Thermistors gone out together

IS it possible for a motherboard failure to kill thermistors? or is it always the thermistor that kills the board?

The thermistor is a resistive device fed into an analog input on the motherboard. The motherboard feeds either 3 or 5VDC through the analog input and combined with the thermistor and internal resistor creates a voltage on the input which varies with temperature. The current developed on this input is very small and a fuse may be present to protect the input from overcurrent. It would be highly unlikely that the mobo took out the thermistor. It is highly likely that the thermistor shorted out causing a high current that took out the input or fuse. Cleaning the hotend with a wire brush may accidently short the two wires on the thermistor if it got too close to them, or the thermistor wires were exposed and somehow twisted together. This has been my experience.

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How do you know your thermistors are dead? They’re easy to test with an ohm meter and a heat source (your fingers).

As to the answer to your question, yes, a motherboard failure can kill a thermistor, but really only the Negative Temperature Coefficient (NTC) variety. I would imagine it would go something like this:

  • Your motherboard has an internal 5V power supply failure. The 5V regulator internally shorts out and passes the full 24V main power supply voltage to the controller circuitry. That should kill just about everything.
  • Now that the thermistors 5V supply has jumped to 24V, the thermistor now has to dissipate 25 times the power (power is a function of E (voltage) squared)
  • If it’s of the NTC variety, the thermistor’s internal resistance will actually drop as it heats up with all the extra current. The drop in resistance will increase the current it draws causing it to heat up more in a vicious spiral until it finally pops.

Positive Temperature Coefficient thermistors will actually self-limit the current they draw as they get hotter, but I suppose it’s remotely possible for them to be killed off as well. Then again, the thermistor in your hot end can survive temperatures of 250C and up, so…

As for the thermistor killing the motherboard, I would say absolutely not. Even if the thermistor is connected directly to the microcontroller’s input, that input can be shorted directly to the 5V (or 3.3V depending on the model) and GND rails without harming the CPU. The actual minimum and maximum values, according to the data sheet, are -0.5V to Vcc+0.5V. I’m hard-pressed to think of a way you could kill it except…

…if you committed the cardinal sin of cleaning the hot end with a wire brush with the heater power on. That could subject the thermistor input pin to the full 24V which will definitely kill the CPU’s input. It may not kill the CPU entirely, perhaps just the input circuit for the thermistor, leaving the rest of the printer’s functions appearing to work normally.

both thermistors read infinity and new ones that read as good don’t register at all on the board. I didn’t probe the board for output voltage though but I think it’s dead. I’m reading about the V2.0 board having circuit protection for these but 3dpc doesn’t have any and they are waiting on a V3 apparently, so hopefully, the V3 board does too.

Edit: it is in fact NTC 100K thermistors in this

I’m not sure what happened to this printer. the symptoms were no heat and an. ABL error. The firmware didn’t seem to be configured correctly when I powered it on. I compiled a new firmware the for it and for it’s screen and that solved every issue including heating but the thermistors were both still not functioning. Checked all the wire connections and pass-through boards and there was no problems there. like I was saying the only thing I haven’t done is probe the inputs on the board. I have spare parts here for everything except a thermistor with a long wire for the bed, motherboard or screen.

Lego, how would touching the heat block and cartage complete a circuit? Where would 24v come from? everything is insulated. Not that a wire brush on a power device is wise, its not. I don’t use them at all but I still don’t see where you would get power from.

If you use a wire brush on a hot-end it is possible for a strand of the wire brush to come in contact with the heating element’s 24V supply while that supply is being energised. Obviously this wouldn’t happen if the insulation on the heater’s wires is intact and running all the way to the heater block but this is not always the case if the initial assembly is sloppy or if the insulation gets soft during use and/or gets moved back during the brushing, thus exposing bare wire. Similarly, if the brush is also in contact with the thermistor wires - and let’s face it - they’re side-by-side - then you can have a short circuit between the exposed heater wire, through the wire brush to an exposed thermistor wire and back into the controller board. I recommend that people either use a nylon toothbrush or my personal preference: round wooden toothpicks or wooden BBQ skiewers to scrape a heater block clean.

bamboo is smart. I have rarely seen exposed wires. It sounds like the heater is faulty if there is exposed wires!

Actually, if you follow the various forums on Facebook, you’ll see several references to this from this summer alone. People cleaning the print heads with a brass brush and getting sparks followed by a dead controller.

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that would kill the 24v on the controller though not just these little inputs I think. Sure would like to know what caused this though. If this board had the wrong firmware could the thermistor settings in the firmware have killed them? like if the wrong type was selected for example?

In a word, no. Even if the firmware had the themistor input pin configured as an output and even if the firmware then applied the maximum output voltage that pin can deliver to that (now) output, that’s only 5V. The current through the thermistor (at room temperature) would then be 5V/100,000ohms = 50 micro amps (millionth’s of an amp). Even if the thermistor was then heated (and it’s a NTC type) and let’s say it’s resistance was reduced to 0.1% of nominal, that’s still 1000 ohms which would mean the current would be 50 milliamps. Still trivial. Assuming the wires to the thermistors didn’t get pulled off during the brushing, the most likely cause of failure is the thermistor coming into contact with the 24V supply. It would then experience a current 5 to 8 times normal which, given it’s already heated state, might be enough to burn it out.

Ok. Also, it’s not one thermistor it’s both and whatever happened took out the inputs on the board too. I see there are 3 inputs and one isn’t populated.


What I would do is:

  1. At room temperature, check the resistance of your thermistor with ohm meter. It should be around 117kohms.
  2. Measure the open circuit voltage across the thermistor input. You should get 5vdc. Check each input and confirm. If you do not see the 5vdc, find another ground point on the motherboard and measure again from the + pin on the thermistor input to the new ground point. If the first is true, your inputs are most likely good. If not and the latter is true, the protection fuse on each input is blown.
  3. Assuming you got to this point because thermistors are good from step 1, and 5vdc is good in step two.,your firmware is likely mapped to the wrong inputs to read the thermistors, assuming the motherboard was setup for NTC100k thermistors and not something else like PT100.
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1} Yup already determined both are dead.
2} Haven’t done this yet. but I did try a new thermistor on it and still didn’t read it.
3} Already flashed good firmware to the board. and it solved some errors like the ABL error and the heaters started working again. but it still doesn’t have input from the thermitors