Thermal Runaway Issue
Hello,
This is just to possibly help others who are having a similar issue to myself. I have five Prusa MK3S printers, two of which arrived in the last month. The issue I have noticed on one of the older printers and also on one of the newer printers is related to a failure in the bed thermistor wires.
I first noticed something was wrong because I would sometimes get a "thermal runaway error." I have printed literally thousands of pieces on these printers and it doesn't happen too regularly but it was enough for me to investigate. I measured the resistance at the connector end while I slid the bed back and forth I noticed on rare occasions the thermistor would become an open circuit. I never would have anticipated this would be the problem but the conductors inside the thermistor wires were sheared from the clamping force of the strain relief. The sheathing on the thermistor was undamaged so the wire appeared fine but by manipulating the short section of wire that was previously clamped by the top plastic plate on the bed by about +/- 15 degrees I was able to see on my multimeter, that was connected to the connector that plugs into the PCB, that an open circuit could be regularly produced. I cut that section of wire out from the thermistor, soldered the wires back together, and didn't tighten the strain relief all the way which has resolved my problem. If you have a similar problem make sure the solder joints aren't clamped by the strain relief.
Something else to note is how this problem manifests. I noticed the temperature would suddenly fluctuate three degrees or so from the current steady temperature but just for a single measurement then it would snap back to where it was. I assume this is generally not a problem because the temperature sensor is likely oversampled to a high degree. I think this is likely the reason that the thermal runaway error is often thrown a few hours into the print. I suspect the fault condition has to remain present for a certain length of time to overcome the oversampling and throw the error.
I have had considerable success with Prusa printers for years but this is one issue I have with the design. Perhaps a serpentine impression in the top plate where the wires could be pressed into would require minimal clamping and provide suitable strain relief. Hopefully this is useful to at least one person.
-Scott