Thermal Runaway Protection (Dual Extruders)

[paulpangrazzi]

I enabled thermal runaway protection after reading the comments in the "Configuration.h" file about fires and mayhem. It seemed prudent. Surprisingly, I can't seem to find much info about the features in this forum.

- However, my first print was halted by the feature (it works!) and now I'm wondering what actually triggered it.

I have just completed the excellent "Itty Bitty Dual Extruder" build for my Makerfarm 10' i3V, and think that perhaps the extruder that was NOT engaged (read: heated) triggered the shutdown? The close proximity of the two hot ends could mean that the working temp of one sends the other one out of bounds... am I off base?

Should I also increase the "thermal_runaway_protection_hysteresis" by a few degrees in the meantime?

--

Edit: It's not the hot ends, it's the heated bed... over the course of printing the temperature slowly drops out of range. This is new; it's printed for dozens of hours at temp - I'll have to figure this out separately.

[paulpangrazzi]

Solved.

My print fan is new for the double extruder build, and is apparently quite effective.

When a small part is printed in the center of the heated bed (which is directly above the thermister) the spot on the bed cools down enough to trigger the shutdown. Ok then!

[TopJimmyCooks]

I'm not sure you need to protect yourself from heated bed runaway. It's for the hotend, which if it runs away while extruding a bunch of plastic or melts the extruder and falls on the part, could be serious.

[truly_bent]

I'm confused, or someone else is. Doesn't "thermal runaway" imply higher temperature than you want? This might occur if the MOSFET on the board shorted and kept feeding current to the bed when it was suppose to be shut off, or some such. Paulpangrazzi is talking about having the fault trigger "when the spot on the bed cools down". That's not how i understand thermal runaway.

TopJimmyCooks' explanation sounds right.

[AbuMaia]

If your thermistor shorts out or has a line break, it won't report properly. If it reports as colder than it should, the printer will put more power to the heater to warm it back up. This can also be a thermal runaway.

[truly_bent]

Makes sense. Glad you clarified that for me AbuMaia.

I guess the only way to monitor thermistor failure would be to have two thermistors for each hot spot. If the two weren't within reasonable readings of each other, a fault could be called. I don't think the boards allow for that number of thermistors, without adding an additional expansion though.

In the same vein, current sensors could be added to monitor the MOSFET health for each hot spot, but i'm talking about adding a lot of extra analog inputs to the controller.

[TopJimmyCooks]

You can do dual thermistors on one hotend with ramps and marlin and it will fault on a discrepancy between the readings. Ramps/marlin do not support dual extruders with redundant thermistors (five thermistors total) out of the box. Pins can be assigned on ramps/arduino but Marlin doesn't handle it at the moment.

The only situation where this type of protection will help is if one of the two thermistors comes out of the hotend. Since most are using furnace cement or set screws rather than just tape these days, its getting to be a rare problem.