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.

Edited 2 time(s). Last edit at 04/26/2021 09:15AM by Tryptamine.