Warning: Heater 1 appears to be over-powered

[bondus]

I just install one of those tiny ceramic heaters without a large heat block.

After a PID tune (M303 H1 S250) I got the message:

Warning: Heater 1 appears to be over-powered. If left on at full power, its temperature is predicted to reach 832C.
Auto tune heater 1 completed in 180 sec
Use M307 H1 to see the result, or M500 to save the result in config-override.g

832C sounds pretty scary!

M307 H1 gives:
Heater 1 model: gain 807.1, time constant 130.9, dead time 3.8, max PWM 1.00, calibration voltage 23.9, mode PID, inverted no, frequency default
Computed PID parameters for setpoint change: P7.6, I0.340, D20.3
Computed PID parameters for load change: P7.6,

The thing heats up really fast, even if it's just a 40W heater. The thermistor is mounted in a little hole right in the nozzle and should get good readings. In theory this setup should work very good, it's light and reacts fast.

I realize that if something goes wrong it will go bad pretty fast. But the firmware seems to have a lot of protections to keep us safe. Even switching a nozzle hot tends to trigger an emergency shutdown.

Should I be worried?

[T3P3Tony]

@bondus

The firmware is doings its best, with the information you have given it, to build a model of how the hotend will behave. That model depends on the inputs (so you need to have the correct Beta for this thermistor).

If you are sure that the thermistor is set correctly and the temperatures the firmware is seeing during the PID tune are accurate then the model will be relatively accurate. That means that if there is a fault that causes the heater to receive 24V solidly it could reach higher than the melting temperature of aluminium. For example the heater mosfet could fail short - the firmware would not be able to turn it off, it report a fault but if you are not around that would not stop the hotend from continuing to heatup.

With such a tiny heated block, a 40W heating element is probably overkill!

There are discussions on this forum about fitting a thermal fuse of some description that might help reduce the risk.

[bondus]

Overkill is a usually good thing, as long as it is controlled

I can imagine a scenario where the thermistor falls out of the little hole and shows a much lower temp than the actual temp. It's just stuck into a little hole held in place by the heat isolator. It would be safer to glue it in there with some thermal glue.

[T3P3Tony]

@bondus that scenario should be picked up by the firmware as a heater fault is it would be commanding a power increase and not seeing it.

[sinned6915]

bondus-

i was using the same hot end heater- be careful with the brazed power connections. those went bad on me, twice. i am thinking of brazing on a support arm to hold them now if i get another one.

iirc, i changed the high temp on the hot end and redid the PID tune and the error went away.

i reset the high limit after i tuned.

what i think happens is it is increasing the temp so fast, it overruns the limit and throws the warning before it can wait for the temp to come down.

[zapta]

@bondus said in Warning: Heater 1 appears to be over-powered:

The thing heats up really fast,

This is also why it can reach that insane max temperature.

The risk is if the power mosfet will fail and become a short. The CPU will not be table to turn it off unless it can shutdown the power supply.