Occasionally temp sensor defects - Tried several thermistors

Printer: Prusa i3
Controller: Teensylu 0.8
Thermistors: Several thermistors from different hotends.
Software: Repetier
Firmware: dunno. used to run for the last 1-2 years perfectly.

I have this problem that sometimes when I want to print the printer says "Extruder 0: temp sensor defect" and switches into dry run mode.
If I restart everything (e.g. power off the printer completely, remove usb, restart Repetier) it works again.
The temperatur readings are normal. The thermistor(s) work perfectly. Some prints print out how they should. I have tried several thermistors but all get the same message occasionally. This is very strange.

I just installed a new hotend including extruder (Sintron all metal extruder). I have printed successfully some parts. The temperatur is constant at 200°C while printing. If I measure the resistance of the thermistor on the connector it reads nothing.
My old hotend's thermistor from an E3D v6 which printed nicely before but also had this issue of occasionally temp sensor defects also reads no resistance.
My multimeter is ok. I have checked it with a third hotend and it reads 113.6kOhm.

Why can I someprint without temp sensor defects and sometimes it messes around?
Could my controller board fry the thermistors? I have checked the connections and there are no shorts.


UPDATE:

At 200°C the thermistor from the Sintron All metal extruder reads 35kOhm.

Edited 1 time(s). Last edit at 08/01/2015 06:52AM by ianmcmill.

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By reading "nothing" do you mean it reads zero Ohms, or does it read infinite, like an open circuit?

If it reads zero Ohms, the wires connection to the thermistor is shorted somewhere, most likely at the heater block. How is the thermistor held in the block? If there's a screw, the clamping pressure it is applying may be biting through the insulating sleeve and shorting out the leads. Inspect the insulting sleeving that should be covering the thermistor leads. If it reads infinite (open circuit) the wire connection to the thermistor or the connector at the controller board is probably at fault.

If it reads infinite resistance, the cable between the controller and hot end may be intermittent due to excessive flexure while printing. Since you said it's a new hot end, I assume you replaced the entire cable to the controller, but if you didn't, the cable may be the problem. If that proves to be the case and the problem goes away by replacing the cable, you'd better replace all the other wires that go to the extruder carriage because they, too, may be on the verge of failure.

Your controller board can't "blow out" a thermistor. Resistance is about 100k at room temp. You'd have to apply a couple hundred volts to it to burn it up. You're not doing that, are you?

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I am not applying couple of hundred volts No.

With "it reads nothing" I mean there is the same number on the multimeter as when both pins point into the their. I don't read 0 ohm.

I just finished another print and I updated my first post above. At 200°C the thermistor reads 35kOhm on the connector to the board.
Yes the thermistor on the new hotend is completely new. It came with the extruder so I assume this one cannot be damaged. But then again Murphey's law. But then again it just printed without problems. Despite the heavy oozing in the end (See [forums.reprap.org] )

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I realize the thermistor is new. What about the cable to the controller? Did the new unit comes with a full length replacement cable, or did you just connect the new thermistor to the end of the old cable?

What you're reading with the meter is called "open circuit"- i.e. infinite resistance. That means there's a bad connection somewhere. There are only 4 possibilities:

1) the pins on the circuit board could have a bad solder joint resulting in an intermittent connection. Careful inspection of the connections may reveal a problem. The solder should be shiny and smoothly connect the pins to the pads on the circuit board.
2) the cable /socket connection at the controller end of the cable is intermittent. Maybe the wire wasn't crimped properly, or it got flexed a few too many times and is now broken- take the metal inserts out of the connector and inspect the wire connections to them.
3) the connection to the thermistor is intermittent- is it crimped or soldered? Visual inspection is all you can do here. Again, solder joints should be shiny, and smoothly connect both wires.
4) the cable itself is intermittent because of repeated flexure. You won't be able to see it because the metal conductor can break inside its insulation. What you will see is that the intermittent connection is a function of the position of the extruder carriage. Hook up the meter to read the resistance at the controller end of the thermistor cable and move the extruder or even just move the cable that goes to the extruder around a little. If the connection opens and closes when you move the cable/extruder you have found the problem. You'll need to replace the thermistor cable and it would be wise to replace all the cables connecting to the extruder carriage.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

The thermistor came with a wire attached.

1) Solder joints look okay.
2) I crimped the connector at the end of the cable myself. I'll check on that. Maybe some bad connection there.
3) Wires to thermistor glass bead seem to be soldered. The connections are covered with shrinking tube.
4) Cable is okay. I just did a print without temp sensor defect issues. Also changing temperature while printing worked out.


Thanks for your help digital dentist!

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Filament Factory - How to build your own cheap filament extruder
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