Thermal Runaway Concerns
I'm in the process of replacing the hotend on my mk3, and figured I'd try out a little test of the printer's thermal runaway protections.
I removed the heater cartridge from the heater block, to simulate it becoming dislodged. I then heated up the extruder to pla temps.
Within seconds the heater cartridge was glowing red hot, and the wiring insulation was beginning to smoke. I immediately turned off the printer.
Perhaps I hadn't waited long enough for the thermal runaway protection to kick in, but it was already at the point of being a fire hazard.
Did something go wrong here? Is this simply not how the runaway protection works? I'm nervous about letting it out of my sight now while printing.
Re: Thermal Runaway Concerns
It works the other way.
With the heater in place, take out the thermistor to see what happens.
Peter
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage…
Re: Thermal Runaway Concerns
Thermal runaway protection is based on the difference in time between applying a power to the hotend heater and it's responce in temperature. If the heater is on and the temperature doesn't change for (x) seconds, the runaway protection is triggered. This is because the heater cartridge takes a second or two to affect the thermistor and because of the thermal mass of the hotend. You can test this by removing the thermistor from the hotend and seeing how long it takes to trigger thermal runaway protection - that is much more likely to fall out than the heater - and that's what's caused all the Anet A8 hotend-related fires as far as I know.
Re: Thermal Runaway Concerns
Thermal runaway protection is based on the difference in time between applying a power to the hotend heater and it's responce in temperature. If the heater is on and the temperature doesn't change for (x) seconds, the runaway protection is triggered. This is because the heater cartridge takes a second or two to affect the thermistor and because of the thermal mass of the hotend. You can test this by removing the thermistor from the hotend and seeing how long it takes to trigger thermal runaway protection - that is much more likely to fall out than the heater - and that's what's caused all the Anet A8 hotend-related fires as far as I know.
But it should still trigger if the heater cartridge were to fall out right? So likely I just didn't wait long enough?
Re: Thermal Runaway Concerns
With the aluminium block clamped around it along its entire length there is no way that heater cartridge is going to fall out. All you are going to do by pushing power through the heater when out of the block is to burn out the heater element.