M303 PID Tuning Fails - "bad curve fit"

[Tryptamine]

Hey guys,

My printer is using a 12V heating patron on a 24V PSU.
Suboptimal, i know, but hard to change due to the design of my printer.
According to ohms Law, the heat output is 4 times higher as if used with 12V, which i could compensate for with a PWM Duty cycle of 0.25.

I used RepRap 2.x before. Back then i was able to autotune the Heater and it was completely stable.
In RRF 3.x this changed.

I updated firmware and PID tuned, which oddly worked back then and lead to stable temps. Settings were:

M307 H1 R10.72 C150.0 D3.34 S0.60 V24.0

Today the temp overshot by 20ish Degrees. So i tried to autotune again and again, using

M303 T0 P0.5 S220 A24

and i always get

Warning: Auto tune of heater 1 failed due to bad curve fit (R=10.395, 1/C=0.0067:0.0127, D=3.3)

Also for P of 0.3, 0.6, etc.
Using the report I calculated C to be 113.996. With the settings

M307 H1 R10.395 C113.996 D3.3 S0.50 V24.0

I get a perfectly flat curve at 220°C, if heated from like 200. If i heat from ambient, i overshoot by 20 degrees and the printer shuts down.

Why so?
Are the M307 settings of config.g used when i run M303?
Can i somehow fit a model by myself?
I can't find a proper explaination for R,C,D and how they relate to P,I,D so i can maybe tune them manually.

Any help is greatly appreciated.

[Phaedrux]

@tryptamine said in M303 PID Tuning Fails - "bad curve fit":

My printer is using a 12V heating patron on a 24V PSU.
Suboptimal, i know, but hard to change due to the design of my printer.

Suboptimal is an understatement. It's downright dangerous. Reducing the PWM doesn't turn 24v into 12v, it just applies 24v a quarter of the time it normally would. If the controller or mosfet were to fail in a closed state your 12v heater would probably melt your block and start a fire.

Is the heater really impossible to switch out? If so using a separate 12v supply and a relay would be a safer option.

[dc42]

@tryptamine, as @Phaedrux says running a 12V heater from 24V is very dangerous. RRF has estimated that the hot end would reach a temperature of over 1550C if the heater was run at full power. You should replace the hot end heater by a 24V one if possible; otherwise, use a 12V PSU and connect the heater between +12V and the negative output of the heater port you are using.

You could probably fool RRF into accepting parameters close to those, although I don't recommend it. You would need to reduce the R value to about 10 to bring the predicted temperature increase below 1500, and temporarily use M143 to make 1500C an acceptable temperature before the M307 command.