working with thermistor

Discussion in 'Embedded Systems and Microcontrollers' started by sairfan1, Mar 2, 2013.

  1. sairfan1

    Thread Starter Member

    May 24, 2012
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    Hi,

    i'm learning thermistors. in equation there are three variable to calculate temperature A,B and C. i want to know where from to get values for these variables. do i have to calculate these or these are some constants. formula image attached.
     
    Last edited: Mar 2, 2013
  2. t06afre

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    May 11, 2009
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    So you will be using a microcontroller for this? That kind of make it more tricky
     
  3. sairfan1

    Thread Starter Member

    May 24, 2012
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    yes i'm going to use PIC for a small project, and will use NTC thermistor for it. there is image of equation, where variable A, B and C are mentioned to calculate temperature.

    i do not understand where to get from values for A, B and C.

    thanking you.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    A, B and V are called the Steinhart–Hart parameters, and must be specified for each device. T is the temperature in kelvin and R is the resistance in ohms. You need to get the datasheet for your NTC thermistor. In order to find the correct values for the constants
    The wiki page is not a bad place to start http://en.wikipedia.org/wiki/Thermistor. Do you have to use a NTC thermistor. Depending on temperature range. You may use a linear sensor
     
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  5. ErnieM

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    Apr 24, 2011
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    You get these values from the manufacturer's data sheet for the particular device you are using. If you can't find that data, pick a different part and/or manufacturer.
     
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  6. John P

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    Oct 14, 2008
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    Do you actually need constant measurements, or are you really interested in a couple of thresholds? If it's the latter, then you can calculate those in advance and have the processor do the comparison. Floating-point math in a typical microcontroller is massively expensive in terms of program storage and time. But then, if this processor has no other work, maybe crunching numbers wouldn't be a problem.
     
  7. tgil

    New Member

    May 18, 2011
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    You are probably better off creating a lookup table (avoid using floating point values) than calculating the value. Most thermistor datasheets include tables that list temperatures and resistances. You will want to create a table that corresponds temperatures to ADC values. You can get better accuracy by linearly extrapolating between points.

    This might be helpful:
    http://www.coactionos.com/embedded-design/114-adc-thermistor-circuit-and-lookup-table.html
     
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  8. sairfan1

    Thread Starter Member

    May 24, 2012
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    thanks to all above, i studied at wiki but could not get the point, now its clear. i was working on project just to learn thermistors.
     
  9. MrChips

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    Oct 2, 2009
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    Most manufacturers will provide you with a table of resistance vs temperature for their thermistors.
    You really don't care about resistance.
    What you need is a table of temperature vs ADC counts.

    To arrive at such a table, get about ten precision resistors that cover your full temperature range.
    Connect each of the resistors to your hardware and record the ADC counts.
    Use the manufacturer's data and map out a table of temperature vs counts.
    From here I will show you how to arrive at a formula.
     
  10. sairfan1

    Thread Starter Member

    May 24, 2012
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    can you please explain method in brief, what should be value for each resistor,
     
    Last edited: Mar 5, 2013
  11. sairfan1

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    May 24, 2012
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    MrChips i'm waiting for your kind reply.
     
  12. ErnieM

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    Apr 24, 2011
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    I believe MrC's method is to use fixed resistors to simulate the thermistor at various temperatures. These can be inserted in your circuit and used to determine what the A2D will output for a given temperature. A single potentiometer would also work, I'd see if I could get my hands on a 15 turn trim pot, not vrey expensive but easy to set.

    However, with a given thermistor (and a spec sheet), and knowing how you use it (ie, connection to micro, A2D resolution and ref voltage) you can produce a table of temperature to A2D value "on paper" (by calculation).

    Have you picked a thermistor yet?
     
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  13. sairfan1

    Thread Starter Member

    May 24, 2012
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    Thanks, i'm clear now, i'll work on it and will get back with results.

    is there any method to calculate expected ADC results? for example if i use 15 turn trim pot, and there are 3.7v on its out pin which is connected to ADC. is it possible that i can calculate expected ADC result?

    yes i have thermistor,
     
  14. MrChips

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    What Ernie meant was, what is the part number of the thermistor?
    Are you going to keep it a secret?
     
  15. ErnieM

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    No it is impossible as there are way too many unknown. There is no information about this ADC so no one can determine anything about how it works.
     
  16. sairfan1

    Thread Starter Member

    May 24, 2012
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    sorry it took some time to confirm part number and manufacturer, its RA 103F 3435F04 by Joinset (www.joinset.com)
     
  17. MrChips

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    Oct 2, 2009
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    Again, what ErnieM is hinting at, you must think we are mind readers and can read your mind across the internet. You want us to determine the ADC results?

    What ADC?
    What are the specs for the ADC?
    How many bits?
    What is the ADC reference voltage?

    All of these can be found in the ADC datasheet, assuming you know the model number of the ADC.

    Details, details, details.
     
  18. sairfan1

    Thread Starter Member

    May 24, 2012
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    im very sorry, its my mistake i asked question that is not topic of post, will take care next time. Because not very good in English thats why did not understand ErnieM's question.

    as in my first post i want to learn thermistor, and wanted to know how to calculate values for A,B and C.
     
  19. ErnieM

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    Apr 24, 2011
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    I took the time to wander thru the seller's page to find what they say about that thermistor. They do have an excel sheet that gives you the values for every degree between -40 and +125 inclusive. It seems to be a ratio from the +25 degree value which is 10K for your part.

    Unless you have some other need for the theoretical constants that should be all the info you need to calculate what voltage you get for what temperature.
     
  20. sairfan1

    Thread Starter Member

    May 24, 2012
    46
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    thanks ErnieM, you guys are right that datasheet and manufacturer's resources are best to learn.
    what is the way forward to calculate values for A, B and C
     
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