[Solved] Thermal rundown, mintemp, extruder thermistor  

Thermal rundown, mintemp, extruder thermistor

Hi Folks,

I ordered the MK3S printer some time ago. Assembling it was a nice
experience due to your excellent manual 😉

However, I unfortunately do have some issues.

I can not get the first layer quality to be similar to some other similar prusa printers my friends have particularly with the TXT sheet. PEI quality is better, TXT does not stick "at all" unless pressed way "too thin".

But let's skip this quality issue for now since I have a more severe issue, which actually may be the reason for the first layer issues.

When the blower starts (on second layer) the printer enters THERMAL RUNDOWN after the extruder temp drops rather quickly from 215 to around 190 and then stops.

After resetting, the printer enters MINTEMP condition and remains there.

I suspect that the extruder thermistor has issues.

At room temp 22 C the BED temp is shown to be 22 C. The extruder temp is at the same time shown to be between 0 C and 4 C (after power up, printer is fully room temp).

The EXTERUDER NTC thermistor resistance is (at 22 C) about 490 Kohm, which appears to be way off the nominal 100 Kohm@25C.

I do not know if the error is linear or not, but the extruder temp easily gets to 215 when heated for PLA when doing first layer wizard; but as said I have "first layer" issues. I unfortunately do not have a suitable thermometer to verify the actual extruder temp at this point.

Please comment on this and/or if there is anything I can do to fix this condition?

Or help you to figure out the reason if you think it is not a failure in the thermistor by measuring/analyzing something?

The MINTEMP condition is rather persistent but it gets solved either by turning off the printer for e.g. 24 hours. Then the same repeats again as above when trying to use the printer ;-(

OTOH, if I actively try to resolve the MINTEMP like above e.g. by factory reset: printer enters self test.

At some point in the sequence the self-test starts BED testing: now BED TESTING fails even though there is nothing wrong with the BED or BED thermistor. It seems like the self-test is NOT starting to heat the bed at all. Possible because the extruder thermistor causes an error condition. Instead self-test now reports a BED error.

This is slightly misleading, so please consider fixing it.

I verified the above by warming the EXTERUDER thermistor with a hair dryer during the BED self test => now the bed self test passed OK, since the extruder thermistor is warmer and appears in correct range: no MINTEMP reasons 😉

I was able to print an object like the PRUSA text plate by just turning the blower away from the bed; but I do not think this is a long term solution particularly since I need to try to solve the first layer issues.

Thanks much for any help!

BR,
Juki
[email protected]