RE:
Hi, I believe I have the exact same problem. After a few minutes of heating, I can see that the LEDs indicating power to the bed and to the heater goes out. Sometimes the printer also registers a power panic and resets itself.
What is the easiest way to measure nAC_FAULT?
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
You can measure from J19 connector pin 3 (have a look in the picture from my last post). When the bed is turned ON, voltage between GND and J19/pin3 should be greater than 3volts (near to 5Volts).
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
Ok, I see. So what's happening is basically that the nAC_FAULT signal voltage is low enough to close the AND gates (and this is why the heaters shut down), but not low enough to actually trigger the power panic input pin to the CPU?
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
Ok, I see. So what's happening is basically that the nAC_FAULT signal voltage is low enough to close the AND gates (and this is why the heaters shut down), but not low enough to actually trigger the power panic input pin to the CPU?
I dont know if the CPU should do something with power panic signal... I just can tell you that the problem in my case was caused by a low voltage in that signal and I solved it inserting a resistor in the connector... this modification is not the best way to run, there is a chance that the power panic should not work anymore.
RE:
Not sure exactly how it's detected, but I'm assuming the CPU is involved in the shutdown procedure that happens on power loss (saving current state of print), as well as sounding the loud beep.
Anyway, I tried your fix and now my printer works fine, so thanks a lot! 🙂 🙂 I will check if power panic/resume still works. I have not needed the function myself since we actually have very stable power here where I live so I won't be too upset if it doesn't work.
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
Not sure exactly how it's detected, but I'm assuming the CPU is involved in the shutdown procedure that happens on power loss (saving current state of print), as well as sounding the loud beep.
Anyway, I tried your fix and now my printer works fine, so thanks a lot! 🙂 🙂 I will check if power panic/resume still works. I have not needed the function myself since we actually have very stable power here where I live so I won't be too upset if it doesn't work.
Thanks a lot Mhacker, Now I know Im not the only one with this isuue!!
I ordered a main board replacement just in case (quite expensive =o )... but up to now it was not necessary to replace it. More than 2 years running with the "resistor fix" so it seems quite reliable... hahah
Kind Regards
Juan Gabriel Vazquez
Puerto Madryn, Patagonia, Argentina
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
My MK3 has been collecting dust for months because of this issue, and a simple resistor in a dupont connector made it work like new again. Thank you for figuring out this issue and taking the time to document it with diagrams and photos. My nAC_FAULT was 2.7v, and I felt such a relief measuring the voltage at 4.5v after installing the resistor! I've been running it for over a week now with no issue, when before it would stop heating and panic while just preheating. Now to give the machine a much needed cleaning.
This has driven me nuts for such a long time. Thank you.
Seth
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
My MK3 has been collecting dust for months because of this issue, and a simple resistor in a dupont connector made it work like new again. Thank you for figuring out this issue and taking the time to document it with diagrams and photos. My nAC_FAULT was 2.7v, and I felt such a relief measuring the voltage at 4.5v after installing the resistor! I've been running it for over a week now with no issue, when before it would stop heating and panic while just preheating. Now to give the machine a much needed cleaning.
This has driven me nuts for such a long time. Thank you.
Seth
Happy to know that!!!
I just received another MK3 with the same issue. Im starting to suspect that this is related to RV27, that seems to be a component for voltage protection that is failing. Im wondering if it's possible to remove RV27
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
That is great news.
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Chuck H
3D Printer Review Blog
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
I had this happen today on my MK3S+ running 3.11. happened while printing from SD with cable disconnected too.
I did a factory reset after reading this and another post that @cwbullet is in. I am printing now and it went past the time where I would get the M112.
finger are crossed.
RE: Hotend temperature drop down till trigger THERMAL RUNAWAY
I had the same error messages for particular models that contained large continuous surfaces. The fan speed increased too much and the PID / heater could not compensate for the higher level of cooling. Hence causing the the thermal runaway error when the hotend temperature dropped to below 15°C the set temperature.
I followed the advice of another forum member (Jimbo70) and reduced the max fan speed in the filament settings to 50% and the minimum fan speed to 15% for PLA in the PrusaSlicer. Both were originally set to 100%. This change seemed to do the trick for me. No need to change any board or add a resistor in my case.
Thanks to Jimbo70: https://forum.prusa3d.com/forum/postid/45095/
Give it a try before soldering or changing of the main board.