Heater Temperature Problems (thermistor shot?)

Hello,

After a few months of smooth sailing, I have come up against a small problem with the temperature readings in my extruder. In pronterface, I am constantly getting readings of 240-295 C when the extruder is cold to touch which obviously makes it impossible to print. I have been printing without problem for several months and my last print was a few days ago and went perfectly so this problem occured sometime during or after cooldown. I have tested the resistance of the thermistor and found it to be ~150k. The leads of the thermistor do not get warm so it is not an obvious short (I am a little hesitant to remove the silicone tape to check the connection at the thermistor itself as I do not have replacement tape). At this point, I'm not sure what my next step should be in terms of testing or diagnosis; any suggestions? Do I need to replace my thermistor entirely or could this be some other issue?

Thanks,

A

If it measures150K then it should report a temperature lower than 25C.

What does it report with the thermistor disconnected?

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[www.hydraraptor.blogspot.com]

If I fully disconnect the thermistor I get a mintemp error. Should the reported temperature be inversely proportional to resistance like that?

The higher the resistance the lower the temperature is. If you get MINTEMP with it disconnected then I don't see why 150K gives 240C. Are you sure it is not 150R?

Check to see if there is any connection between the thermistor and the heater.

The only fault that affects thermistors is leaking plastic getting onto the wires and reducing the resistance which causes the temperature to read high but also the resistance reads low.

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[www.hydraraptor.blogspot.com]

I retested the resistance of the thermistor and am now getting readings of > 1 mega ohm resistance so that is in line with the high temperature readings. I also bit the bullet and removed the silicone tape to get a better look at the thermistor. I also removed a small amount of putty which was obscuring the connections. Although I can't see any visible breaks from the outside, the resistance continues to be very high right at the point where the leads enter the thermistor. My conclusion is that something happened inside the thermistor itself. I'll go and buy a new one tomorrow and see if a replacement solves the problem.

Thanks for all the help,

A

Edited 1 time(s). Last edit at 08/24/2014 03:27PM by afsmith.

Quote

I retested the resistance of the thermistor and am now getting readings of > 1 mega ohm resistance so that is in line with the high temperature readings.

No 1M would give a very low reading or MINTEMP. To get high temperature reading you need a low resistance. Are you sure your meter is working?

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[www.hydraraptor.blogspot.com]

You are right, I had that backwards. I have plugged everything back in and I guess I must have had a short because now temperature readings are back to normal (give or take a few degrees)...I'll pick up some silicone tape tomorrow to replace what I took off then give it a spin (maybe a new multimeter too )

Thanks again,

A