Failure to detect open-circuit thermistor #3
Description
I don't know if this is the right place to report this but I have found an issue with the design of mini-Rambo board supplied with original Prusa i3 Mk2s where the board would not detect an open-circuit heat-block thermistor, thus risking overheating or worse.
The problem is caused by the voltage on zener diode D12 being too low due to insufficient zener current and/or device tolerances. On my board the voltage was about 4.4V rather than 4.7.
A new diode did not help but increasing the zener current to just over 3mA by reducing the value of the 10K dropper gives the correct voltage. The thermistor failure is now detected.
I started a thread in the Prusa Research support forum here:
https://shop.prusa3d.com/forum/hardware-firmware-and-software-help-f44/danger-extruder-thermistor-failsafe-didn-t-work-t27201.html