Melted Hot End

Hi all.

I have a reprap mendel, bowden hot end. And boy do I have problems. The other week i started the printer and was letting everything get up to temp for the first print of the day. Rather than sit and watch it, I went upstairs to take care of some quick buisness, a few minutes maybe. When I came back down, the hot end aluminum extruder was a puddle laying on the smoldering tape and cracked hot bed glass. It got so hot that it melted the aluminum into a puddle and dropped from the print carriage. I am lucky the house didn't burn down. Now, I am at wits end with this thing. I have had her built for a few months now. I have been printing with it. Not well, but printing none the less.

So anyways, i'm thinking the 100k thermister went bad and temps spiked. So I just now ordered and installed the new hot end, all new parts. I connected to the printer with ponterface and began testing temps on the hot end. Well, the thermister temps were very very slowly rising. I didn't see it get over 100c on the ponter face moniter. When I leaned over to inspect the hot end, the heater resister was glowing! This thing really needs an emergency stop switch. Needless to say, I shut'er down before any serious damage or melting metal occured. Well anyways, I figured, with my luck in electronics, another bad thermister. Put a new one in, same thing happened. I put a third 100k thermister in, this one from a diffrent manufacturer, and got the same result. So it is a repeatable problem, not thermister dependant.

I can ground out the two thermister wires and the temp reading in ponterface will spike, so a signal is getting from the printer to the computer. I tried applying heat to the thermister and not much happened. If temp was regestering and rising, it was doing so slower that I could observe. Also, I am not sure if the hot bed temperature is regestering properly either, for what it is worth, maybe it could help narrow down a common problem area.

Can anyone please help me out? Could something have gone wrong with the controller board? I really don't know what else to check and I don't understand what would have caused it.

Thanks for the help in advance!
John



When I connect to the printer, this is what displays in the console:
Connecting...
start
Printer is now online.
echo:Marlin 1.0.0 RC2
echo: Last Updated: 2012-06-25-1 | Author: RepRapPro
echo: Free Memory: 11678 PlannerBufferBytes: 1232
echo:Using Default settings:
echoteps per unit:
echo: M92 X91.43 Y91.43 Z4000.00 E875.00
echo:Maximum feedrates (mm/s):
echo: M203 X300.00 Y300.00 Z3.00 E45.00
echo:Maximum Acceleration (mm/s2):
echo: M201 X800 Y800 Z30 E250
echo:Acceleration: S=acceleration, T=retract acceleration
echo: M204 S1000.00 T1000.00
echo:Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum xY jerk (mm/s), Z=maximum Z jerk (mm/s)
echo: M205 S0.00 T0.00 B20000 X15.00 Z0.40 E15.00
echo: M206 X0.00 Y0.00 Z0.00
echoID settings:
echo: M301 P3.00 I2.00 D80.00 W100
FPU Enabled no
transform correction not enabledechoD init fail

Does It read room temperature at room temperature?

Ahhh. Good question. No. It is reading 83C. Hmmmm Got so frustrated I didn't notice this.

It reads 83C even with the thermister disconnected.

If you plug the Hotend thermistor into the heated bed thermistor port, what does it read at room temperature?

When i switch the hotend thermister wires to the hot bed connections on the control board, it gives a reading of -10C. Wow.

Do they have the same thermistor setting in the firmware?
Assuming the thermistor reads something sensible when you measure the resistance.
Either the firmware has the wrong thermistor setting, or the input on the arduino is bad.
If the input is bad, you can peobably change the pin condiguration and use a different one. THough it's possible more than one input is damaged.
The bigger issue would be what damaged it in the first place, the most likely cause would be a short between the thermistor pins and the heater.

To be honest, I do not know about the thermistor setting in the firmware. I have the Melzi board that came with everything preloaded and it worked for quite awhile before the hot end hit critical mass. I can see where its possible that the thermister end leads might have shorted out on the heater block, but since it is a puddle of aluminum now, the evidence of that happening has dissappeared.

Is is possible that a shorting out termister wrecked the board? There is no visable burned damage to it. Not really sure if I should attempt reloading firmware, thats the only thing i can think of that may help. Other than ordering a new board, I'll have to wait till I get home. What do you mean when you say change the pin configuration?

Thanks again for the help so far!
John

If the thermister shorted to the power on the resistor, yes it's possible the board was damaged, it's possible to damage just some pins on the Arduino.
If that is the case you can sometime reconfigure the firmware to use say the second thermister input for the hotend, and work around the damage.
It would require modifying pins.h in the firmware.

First you need to figure out what is not working, the best way to do this is to test things in isolation.
Measure the resistance of the thermister at room temperature.
Connect known resistances to the thermister inputs (the thermister itself is fine for this if it is good) and verify they read appropriately (this depends on firmware settings).

Dear John,

I just had a similar issue with my emaker Huxley. Fortunately I noticed the wrong temperature reading before turning the thing on so I did not melt my hot end and broke the glass. After a careful comparison of my ATMEGA644P to another one I noticed some weird resistances on some pins, so I replaced it, just to break the new IC too.

So, basically, if you do not insulate the wires around your hot end properly, you may short the heater and the resistor. This causes the thermistor to get a much higher voltage than intended, which burns the microcontroller on the board. I think it's reference voltage gets scewed up and all of its ADCs start failing. In my case, no matter what I did to the thermistor (having it connected, having it disconnected, shorting the pins, etc) resulted in completely random and different readins. Even with thermistors off it was reporting 6C on the bed and 190C on the hot end.

I ended up replacing the microcontroller and insulating the thermistor wire properly so that it cannot get into contact with the resistor. As there is almost nothing between the thermistor and the ATMEGA (at least on my Sanguino), only the microcontroller could be and had been damaged.