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Difference between extruder MAXTEMP variable and temp you can set the hotend to

Posted by Brien Allison 
Difference between extruder MAXTEMP variable and temp you can set the hotend to
November 22, 2018 08:11PM
Just a quick question about the maxtemp variable. When this is set to a given value, when I heat up my hotend manually it can only reach a value 15 degrees below the MAXTEMP setting. I just wanted to make sure this is an intended feature in Marlin and not a bug where it's displaying the incorrect temperature or something. Looking into the documentation on the MAXTEMP variable I'm guessing it is an intended feature since it shuts down the machine after reaching MAXTEMP, which would be very impractical if you'd just set the printer to that temperature.

I've just installed an E3D Titan Aero with a V6 hotend and the documentation said to set the MAXTEMP to 285 in the marlin firmware setup guide. However, earlier on in the installation guide it says to set the extruder temperature to 285 in order to hot tighten the hotend, but with this 15 degree difference it can't reach that, hence my confusion. This 15 degree difference is something which should probably be updated on the E3D guide page, but it would also probably help if that was mentioned in the Marlin documentation on MAXTEMP values.
Re: Difference between extruder MAXTEMP variable and temp you can set the hotend to
November 23, 2018 01:48AM
Yes I think with any pid controller if you want to reach the desired output quickly and not wait for ages you will have to overshoot it first, so it makes sense to have a 15 degree so that the machine doesn't halt when trying to heat the head. I didn't check the documentations yet but this makes sense to me.
Re: Difference between extruder MAXTEMP variable and temp you can set the hotend to
November 23, 2018 02:43AM
It's entirely possible to design a PID controller that doesn't overshoot significantly. However the PID constants you need to do that are not optimum for maintaining the temperature once it has been reached. That's why RepRapFirmware calculates two different sets of PID parameters from a model of your heater, and switches from one to the other when the temperature is close to the target.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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