had rubbed against the Negative (-) post bolt under the corner of the Heat Bed and exposed the copper inside shorting it against the negative post. After repairs all is good.

Though as a result of the second bed error, I had a Hot End temperature discrepancy between what the LCD was reading and what my Thermal Thermometer was reading. As long as I had the printer out of the enclosure, I replaced the entire Hot End with a new OEM part.

The problem now is I am getting a Filament Sensor Cable Error. I have checked continuity of the cable and all is good. I ran a "Unload / Reload" Filament in the Menu and the sensor reads the filament normally.

Also, I did a "Factory Reset" after the repairs and re-"Flashed" the printer with the latest firmware.

I can't run the "Calibration Wizard" with the continued Sensor Failure. Is there a work around? Should I just calibrate the Printer without the Wizard as a work-around? 

Thank you for your help.

Best, Steve Thomas USA

This post was modified 5 months ago by sglt1
Posted : 19/05/2020 4:11 pm
sglt1
(@sglt1)
Active Member

I have incorrectly called the "Wire Loop" that is taped under the Heat Bed a thermistor. the culprit here is that "Wire Loop" that abraded against the bolt that holds the Negative (-) Lead to the Heat Bed. 

I do still have an issue with the Filament Sensor Error. This was functioning when I disassembled the Hot End of the Extruder and the only I did was unplug the connecting wire and later reconnect that same wire and this was not my first rodeo with assembly and disassembly of the Hot End. If any one has a solution including "it should be replaced" I am all ears.

I have calibrated the printer and everything is working great except the filament continues to flow when I do a filament spool change and "load filament'.

Best, Steve Thomas USA

Posted : 19/05/2020 11:55 pm
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