bad temperature readings on Rostock max v3

[mark.gesing]

Hi,

I'm trying to get started with my newly assembled Rostock max v3, but I'm having a nightmare getting reliable temperature measurements from the thermistors.

Right now, the hot-end is reading room-tempaerature as 90C, and the bed is reading room temperature as 167C, the actual room temperature is somewhere between 19 and 20C.

To determine weather the problem is in software, or hardware, I soldered a 100 kilo ohlm resistor to one of the spare connectors which came with my kit, and plugged it into the rambo board in place of the thermistors (one at a time). With my resistor plugged in (instead of the thermistor) the measurement is 24.8, close enough to the 100kilo ohlms at 25C that I'm convinced that the rambo board is doing what it should for the temperature measurement.

I have inspected all the solder joints on both thermistors, and both seem OK. (and if they broke we'd be expecting a low temperature, not a high temperature right?)

I have gotten reasonable measurements from both these thermistors before, and for a brief, glorious, moment they were both working at the *same time* and I was able to print about 1/3 of a calibration box before the temperature measurement got screwed up, and the hot-end got too cold to keep extruding.

For a longer stretch, I had good measurements from the hot end, but not the bed. I managed to print a full calibration cube by plugging my 100 kilo ohlm resistor into the bed's port on the rambo board, and setting my print settings to a bed temp of 23C, putting some masking tape on the bed for traction I got a print to work.

Now, I'm back to neither thermistor giving me good data (maybe the one I now have installed was also damaged in transit, and the vibrations from printing finished the job? who knows?)

Have any of you had similar problems with thermistors?

Are there any precautions I can take to prevent my thermistors from breaking?

Are there other possible explinations for my temperature measurements being wrong?


Thanks,

Mark.

TL;DR: My temperature measurements are too high, I think my thermistors have gone bad, why might they have gone bad, what can I do to fix the problem and /or prevent it from recurring in the future.

[Polygonhell]

It's very unlikely you have 2 bad thermistors.
It's likely that there is a short in the wiring somewhere, I know I had a bed thermistor read very high at room temperature, when one of it's legs was touching an exposed metal piece on the beds heating element.
You can do the same thing if the thermistor leg is touching the heater block on the hotend.

The way to trace it down is measure the resistance, then trace the wiring all the way back to the component, and check at each connection, eventually you'll end up removing the thermistor and checking it, you'll know for certain then if it's good or bad.

I bought 10 replacement thermistors 5 or 6 years ago when I broke the leg off one on one of my first printers, I think I still have 8 left.

[mark.gesing]

It's very unlikely you have 2 bad thermistors.
It's likely that there is a short in the wiring somewhere, I know I had a bed thermistor read very high at room temperature, when one of it's legs was touching an exposed metal piece on the beds heating element.
You can do the same thing if the thermistor leg is touching the heater block on the hotend.

The way to trace it down is measure the resistance, then trace the wiring all the way back to the component, and check at each connection, eventually you'll end up removing the thermistor and checking it, you'll know for certain then if it's good or bad.

I bought 10 replacement thermistors 5 or 6 years ago when I broke the leg off one on one of my first printers, I think I still have 8 left.

Thanks for the reply.

I have traced the connections as you suggested, I wasn't able to find a short. I'll get some replacements soon, in the meantime I might test the wiring by replacing the thermistors with resistors and seeing that I get reasonable results through my wires.