Thermistor Problems

Hi all,

I noticed that my extruder wasnt heating to the correct temperature. If I have it set to 200C in pronterface it will stay around 220C and rise and fall by about 5 degrees without ever reaching 200. However the actual temperature is far lower than that as I can touch the hot end with bare hands. So I investigated and measured the resistance of the thermistor by disconnecting it from the PCB to be around 6.3k ohms when it should be around 100k Ohms at room temperature. Does this mean I have to fit a new thermistor? Any ideas what may have caused this as it is a pretty new build with only a few prints done on it.

Also I am measuring the resistance of the bed thermistor and only getting 6.3k ohms also. I measured this at the Melzi board.

Seems strange that they both measure the same.

Any help would be great.

Thanks.

Did you disconnect from the Melzi when making the measurement?

Extruder thermistor problems are usually caused by molten plastic getting onto the wires and into the cement holding it in.

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[www.hydraraptor.blogspot.com]

Yes I disconnected it. Could I damage the board by measuring while connected?

Ok I will take off the silicone tape and have a look. Is it repairable? Could I heat the Extruder to soften any excess plastic and then remove it?

Thanks,

If you measure it while connected you wont get an accurate reading because the input impedance of the board is much lower than 100K. It shouldn't damage it as long as the power is off.

I don't understand how both the bed and the extruder could be 6.3K and 6.3K is probably about what you would measure when connected to the Melzi. Does the bed temperature read correctly?

If the bed measures 6.3K because it was still connected to the Melzi then it would be an amazing coincidence that the hot end reads the same when disconnected.

If the extruder leaks molten plastic onto the thermistor it soaks into the cement and turns to carbon, lowering the resistance, causing the temperature reading to be too high. While it is still plastic you could heat it to get it off but it is an insulator in that state. It is only when it has burnt black by being heated for a long time that it becomes conductive. At that stage you can't melt it. You might be able to scape it and a little of the cement away.

If the extruder has leaked then that is a problem in itself. However I don't know any other way thermistors can give the wrong value other than contaminated cement.

Edited 1 time(s). Last edit at 09/22/2014 05:01AM by nophead.

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[www.hydraraptor.blogspot.com]

Apologies for the confusion, you are correct I had the thermistors connected to the Melzi when I got those figures. When I disconnected the extruder thermistor I got around 400 ohms I think but I'm not 100% sure was it ohms or k.ohms, that was when it was still relatively hot. I have come back to it today and disconnected both thermistors and got readings around 130k which is what they were originally when I first built the machine. I also peeled back the silicon tape to look at the cement and it doesn't seem to be in contact with much plastic but I will clean the hotend anyway as there is some plastic on the bottom surface. I will let you know how this goes

I have cleaned the hot end, there didnt seem to be any plastic affecting the cement. I have started a few prints but am still adjusting my z height to get a good first layer. While starting these prints I have to wait a lot longer than normal for the hot end temp to set.It is very erratic and the countdown from 60 seconds has to restart over and over again. I have checked all the wiring connections related to the thermistor.I have tried to make sure there are no causes of temp fluctuation in the room. The temp fluctuation is up to +/- 10 degrees. The bed temp stays pretty constant during this time. Any ideas what else I can check?
Thanks

I think I have solved the problem, I tightened up the extruder heater wires in the Melzi and this seems to have solved the problem for now.

Hi, I am having this problem again. I was running the printer last night and the extruder heated up fine for a few hours and then all of a sudden as I was starting a new print it went back to being very erratic. I have checked the resistances of the heater and thermistor and both are normal. I have checked the connections for both and they are fine. Please see attached graphs of the extruder heating up. Notice the large jumps it makes. This is definitely a false measurement as the extruder while hot is ok to touch and does not ooze. I left the temp set for a few mins and it even seemed to cool down as i could touch it with bare hands. So I turned off the machine and disconnected the thermistor and the resistance was only 350 ohms. Do you have a temp vs resistance curve for the thermistor? Please let me know what you think.


****************Update
I peeled back the silicone tape and it turns out I was looking at the extruder connection rather than the thermistor connection. The thermistor connection is indeed covered in black carbon. I have scraped it off but its is still behaving the same. What is the best course of action from here? Do you mean that the extruder has a leak in the hot end? Or could the plastic have gotten under the silicon tape when I have been adjusting the z-height?

Edited 4 time(s). Last edit at 09/23/2014 03:47PM by ballyboy.

Does it look like the black as come down from above or up from below?

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[www.hydraraptor.blogspot.com]

I would say it came up from below but found it a bit strange that there wasn't a build up on all four sides.

Edited 1 time(s). Last edit at 09/23/2014 06:06PM by ballyboy.

See attached photos after scraping off carbon.

If there is carbon at the junction between the brass and the PEEK then I think it has leaked. I can't see from the pictures because it is in shadow and I am looking for black on black!

Is the resistance correct with the carbon scraped off?

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[www.hydraraptor.blogspot.com]

There doesnt seem to be any carbon between the brass and the PEEK. The thermistor resistance is only 72kohms now. It was 130k when I built the machine.

Try scraping some of the cement away so it is flush with the surface of the block. If that doesn't fix it then the plastic has soaked all the way in to the cement so you would need to drill it out and replace it.

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[www.hydraraptor.blogspot.com]