Safety Problem with thermistor circuit

[RetireeJay46]

The clamping circuit for thermistor inputs uses a zener diode to theoretically limit the positive voltage excursion at the ATMega input to just a shade over 5.0V. However, there are two problems with the circuit: (1) the current through the 4.7V zener, 0.73mA, is not sufficient to put the zener fully into its rated clamping voltage. In practice, one user has seen 4.4V. This was solved by increasing the current to about 3mA, well above the "knee" for his particular zener.
(2) Even with sufficient current, some zeners with this part number will be as low as 4.4V due to manufacturing tolerances.
The problem is this: In either of the above scenarios, the analog voltage at the ATMega input is constrained to slightly less than 5V. (Note that the clamping diode is operating at a very low current, so its voltage drop added to the zener voltage is less than the canonical "0.6V". At 0.01mA, the 1N4148 has a forward drop of about 0.4V). If the input voltage at the ATMega is not allowed to go all the way up to VCC of 5V, then the A/D conversion will never return a full-scale reading. But if (when) the thermistor leads break, the firmware requires a full-scale reading in order to detect the break and shut down the associated heater. At room temperature of 20C the input voltage is 4.819V, and increasing slowly; 0C yields 4.930. The open-circuit voltage should always, unconditionally, be allowed to go above 4.930 regardless of zener tolerance and other part tolerances.

This concern is not theoretical. A user of a Prusa MK2S has actually experienced a thermistor break that did not trigger a firmware shutdown and did result in an extremely hot extruder.

Failure to trigger the firmware shutdown is a fire hazard.