I have a Prusa i3 printer with a J-Head hot end and a PCB MK1 heated bed running Marlin from December 3, 2014 on a RAMPS 1.4 / Arduino Mega 2560 rev3. Everything has been working fine for 100+ combined hours of operation. Very early into my last print the LCD displayed THERMAL_RUNAWAY and everything shut down.

I have done some troubleshooting, and it appears to be an issue with just the heated bed. If I leave the hot end off and set the bed temperature to 60c it will come up to temperature in the normal time (several seconds), and hold that temperature very steady for as long as 5 minutes. Then, without any spike on the temperature graph, it goes into THERMAL_RUNAWAY and shuts down. As a secondary test I monitored the bed temperature with an IR thermometer. The thermometer and the LCD temperature stayed in agreement +/- 1.5C until right before the failure. In the few seconds leading up to the failure, the bed temperature actually started to go down several degrees while the LCD display stayed constant at 60c.

The thermistor is connected to the bottom of the PCB with Permatex Ultra Copper and kapton tape. It appears to still have a solid connection.

Any ideas on what could be going on? Any opinions about what the point of failure might be (PCB, thermistor, RAMPS, etc)?

Thanks!

I am guessing it is a terminator issue. Also look for a short between thermistor leads.

Despite its name, thermal runaway protection triggers when the temperature is too low for the specified time.

Too low is defined as the set point - THERMAL_RUNAWAY_PROTECTION_BED_HYSTERESIS.
Too long is defined as THERMAL_RUNAWAY_PROTECTION_BED_PERIOD.

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[www.hydraraptor.blogspot.com]

That makes perfect sense, thank you for the explanation! I am going to order a new PCB and thermistor and I suspect I will be in good shape.

It could just be the defaults in Marlin are not suitable for your setup. The code is supposed to detect thermistors that have become detached. But it will also trigger if your bed is under powered relative to the two constants above.

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[www.hydraraptor.blogspot.com]

The defaults were not suitable for my heated bed either. I think the default is a 2'C drop and I had it faulting nearly every print so I changed to 5'C and no issues since.

Also if you have a PCB heater running from 12V you don't need thermal runaway detection as even at full power the temperature is self limiting because the resistance of copper goes up rapidly with temperature. If it takes 10A when cold it will only take ~7A when at 110C and won't go much above 130C even under a fault condition. So you can safety disable the firmware protection.

If you have a constant resistance bed heater then you should really fit a thermal fuse as they are readily available for bed temperatures.

The runaway protection in Marlin only protects against thermistors that have fallen off and if you glue them to the bed with JB weld and to the hot end with high temperature exhaust cement that will never happen.

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[www.hydraraptor.blogspot.com]

Thank you everyone for the feedback, I am now printing again! I removed the silicon from the thermistor and heated bed and reconnected it using Arctic Alumina thermal adhesive which did a much nicer job. I don't think I could disconnect it now if I wanted to. I also changed my THERMAL_RUNAWAY_PROTECTION_BED_HYSTERESIS from 2 to 4. I have confirmed operation with an IR thermometer and am now 15 minutes into a print with no issues.