Possible firmware bug for MK3S
I'm having thermal runaway errors occur on large prints at consistently the same location within the print. Small prints seem to work fine. Large prints not. For example; extruder set point is 250 but the actual temperature will change from 250 to around 235 at a consistent point in the print; however, the setpoint is still 250. Consequently, the feedback algorithm can never achieve the 250 temperature and throws a thermal runaway error. I have verified this behavior with MK3S firmware 3.9.0 and 3.9.1. Also, the behavior is present with and without Octoprint. I have printed off of SD card and same thing happens. I have checked the GCODE file and nothing pops out. Again, I must reiterate that the print seems to behave as it should up to a certain point. I have printed the same part 3 times and the error consistently occurs at the same location in the print. So, a hardware issue can be eliminated as the cause. It's either a firmware issue or a PrusaSlicer issue. Please help!
RE: Possible firmware bug for MK3S
Photos tell more than thousand words...
Many thermal runaway errors is typical caused by broken wires to thermistor or heating element. A broken wire can be hard to detect as it only fails when it is bend the right way. Part cooling fan is another source of runaway as it can trigger a runaway at high to full speed.
At the firmware level bad PID regulation setup can can give over/under shoot of temperature. Try running a PID configuration on the printer in firmware to calibrate the hotend PID around 250C.
Have a look at my models on Printables.com 😉
RE: Possible firmware bug for MK3S
Area51, thanks for replying . You bring up an interesting point regarding the PID calibration. I'll have to dig into that. I have a hard time believing it's a bad wire or thermistor issue since the error occurs at EXACTLY the same point in the print 3 times in a row. A hardware issue should be more sporadic than that. That said, I'll check the resistance of the thermistor and look for loss of continuity in the wiring.
RE: Possible firmware bug for MK3S
OK, I've run the PID calibration routine. That was successful. The thermistor resistance is a bit low if it's supposed to be 100 k Ohms as I've seen elsewhere on this forum. It's hovering between 75-80 k Ohms. Anyone think that's my problem?
ADDENDUM: Possible firmware bug for MK3S
More information....I took the thermistor out of the system completely and measured it's resistance. Out of the printer, it initially read 130 k Ohms but depending on how I hold it and the wires entering the thermistor, it would drop to ~100 k Ohms. I tried this on another spare thermistor I have and it does the same thing. Is this normal for a thermistor? Shouldn't it be a constant 100 k Ohms regardless of how it's held or how the wires are oriented? In any case, I've ordered some new thermistors.
RE: Possible firmware bug for MK3S
Hi Wayne,
A thermistor is specifically designed to change resistance significantly as the temperature changes.
so it sounds like it's doing the right thing.
I believe the 100K ohm value is expected at a thremistor temperature of 25Centigrade.
So if you are holding the thermistor bead / cartridge, it is reasonable for the temperature to be higher than 25C and therefore the resistance to be below 100Kohms...
Different manufacturers have different temperature response curves for their thermistors, so it's always a good idea to get one from the same company as the manufacturer of your printer uses. Which for Prusa, I believe is Semitec...
regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: Possible firmware bug for MK3S
Thanks Joantabb! I'm still going to install a brand new thermistor just to absolutely eliminate that variable. Do you or anyone know how to verify the heater cartridge operation? Should it have a specific resistance as well? That's the only other hardware item I can think of that might cause my thermal runaway error.
RE: Possible firmware bug for MK3S
RE: Possible firmware bug for MK3S
Hi Wayne,
24 volt 40 watt heater, should measure about 14 ohms when cold, the resistance will increase slightly when it heats but not a lot...
Be careful not to get a 12 volt heater (about 3.5 ohms cold), that would bee bad news it would try to draw 160 watts...
Regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: Possible firmware bug for MK3S
Thanks Tim and Joan! Much appreciate the help.
W
RE: Possible firmware bug for MK3S
More information....I took the thermistor out of the system completely and measured it's resistance. Out of the printer, it initially read 130 k Ohms but depending on how I hold it and the wires entering the thermistor, it would drop to ~100 k Ohms. I tried this on another spare thermistor I have and it does the same thing. Is this normal for a thermistor? Shouldn't it be a constant 100 k Ohms regardless of how it's held or how the wires are oriented? In any case, I've ordered some new thermistors.
Don't touch the wires while measuring the thermistor resistance as your body/fingers will be part of the measurement. If untouched the measured value should be around 100k and be constant if temperature is constant.
As @joantabb points out it is important to get the same type/make as the original as they measure differently. It also has to be one of the high temperature ones used by E3D and Prusa.
Have a look at my models on Printables.com 😉
RE: Possible firmware bug for MK3S
OK, problem solved! Turns out it wasn't a hardware or firmware issue. It was a user problem! When I sliced the part in Prusaslicer, I had the bridging fan speed set to 100%. So........when my extruder is trying to maintain 250C and a bunch of air is being blown across it, it can't keep up with the heat loss and eventually declares a thermal runaway error.
It's always the little things.
Thanks for the help anyway.
Wayne
RE: Possible firmware bug for MK3S
Get yourself a silicon sock to go around the heatblock. e3d ones last longer. Then run the pid again as now its will have a nice warm coat 🙂 Gets rid of all that temperature drop. I dont run printers without them now and I think they should come as standard.
RE: Possible firmware bug for MK3S
@neophyl
Good idea. I'm on it.
W