Odd Thermistor Behavior

[DGBK]

Equipment: Rostock Max V2, stock hotend

Last night I was attempting to start a print. I had to give it a couple restarts due to poor adhesion on the first layer. When I was attempting a third try at the part, I noticed that the hotend temperature was reading 10 C on the matter control interface. The hotend had just been up to 230 C, and it was in the 30s in my room, so I knew that reading was erroneous. I have had it happen a couple times that the matter control interface gets out of sync with the machine. Restarting it usually fixes the sync issue.

After restarting matter control, it was still reading 10 C. I looked at the LCD display and noticed it was getting the same reading as matter control. At that point I gave the hot end a close up look. The wires all appeared to be intact. The thermistor did not pull out, and it would have read at least room temperature if it had. My first thought was that there was a short in the thermistor, but after reading about how they work, that theory didn't make sense. A short should read a higher temperature since the resistance goes down when the thermistor heats up.

I also noted that when I manipulated the hotend with my needle nosed pliers, it would temporarily read the correct temperature. Any ideas what this is a symptom of? Is it possible I have a break in the wire?

I turned the machine off, and tried it again a couple hours later. At that point, it was back to reading the correct ambient temperature. I'm now afraid to use the stock hotend without consulting here first. Luckily, I have all the parts needed to install an E3d hotend. I think I will go with that until I can figure out my issue with the stock.

Seeing this issue also gave me a thought. Is there any way to program a fail scenario into the printer? What i mean is, if the printer detects a large drop in hotend temperature, can we program an abort into the code? I can't imagine a scenario where a sudden temperature drop (of say more than 10 degrees) would be expected without a thermistor pulling loose from the hotend. If that was detected, the machine could simply shut down the hot end to avoid a fire.

[Dale Eason]

You probably have a bad solder joint connection from the wire to the thermister. That will give you an intermittent connection. I had the same thing happen to me to start with. It can be hard to get a good solder connection to the thermister wires.

Dale Eason