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Thermistor help

Discussion in 'Sensors and Actuators' started by TheQuestor, Aug 2, 2018.

  1. TheQuestor

    TheQuestor

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    Aug 2, 2018
    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
    .
    Thanks in advance.
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    No.
    The way thermistors work is they change their resistance throughout their range over the temperature change.
    Some are PTC while others are NTC.
    Which one is yours?
    Why replace the thermistor in the first place?
    Show details of the original and the new one.
     
  3. TheQuestor

    TheQuestor

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    Aug 2, 2018
    It's NTC and I broke the leads on the original one.
    I found another that WILL work but I'd like to play around with all these other ones :p

    The original one looks pretty much like this, This one WILL work and I will be using it, but still.

    WP_20180802_18_35_28_Pro_LI.jpg

    The ones I have look like this which is not what they looked like on ebay :(
    WP_20180802_18_35_17_Pro_LI.jpg


    I also have a bunch of these but they are for something else and don't fit the hotend and would have to be attached to the outside of the hotend, Sigh. Not Ideal WP_20180802_18_35_24_Pro_LI.jpg

    The fact that I found one to use kind of makes this a mute point but I need to order more [I prefer to have a ton of parts on hand instead of waiting to order when I need them since it takes a couple weeks + to get them from china,

    I need to order ones that look like the top one but Zonestar [the maker of the printer I have] have not been extremely helpful other than saying they are 100k Ohm thermistors and supposedly so are the other 2 batches I have :(

    I have multiple hotends that I swap out [single nozzle direct, dual filament single nozzle bowden, and soon a laser etcher] and I REALLY want to have 10 or so on hand just in case :)
     
    Last edited by a moderator: Aug 3, 2018
  4. Bluejets

    Bluejets

    2,976
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    Oct 5, 2014
    mmm...well,.... when I said show details what I meant were the specifications, perhaps I should have been clearer.

    As you can probably appreciate, there is not a lot one can do to narrow down any detail from the difference of 2 "blobs" on the end of 2 wires.

    Other than that, the only advice I can give is get the same type of same resistance.
    Looks are not everything.
    Why you need "bags" of these is beyond me I'm afraid.
     
  5. TheQuestor

    TheQuestor

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    Aug 2, 2018
    NTC 100k Ohm Thermistor is what the manufacturer said they were,

    Why buy bags? Well; because why buy "1" when I can get 10 or 50 for nearly the same price.

    Do you ever buy just 1 resistor? Or do you buy a row of them., I buy the row :)
     
    Last edited by a moderator: Aug 3, 2018
  6. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir Questor . . . . .

    ANSWERS . . . .

    Without further information and confirmation . . .I would have assumed its use as being a hotbed sensor..
    To confirm what you DO have . . .take a DVM and use its ohmmeter function and measure out the resistance and see how close it is being to 10 K? or the expected 100k that the unit is now reading out at an ambient 25 Degrees room temperature.
    Then swing a thumb to index finger across its center and pressure grip across it and watch the resistance slowly creep.
    If the resistance lowers you have yourself a Negative temp unit, however, if its resistance rises, it is being a Positive temp unit.

    73's de Edd
    .....
     
  7. TheQuestor

    TheQuestor

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    Aug 2, 2018
    aye good info. It's actually for the hotend but it's the same thermistor for the hotbed.It's an old Prusa i3 clone [Zonestar] running stock Repetier firmware. I'll test the last good one I have and will see what the resistance is at room temp and whether its resistance goes up or down in a while. I need FOOD really bad right now and will get back after I stuff my face.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You may have to recalibrate the temperature readings on your 3D printer. Even two thermistors of the same type will differ slightly in their characteristics, and removing and remounting the same thermistor can result in different performance.

    The difference you're seeing is larger than I'd expect and may be a result of the new thermistor having a different slope. However, considering that the temperature (29C) is close to the temperature for nominal resistance (probably 20C) then your conclusion that it's the wind resistance is probably correct.

    Thermistors are often used with either a constant current or as a leg of a voltage divider. If it's the latter, you could chance the value of the other resistor in the voltage divider.
     
  9. TheQuestor

    TheQuestor

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    Aug 2, 2018
    I've run about 5 different hotends on this thing and at least as many blocks and thermistors and they have all been pretty much on spec. I don't care about a 5 degree diff really as long as it works and so far it always has done just that :) it was a cheap printer [back then it was around 300 bucks which, back then, was quite a bit cheaper then a lot of them on the market] and has printed dozens and dozens of spools of PLA and ABS and I really feel I have more then got my money out of this thing :)

    I am going to test one of the large green thermistors just to see what it says but the small black ones are so WAY off it's not even funny. Ambient on the little clear thermistors read about 29C [+-5 depending on how warm the room is] but these small black thermistors are saying 81C which is what was leading me to think that they sold me 10k Ohm instead of 100k Ohm but like I said I bought them over a year ago and just got around to messing with them [had some SERIOUS health issues which pretty much took a year away from me :( but all is good now more or less] and now that I am playing around again it's been fun.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    For me the question would be whether to get the correct thermistors or considering hacking the printer.

    There are plenty of factors going each way, but waiting for the correct thermistors is going to be lower risk and it leaves you with a standard printer that won't surprise you in potentially new and unusual ways.
     
    TheQuestor likes this.
  11. TheQuestor

    TheQuestor

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    Aug 2, 2018
    I want the proper thermistors really I just need to FIND them lol.

    and THESE look like the proper ones. NTC 3950 which now tells me more than I knew before. I didn't know they where 3950's

    https://www.ebay.com/itm/5pcs-NTC-3...d-extruder-for-Reprap-3D-Printer/322540970318
     
    Last edited by a moderator: Aug 3, 2018
  12. BobK

    BobK

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    Jan 5, 2010
    I have an I3 clone and it is easy to find thermistors sold specifically as replacements for it.

    Bob
     
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