geoff.b2
Posts: 12
Joined: Sep 14, 2017 1:51 pm
Contact: Send private message

Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 10:24 am

Prusa Mk2.5

My Mk2.5 recently started to produce prints with large chunks of missing extrusion. I found that this was caused by an intermittent break in the wire from the heat-block thermistor (about half way along the cable bundle).
I have replaced the lead and now all is well. However, I am concerned that the firmware (3.5.0) did not detect the failure, because when present the indicated temperature dropped to 27C and the head heater tried to ramp up the power to compensate. I caught it at one point (when connection momentarily restored) up to 270C. Luckily the break was intermittent and dependent on head position, so at intervals the correct reading would be measured and the head would cool again for a few minutes.
The symptoms on my printer when the wire was o/c were: the indicated temperature dropped to 27C, the heatsink cooling fan stopped, the extruder stopped, the printer X,Y and Z actions continued, the heater power increased to try and restore the erroneous temperature.
Now I thought that the Prusa printers were smart enough to detect a faulty thermistor in order to prevent thermal runaway and possible meltdown or fire. This did not happen on my printer - does anyone know why?

RetireeJay
Posts: 216
Joined: Feb 13, 2018 10:47 pm
Answers: 4
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 1:33 pm

Safety is always a priority! Thanks for bringing this up; it may indeed be a problem.

Normally when a thermistor wire breaks completely, the indicated temperature drops to zero. Technically, a thermistor of the type we use is 100k ohms at room temperature, and the resistance goes down significantly as the thermistor gets hotter, or up if the thermistor gets colder. So technically, an infinite resistance corresponds to an undefined temperature well below zero, but the table of values in Marlin firmware stops at zero (Prusa firmware is based on Marlin).

Normally, if the firmware detects a temperature of zero, it will declare a fault and stop the print.

You could try unplugging the thermistor from the board and see if the indicated temperature drops to zero (and/or causes the board to report a fault). If it does not, then there may be a problem in the firmware configuration because open circuit is definitely a fault. If your indicated temperature when unplugged does drop to zero, that probably means that in the earlier case your broken cable was not truly and absolutely broken but was maintaining a weak connection of about 100k ohms.

I can't run the same experiment for you because my MK3 uses a different board and therefore also slightly different firmware.

geoff.b2
Posts: 12
Joined: Sep 14, 2017 1:51 pm
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 1:50 pm

Yes I will try that when I can. I have updated firmware now to 3.5.1, but I don't think there have been any changes that might affect the situation above.

Certainly the break could well have gone through a high-resistance phase as it made & broke connection so might not have given an instantaneous step change in resistance.
The firmware must have been detecting the temperature change because the Noctua fan stopped and started as the open circuit came and went - presumably because when the apparent temperature was low the firmware decided there was no need to cool the heatsink!

I dissected the wire afterwards and found a clean break in all the strands of one conductor at the same point about halfway along the cable.

Just an addition: when the connection was broken (during the fault) the firmware didn't indicate 0 it indicated 27 degrees.

RetireeJay
Posts: 216
Joined: Feb 13, 2018 10:47 pm
Answers: 4
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 4:22 pm

I've looked at ALL the thermistor tables in "thermistortables.h" and ALL of them carry the temperature down to zero (some of them even below zero). So if your readout says that the temperature is greater than zero when the thermistor is unplugged (or definitively open-circuit) then something is not working the way it should. The logic inside the temperature code (temperature.cpp) is rather large and complex and seems to involve the possibility of an "ambient" thermistor as well as the extruder, bed, and PINDA thermistors, although I am not aware of an ambient thermistor in Prusa printers.

I have had my printer declare a temperature fault when the print fan cooled the extruder temperature more than five degrees below setpoint for more than a few seconds, so I know that generally speaking there are multiple fail-safes built into the firmware.

It's crucial for us to figure out if your printer keeps running happily with the thermistor unplugged. That would be a serious safety issue and needs to be thoroughly understood and fixed. It could be a defect on the board that generates 100k of resistance across the terminals, or it could be a newly-introduced bug in the V3.5 firmware.

geoff.b2
Posts: 12
Joined: Sep 14, 2017 1:51 pm
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 4:44 pm

Right. I've just unplugged the thermistor. The temperature reads 20C. Turned on PREHEAT with the thermistor still disconnected, the printhead and bed both started heating but of course the indicated temperature didn't rise.
Blowing warm air into the electronics box causes the indicated temperature to rise, even with the thermistor disconnected. The temperature indicated seems to be the ambient in the box - as if there was a temperature sensor contained in the electronics.

RetireeJay
Posts: 216
Joined: Feb 13, 2018 10:47 pm
Answers: 4
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 7:14 pm

You said that this is a recent problem. Can you correlate it with any particular event besides the cable breaking? Did the printer work fine before you re-flashed the firmware to 3.5? Can you try re-flashing it with 3.4 or earlier to see if the problem goes away? Did you build this from a kit or buy it pre-assembled? What kind of board is used for the controller?

Obviously you can't use your printer until you get this figured out. If the indicated temperature is greater than zero when the thermistor is unplugged, then you can't trust it to be correct when the thermistor is plugged in.

geoff.b2
Posts: 12
Joined: Sep 14, 2017 1:51 pm
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 9:52 pm

I have never had the thermistor unplugged or disconnected before, so I wouldn't have noticed anything unusual until the lead failure.

The printer was a Mk2s kit, bought end of September 2017, and upgraded to Mk2.5 in April 2018. The board is whatever was being supplied with the Mk2s in September 2017.

The printer has performed faultlessly since I had it both before and after the upgrade, until the recent faulty thermistor lead and I have had no reason to think that the temperatures were outside normal tolerances. All materials seem to print normally.

I have just checked the heat-block with a thermocouple thermometer and it measures about 209 degrees when set for 215 degrees; given that the probe has to be fixed on the side of the block that seems to me to be reasonable.

I haven't yet tried re-flashing with 3.4 but 3.5.0 and 3.5.1 both behave the same. I'll try 3.4 tomorrow.

RetireeJay
Posts: 216
Joined: Feb 13, 2018 10:47 pm
Answers: 4
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 19, 2019 10:22 pm

Based on the Prusa website documentation for the MK2S, your board is a "Mini-Rambo". I've looked at the schematic (https://reprap.org/wiki/File:MiniRambo1 ... ic-pg4.svg) and I don't see anything that would normally prevent the voltage at the input of the Atmel A/D converter from going to full scale when the thermistor is unplugged. Which should correspond to an indicated temperature of 0C. However, I note that all the thermistors have a common "reference" voltage, and maybe if that's not where it should be it could cause the erroneous 20C reading. If that is the case, then if you unplug the bed thermistor the bed temperature reading should also go to 20C instead of zero (or at least the extruder and bed temperatures should track each other if both thermistors are unplugged). Your board has three thermistor sockets; are all three in use?

geoff.b2
Posts: 12
Joined: Sep 14, 2017 1:51 pm
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 20, 2019 9:54 am

I think I have tracked this problem down: the thermistor D12 which provides the reference voltage is very temperature sensitive - a brief shot of freezer spray sends the temperature reading down rapidly, enough that with the thermistor disconnected I get a min temp error, which is as it should be.

I'm debating the difficulty of replacing this diode or whether a new board is going to be needed. seems a shame to replace the whole board for the sake of a component costing a few pence!

Notwithstanding the above, there would seem to be room for improvement in the failure detection. Consider: the printer is printing happily away when the measured heat block temperature suddenly drops from 210 degrees to 27 degrees. The Noctua fan controller says "Aha! the heat block is cold, I'll stop the fan". The extruder controller says "Aha! the heat block is cold, I'll stop trying to extrude". Meanwhile the X, Y and Z drivers carry on merrily as if nothing has happened.

geoff.b2
Posts: 12
Joined: Sep 14, 2017 1:51 pm
Contact: Send private message

Re: Danger: Extruder thermistor failsafe didn't work

Jan 20, 2019 10:15 am

Thinking about it, either way I'm going to have to take the board out so I'll try replacing the diode first and if I don't have any luck I'll order a new board.

Return to “Hardware, firmware and software help”

Who is online

Users browsing this forum: No registered users and 1 guest