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I have an old HBOT printer (now they call themselves Zmorph).

I had to replace the old hotend. From the looks of it, what I need is a J-head MkV type nozzle (from RepRap wiki).

I've bought and put together the entire hotend but unfortunately when I've tried to heat up the nozzle to 150 °C for a test run, it started to heat up very quickly (compared to the old hotend) and didn't stop at the temperature I've set. I've quickly turned off the heating at around 200 °C, but the temperature still managed to climb up to 280 °C from inertia alone.

Thermistor shows the temperature correctly so its probably not a typical thermal runaway situation. I've measured the resistance on both the old and new heater cartridge and since I've never held a multimeter in my hand before, I need help with interpreting results. Basically it turns out that:

  • old cartridge: resistance=14.5 Ω which means it should be: voltage = 24 V and power = 40 W
  • new cartridge: resistance=6.5 Ω which means it should be: voltage = 12 V and power = 20 W

So, it stands to reason that if I've put a wrong heating cartridge into my hotend I will get:

power = (voltage^2)/resistance = (24^2)/6.5 = 89 W

instead of the correct value I got with the old heating cartridge:

power = (voltage^2)/resistance = (24^2)/14.5 = 40 W

My heating cartridge gets more then double the power it needs (89 W instead of 40 W), so naturally it overheats

A logical solution would be to simply buy a new ceramic heating cartridge with correct parameters.

enter image description here

Does my reasoning make sense?

0scar
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Piotrekdoro
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1 Answers1

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Does my reasoning make sense?

Yes this makes sense, the current PID schedule is not able to cope with the higher power cartridges. It remains a question if you will get it stable after a PID tuning session as the heat input is very large.

See e.g. this question with this answer, which is simiar to the behavior you describe.

I have had similar issues when I mixed 24 V and 12 V cartridges in my spares supply and installed the incorrect one by mistake. I was able to get this sort of stable by lowering the power factor of the heating cartridge in firmware, but this is unsupported in the latest version of Marlin. You might have better luck with the "Model Predictive Control for hotend" model. Alternatively, and the best solution (I ended up doing as well) is inserting the correct heater cartridge.

0scar
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