MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
My MK3S worked fine for many hours. But now, the hot end thermistor seems to read too low when the printer is at room temperature. The bed thermistor agrees with a room thermometer, the hot end thermistor is 7-9 centigrades below which triggers the MINTEMP Error:-(
Heating the thermistor with some hot air works to avoid the MINTEMP error and after that, I can print and PETG (face shield frames;-) worked just fine. The fast gcode prints at 265C pretty hot. The thermistor seems to work except with the negative offset:-(
I got a replacement thermistor and before I started to install it, I just plugged it in and it seems to exibit the same negative offset:-(
Any idea??
Thanks,
-- Marco
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
Did you perform this calibration?
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
I don't think a PID tuning will help. Its purpose is to make a set temperature more stable not correct for an offset error.
I would recommend contacting Prusa support via chat to see if they have any ideas. It sounds like there may an error in the firmware but that is just a guess.
Jerry
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
Yeah, I looked at what the PID tuning would do and I agree with Jerry. I'm going to switch the hot end & bed thermistors on the board and see, what that will tell me.
-- Marco
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
I've been having the same error. The hotend temp is about 7-9 deg C lower than the indicated bed temp causing a MINTEMP error. A hair dryer to the hot end for a couple of minutes gets it up into normal range and I can print but I've noticed a lot of stringing I never used to get with the settings I use for PLA and PETG. I'm still a MK3 (I have the MK3S upgrade kit just haven't had time to tear it apart and do the upgrade yet.) I am running the latest firmware and that hasn't helped either.
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
I upgraded to 3.9.0-RC3 and at least that's pushed to MINTEMP error check to 10C, I believe. So I don't see the error any more. But the the temp is still below the bed (when everything is at room temp):-( So the upgrade did not help with that:-(
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
This is the reading with the connectors (bed thermistor <-> hot end thermistor) switched .
The connectors switched back and restarted.
A little thermometer as reference. The bed thermistor on the bed connector is pretty close. I also measured the resistance of the thermistors:
Replacement (not mounted anywhere yet): 108.5 kOhm
Bed thermistor: 104.9 kOhm
Hot end thermistor: 113.1 kOhm
Still not sure, what to make of it:-(
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
The thermistor circuit is a voltage divider from vcc to ground
the 4.7Kohm pullup resistor is in series with the thermometer and then the signal goes through another 4.7K current limiting resistor, to the analogue input pin on the processor there are two diodes to protect the input in the event of voltages outside the expected range and a capacitor to smooth the voltage in an electrically noisy environment.
the thermistor goes higher in resistance as the temperature drops. so effectively the the sense voltage goes higher as the temperature goes lower...
If something had changed in the input circuitry to lower the resistance to VCC, you could get the voltage difference that could cause erroneous results.
is it possible that the circuit has been subject to unespected voltages perhaps damaging the diodes?
is it possible that there is some contamination on the PCB causing a low resistance circuit between the sense wire and the 5 volt vcc rail?
the problem is that we have no idea what the effect of this inaccuracy is, at higher temperatures
these circuits are not linear, if you try to adjust the lower end a tiny bit, it may be possible that the working range would be a lot different to the expected temperature.
is it possible to test the thermistor in boiling water? that should read 100C unless you live at high altitudes where water will boil at a lower temperature
Possibly easier and safer is to put the extruder thermistor, on the heatbed, and cover it with a wadded up tissue, held in place with a weight, and then using the settings option, adjust the heatbed to a number of temperatures whilst comparing the reported temperature. see if the extruder thermistor tracks the bed thermistor... (it would be sensible to unplug the extruder heater whilst doing this just in case you applied power tot he extruder heater as there would be nothing controlling the heater whilst the thermistor was was on the heat bed...
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: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
I tried to build my own little firmware with the raw ADC values logged to the serial link (to a OctoPi) and they correspond to the C values:-(
Send: M105
Recv: ok T:13.4 /0.0 B:21.6 /0.0 T0:13.4 /0.0 @:0 B@:0 P:22.5 A:27.1 ADC B:21.6C->982.43750 Rb->428.22671 Rxb->15719.00000 T0:13.4C->995.18750 Rt0->431.75363 Rx0->15923.00000
Send: M105
Recv: ok T:13.3 /0.0 B:21.6 /0.0 T0:13.3 /0.0 @:0 B@:0 P:22.5 A:27.3 ADC B:21.6C->982.50000 Rb->428.24401 Rxb->15720.00000 T0:13.3C->995.37500 Rt0->431.80535 Rx0->15926.00000
Send: M105
Recv: ok T:13.5 /0.0 B:21.6 /0.0 T0:13.5 /0.0 @:0 B@:0 P:22.5 A:26.9 ADC B:21.6C->982.50000 Rb->428.24401 Rxb->15720.00000 T0:13.5C->995.06250 Rt0->431.71914 Rx0->15921.00000
Send: M105
Recv: ok T:13.5 /0.0 B:21.4 /0.0 T0:13.5 /0.0 @:0 B@:0 P:22.5 A:27.3 ADC B:21.4C->982.68750 Rb->428.29598 Rxb->15723.00000 T0:13.5C->995.12500 Rt0->431.73638 Rx0->15922.00000
Send: M105
Recv: ok T:13.3 /0.0 B:21.6 /0.0 T0:13.3 /0.0 @:0 B@:0 P:22.5 A:27.1 ADC B:21.6C->982.37500 Rb->428.20935 Rxb->15718.00000 T0:13.3C->995.31250 Rt0->431.78805 Rx0->15925.00000
I looked at the board and did not see anything obvious:-( Tried to blow it out with some canned air (with power off;-) and so far, nothing helped:-(
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
@joantabb
Morning from across the pond Joan,
I appreciated your write-up about thermistors and Prusa printers, especially the clever way to test the hot end thermistor.
Later I got thinking about your statement that there was a current limiting resistor in the circuit and that just did not seem right to me. The Arduino analog inputs are on the order of megohms and the diodes will limit voltages to a safe range so current limiting seems to not be needed. After thinking some more about it, it occurred to me that rather than limiting current, the resistor along with the capacitor forms an RC circuit for filtering the input.
What are your thoughts about that?
Jerry
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
@jerry-v2
Hi Jerry, that works for me (probably a better explanation), either way, the series resistors have relatively little effect compared to the high impedance of the input port
however it doesn't explain the different readings...
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: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
@joantabb
Agreed that is not a solution to the problem. I just wanted to have an understanding of how the circuit worked.
Given that thermistors are not linear, I see two other possibilities for the problem: 1. A batch of thermistors are not within spec and don’t match the linearization compensation; 2. The linearization is corrupted in some manner.
I don't how (or if) linearization is done so I'm not any help beyond proposing the possibilities.
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
@jerry-v2 thanks for helping to find the real problem.
To your 1.) I tried with two 'hot end thermistors' guarantied from different batches (the original 1.5 years + Prusa stock time old, one newly sourced as replacement in the US). I also connected the bed thermistor to the hot-end-thermistor plug (a little different curve but should be close around 20-25C) and all of them report lower than expected temperatures:-(
To your 2.) I checked the the raw ADC values against the the C-values and they seem to be consistent with the algorithm described in software. I also see the same issue with version 3.8.3, 3.9.0-RC3 and my own modified version of 3.9.0-RC3.
So, I'm running out of ideas except for trying to replace the board, or live with the offset for now 🙁
RE: MK3S hot end thermistor seems to read 7-9 centigrade to low?! - Replacement agrees ?!
OK, it took a very long time to get around to replace the board. First, I was printing face shield frames pretty mch 24*7, so the printer never cooled down (and I printed in PETG anyhow). Then, there were some other things to think about.
OK, the end result. The new board reports temperatures close together with the original thermistors. Not sure, what to make of it, yet. I looked at the original board and could not see anything in that area.