Hot End Thermister Problem

Hi,

The temperature on the hot end is reading incorrectly. I get a reading of -2.7 where it should be room temperature.
I have checked the wiring and everything seems to be fine. I've unplugged the thermister and immediately got an error and a -275 degree reading. I also put my hands on it and the temperature went up to 9 degrees... So it seems that it is working. Any ideas what might be wrong?

I have the same problem too. I suspect that the thermistor can only work accurately when it heat up to higher temperature, at room temperature it will operate at unlinear region. Mine shows 12 degree, and my actual room temperature is 27 degree. I believe it's normal.

Have you been able to print successfully without the hotend overheating or underheating?

Mine not printing yet , still stuck at commissioning stage. ( Just received the Ormerod 3 days ago) I did check with temperature using industrial thermometer. hotend temperature seems quite accurate .

My hot end thermistor also reports incorrectly at room temp, often between -28° and +6°.

It does however print fine, I usually print with the hot end at about 185°c

Matt

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Limited Edition Red RS Ormerod 1 #144 of 200 - RRP 1.09fw
iamburnys Ormerod Upgrades Github
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The cause of a wrong thermistor reading is most probably with the connector. I just put the thermistor directly to the duet using jumper wires and it reads the room temperature just fine. So try improving the connection or use a separate cable for the thermistor.

I think voltage drop along the cable, and added resistance of connectors, effects the reading at low temperature, when the resistance of the thermistor is at it's highest. Especially as we're using 3.3V. Check connections are good, that pins are not pushed out of the crimp housings.

Ian
RepRapPro tech support

Also, update the firmware on the Duet board if you have not already done so. The current version is 0.57a. Firmware earlier than 0.53 gave less accurate temperature readings.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

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

My thermistor also underreads at room temp, which led me to do some testing yesterday [forums.reprap.org] dmould did similar tests and got results that better accorded between his thermistor and thermocouple, which I presumed to be due to different thermocouple placement -I'll try cleaning the thermistor legs and plugging it in direct (as adsoftware reports above) to the duet and see if high contact resistance is the cause of my offset too! [edit you can see in my table that my low end temperature was way off around room temp, it actually drops to -20 odd at 15°C, but I'd put that down to quantization error and I'm not really worried about accuracy that far from working temp]

Ray

Edited 1 time(s). Last edit at 01/18/2014 07:00AM by rayhicks.

The resistance of the thermistor is 100K at 25C, so it's most unlikely that you would get a high enough contact resistance to affect the reading. OTOH I had my reading drop to -273 a few times, which indicates open circuit. I had to tighten up the receptacles in the 6-way socket to cure this.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

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