Busted thermistor ?

[askernas]

Hello!

All of a sudden from out of the blue (printer printed fine yesterday), I'm having issues. This right after I've done some of the best prints in my printing career with everything perfectly aligned.... *grumble* ..

When turning on my M2 (24V mod with the v4 extruder), the temperature on my nozzle is reported as Err: MAXTEMP. The temperature reported is "jumping" around a bit, however, the nozzle heating element is not even turned on.

When unplugging the thermistor, it reports 9, leading me to believe that it is a broken down thermistor .. Any suggestions or experiences in this area?

Thanks,
Micke

[Tim]

I've never had thermistor problems, but generally they are caused by loose or broken wires, rarely the thermistor itself. The thermistor runs off the digital voltage supply on the RAMBo, so you can take readings off of it without turning on anything else on the printer (the low voltage operation also means that it's much less likely to be something wrong on the controller board). It should reliably read room temperature. Since it's not, try wiggling the wires, all the way down to the connector on the RAMBo, and see if you can get a stable reading. But unless you have a bad connection at the connector end, which can be re-soldered, probably you will end up having to replace the thermistor. Fortunately, it's one of the cheapest things to replace on the machine.

[Mark the Greater]

Yup. I would call six bucks cheap!

[innkeeper]

cheap enough to replace... but try checking the connectors on the board, and at the print head first.
when my printer was new i had a heck of a time with the hotend temp, turned out to be the rambo board

[askernas]

I wiggled all the cables, but it was still up and around 300 degrees. It did vary somewhat, but not down to normal room temperature.. more like *maybe* down to 280 from 340~ish.

When I unplugged the thermistor, the reading on my Viki LCD, did only report 0 degrees when the thermistor was out, as I said.. so hence I thought it was the thermistor (or cable)..

Will remove it and solder and test later on today.. *fingers crossed* ... If I have a bit of downtime, it will be a good opportunity to start experimenting with auto-Z and auto-levelling...

[askernas]

I've done some measuring on my M2:

First off, the thermistor on the v4 extruder reports about 40 ohms at room temperature. Less resistance when temperature gets colder, and higher restistance when exposed to higher temperatures.
(I tested with cooling spray and a lighter)

However, when testing with attaching various resistors on the M2 instead of the thermistor, I find the opposite happens:

Lower ohms reports higher temperatures.

10 ohm - 358 degrees
100 ohm - 310 degrees
220 ohm - 258 degrees
1kohm - 173 degrees.

Can anyone verify that what I'm seeing from the M2 is correct?

Thanks,
Micke

[ednisley]

it was still up and around 300 degrees... report 0 degrees when the thermistor was out

That's the classic symptom of a shorted thermistor or cable: a very low resistance (the short) produces a high temperature, an open circuit (unplugged) produces a low temperature.

The short (or very low resistance) can happen at the thermistor itself, where the wires emerge from the resistive element potted inside the steel tube. A simple twist can wrap the hair-fine wires together and cause problems.

Measuring the thermistor's resistance at the connector on the extruder will identify the problem. It should be around 100 kΩ; you'll probably find a much lower value.

[askernas]

Measuring the thermistor's resistance at the connector on the extruder will identify the problem. It should be around 100 kΩ; you'll probably find a much lower value.

Yes, at room temperature the thermistors resistance was around 40 ohm, as mentioned above

Thanks, then I know it is something shorted or busted there, and not something that hs shorted om my RAMBo =)

Ordering a new thermistor then!

Thanks a lot!

//Micke

[askernas]

Just got word from Josh at MakerGear. It is indeed a busted thermistor, and they are sending me a new one free of charge.

The Customer Service by this company is so great, and that alone would be enough to keep me recommending them to people wanting to buy a 3D printer.
The fact that it is also built like a tank and so sturdy, is just awesome =)