# Thermistor help

#### TheQuestor

Joined Aug 2, 2018
4
I have a 3d printer and it came with what I was told a 100k Ohm thermistor. I bought a bag full of what was described as 100k ohm thermistors but when I hook it up to my printer's motherboard the temp is WAY off. It should read about 29C but it reads like 86C which leads me to believe they are 10k Ohm thermistors, not 100k Ohm,

Can I just put a resistor inline to bump it up to 100k and if so what size resistor
.

#### paulktreg

Joined Jun 2, 2008
784
Your series resistor will not change with temperature and you'll just throw everything out.

What 3D printer do you have? What does this thermistor do? Why did you buy a bag? Does it fail regularly?

#### AnalogKid

Joined Aug 1, 2013
8,487
You can insert a resistor in series. but the result will be correct for only one temperature. For example, if you put 90K in series with a 10K thermistor, it will read 100K at +25C. But the combined resistance will not be correct for any other reading.

Note that thermistors usually are not interchangeable among manufacturers. Two different parts will read the same at the calibration temperature, but the response curve with respect to temperature will be slightly different, because each manufacturer has its own formula for the temperature sensitive material.

ak

Joined Jan 15, 2015
5,499
No, you can't just place a fixed resistor in series with what you have. If the system was designed to work with a 100 K NTC thermistor then you need a 100 K thermistor and you also need one with the same dimensions as the broken one so it fits in the thermo-well the same as the old one.

Here is what I suggest you do. Rather then screw around with Ebay or similar looking to buy a bag in bulk I suggest you look at a data sheet from a reputable manufacturer or distributor like this one or like this one. I also suggest you note what AnalogKid points out where he covers:
Note that thermistors usually are not interchangeable among manufacturers. Two different parts will read the same at the calibration temperature, but the response curve with respect to temperature will be slightly different, because each manufacturer has its own formula for the temperature sensitive material.
The whole order was on the expansive side, and I wavered about buying it but decided to go ahead. Then after clicking "ok", I remembered the exchange rate. So it was about half again more than I was thinking, in $AU. As for your printer temp, if it is stable, just work out what to set it to for a good print and use that. On my Deltaprintr, I have a problem that the fan is cooling the thermistor, and instead of fixing it, I just edit the gcode file to make sure the fan is on first, then print at "175C", and that is around 195C really. It works fine. The gcode was.. M109 S175.000000 ; set temperature to 175C M106 S255 ; start with fan on 100% now.. M106 S255 ; start with fan on 100% M109 S175.000000 ; set temperature to 175C I really should fix it but my Deltaprintr is working really well so I will leave it alone. Thread Starter #### TheQuestor Joined Aug 2, 2018 4 I don't knock TinkerCAD as it is the one I use too. For some reason. I have not been able to get my old head around any other program. Yes, I know about postage! Recently, I purchased some stuff from Sparkfun and, as I live in Australia, the post was a bit over$70.
The whole order was on the expansive side, and I wavered about buying it but decided to go ahead. Then after clicking "ok", I remembered the exchange rate. So it was about half again more than I was thinking, in \$AU.

As for your printer temp, if it is stable, just work out what to set it to for a good print and use that. On my Deltaprintr, I have a problem that the fan is cooling the thermistor, and instead of fixing it, I just edit the gcode file to make sure the fan is on first, then print at "175C", and that is around 195C really. It works fine.

The gcode was..
M109 S175.000000 ; set temperature to 175C

now..