Extruder Thermistor fault again

Hi all.
I have reading a lot about the issue of getting -273 from hot end thermistor of my Ormerod, but can't figure out how to fix it. I'm sorry!
I have soldered the wires, testing the resistance, and it is all ok in the connections.
I have firmware 057z-dc42. Is there anyone that could help me explaining the procedure to fix this fault, to avoid the use of a cigarette lighter to warming up the extruder all the time I want to print?
Many thanks!
Rick.

Hi Rick,

Do you mean that you are getting a reading of -273C when your thermistor is connected but the hot end is cold?

If yes, what is the approximate room temperature, and what resistance does it read when you get the -273C reading? Do you get sensible readings from it when it is not reading -273C?

If no, what temperature reading do you get when the thermistor is (a) cold, (b) disconnected?

---

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].

Hi dc42 and thank you.
Yes, when I'm connecting my Ormerod the hot end temp is -273. The room temp is about 18° - 19° C.
The resistance is 127 Kohm at 18.5° room temp.
If I warming up the extruder with a little flame (not in contact) then after disconnecting and reconnecting the duet, I'm reading a "real" temperature (about 40 or 50°) and I can set the hot end temp to any values I want.
Now printing is possible, and there is no problem in the extrusion process. The object printed is fine!
I noted that when the hot end is cooling down I read values falling down until about -14 than suddenly I get -273, and no way! I must warming up as above, disconnect and reconnect to get a real value.

Many thanks.

Rick.

Rick, you appear to be one of the unlucky few whose ADC in the Duet microcontroller has a large positive high-end offset. Unfortunately, I cannot correct for this in the firmware without risking loss of detection of a disconnected thermistor. So the best suggestions I can make are:

1. Ask RRP to replace your Duet board.

2. Replace the 1K series resistor on the Duet board by a higher resistor e.g. 2K2 or 4K7. This requires SMD soldering and will probably invalidate your Duet board warranty unless you get prior approval from RRP. Then use the facility in my recent versions of the firmware to tell it the new ratio of 25C thermistor resistance to series resistance, and also correct for the high end ADC offset.

---

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].

Thanks a lot for your answer.

I'll try to contact RRP for the Duet board, but in the case of a home made fix, where is this resistor exactly?

Thank you.

Rick.

The resistor is close to the SAM3X chp and is labelled R19. However, I have thought of an easier way. Connect a resistor in parallel with the thermistor, e.g. soldered across the pins on the back of the Duet board. Choose a value such that with the resistor connected but the thermistor disconnected you still get a reading of -273, but with the thermistor connected as well and the head cold you get a higher reading, preferably within 10C of the actual temperature. 100K might be a good value to try. When you have found a suitable value, adjust the H parameter in the M301 command to correct any residual high-end offset, and adjust the R parameter to correct for the effect of the extra resistor. If you use a parallel resistor with a resistance of N kilohms, then you need to set R to 100000 * (N + 1)/N.

---

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].

Ok it seems to be much more easier! I'll give it a try a.s.a.p. hoping to sort it out.

Thank you so much.