I've just had my first uncontrolled heating...result of new firmware?

Yesterday I updated from 65j to 78e version of DC's firmware and all looked good, then today I had a mishap during a print when a thermistor lead came off during a print and the printer went into an uncontrolled heat up...luckily I discovered it within 5 mins, but by then smoke was pouring from the hot end - very scary. No long term damage seems to have been done, but I have now found the problem to be that when the thermistor wire came off, the temperature showed as 1.4C, so the heater came on to attempt to bring it up to temp. I now see that even with all the printer back together the room temperature on the hot end is showing as approx. 10C above where it should be (I've never seen this before the firmware upgrade) and still only circa 1.4C with thermistor disconnected. This may be a complete co-incidence that this has happened straight after the firmware update, but thought I would raise it given both the seriousness of the problem and as a plea for help on what might be going wrong here and how I can correct the thermistor temp reading back to normal. FYI I am using DC's 2nd generation board

Hi MrCrispi, that 1.4C reading with the thermistor disconnected is way out, and explains why the thermistor-disconnect mechanism didn't kick in. When my current print finishes, I'll try disconnecting my thermistors and see what reading I get.

Do you have any M305 commands with H and/or L parameters in your config.g file to correct the Duet's ADC? If so, then you probably have the wrong values for your board.

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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].

Thanks @DC - currently I've not got any M305 commands in my config.g file - for completeness here it is:

M111 S0 ; Debug off
M550 PMy RepRapPro Ormerod ; Machine name (can be anything you like)
M551 Preprap ; Machine password (currently not used)
M552 P192.168.1.14 ; IP address
M553 P255.255.255.0 ; Netmask
M554 P192.168.1.254 ; Gateway
M555 P2 ; Set output to look like Marlin
G21 ; Work in millimetres
G90 ; Send absolute corrdinates...
M83 ; ...but relative extruder moves
M906 X800 Y800 Z800 E1000 ; Set motor currents (mA)
M563 P1 D0 H1 ; Define tool 1
G10 P1 S0 R0 ; Set tool 1 operating and standby temperatures
M92 E420 ; Set extruder steps per mm
M558 P1 ; Use an unmodulated Z probe
G31 P500 Z0.6 ; Set Z probe height and threshold
M557 P0 X60 Y40 ; Four...
M557 P1 X60 Y180 ; ...probe points...
M557 P2 X210 Y180 ; ...for bed...
M557 P3 X210 Y40 ; ...levelling
M556 S78 X0 Y0 Z0 ; Put your axis compensation here
M201 X800 Y800 Z15 E1000 ; Accelerations (mm/s^2)
M203 X15000 Y15000 Z100 E3600 ; Maximum speeds (mm/min)
M566 X600 Y600 Z30 E20 ; Minimum speeds mm/minute
M208 X214 Y210 ; set axis travel

I've just disconnected both my hot end thermistors (one is connected to my differential LED hot end board, the other just to the second hot end loom) and both read -273.1C as expected. So I think your 1.4C reading is down to the offset error in your ADC and nothing to do with firmware. Have you previously tried disconnecting the thermistor and seeing what reading you get?

Do you find that your hot end reads significantly above the actual room temperature? If so, that confirms it.

You can correct for the ADC high end offset using the H parameter in the M305 command. Use P1 to select the first hot end heater. You will need to use a negative value for H to reduce the indicated temperature. There are a few ways to find the correct value:

1. If you have a 100K resistor (preferably 1% or 2% tolerance), connect it in place of the thermistor and adjust the H value to get a reading close to 25C.

2, With the heater block at room temperature, use an accurate thermometer to read room temperature, and adjust the H parameter so that the Duet temperature readout agrees with it.

3. With the thermistor disconnected, find the highest (least negative H value that gives a reading of -273.1, then subtract 0.5 from it. Caution: once you have a reading below about -30C, a temperature fault condition will be triggered and the reading will be locked until you send M562 P1 to reset the fault condition on heater 1.

Edited 3 time(s). Last edit at 07/28/2014 06:47PM by dc42.

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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].

Thanks again @DC42 - good at least to know that it's not a problem for everyone else...as per my last post, hot end temp reads about 10C too high at room temp, so will have a play with M305 to see if I can get it to a point where the fault safety mechanism cuts in if the thermistor gets disconnected...I don't want to risk an uncontrolled overheat like that again as I regularly leave my printer unattended for relatively long periods and this rocks my confidence in doing that

I've just re-read your original post and this bit bothers me:

Quote

I now see that even with all the printer back together the room temperature on the hot end is showing as approx. 10C above where it should be (I've never seen this before the firmware upgrade)

I don't see why your room temperature reading should have changed. So here are some suggestions:

1. Send M305 P1 T100000 R1000 B4200 H0 L0 to make sure you haven't somehow got some adjustments stored in flash, and see if the readout with thermistor disconnected or thermistor at room temperature changes.

2. Disconnect the thermistor wires at the Duet end and see what the disconnected reading is, to rule out a problem on the hot end board.

3. Temporarily roll the firmware back and see what the disconnected and cold temperatures read.

4. If none of the above changes the disconnected reading from around 1.4C, disconnect the bed thermistor and see what its disconnected reading is.

Edited 1 time(s). Last edit at 07/29/2014 03:04AM by dc42.

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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].

Sorted Inserting a M305 P1 H-41 sorted out the hot end to approx. room temperature and M305 P0 H-70 did the same for the heated bed. Both now do indeed register -273 when disconnected so safe again. Many thanks @DC42

If you want me to do some experiments rolling back the firmware to confirm whether or not it was the same prior to the firmware update, I'm happy to - just let me know

I'd prefer it if you could do items 2 and 3 on my list above (with the H factor correction set to zero), because it seems strange to me that without correction, the reading is now 10C too high but it wasn't before.

In theory, you should need the same correction factors for both the hot end and the heated bed. There may be some small residual differences caused by the thermistor tolerances.

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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].

Sure thing - will do tomorrow and report back...

OK - looks like I had the problem prior to the firmware upgrade and just hadn't noticed - rolled back to 65e and old config.g file and got more or less the same readings disconnected and connected. Unplugging at duet end also gave same results, so looks like no problem with new hot end board - still can't explain why I need such different M305 commands for hot end and bed, but at least its working again and now I know the safety override will kick in...many thanks @DC42

Thanks for checking!

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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].