NTC Thermistor: How to Fit Resistance vs Temperature Data to a Formula for MCU Use

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#1 21683520 23 Nov 2019 02:34

John Curtin John Curtin

Anonymous

Post #1
21683520 23 Nov 2019 02:34

HiI have a NTC, and i have measured the following values:Celsius degrees Kohm--------------------------------1 316 2611 2017 1426 1033 741 551 462 3
How can i descibe these data by a formula? I want to use a MCU.Thank you very muchJohn Curtin
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#2 21683521 23 Nov 2019 02:59

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #2
21683521 23 Nov 2019 02:59

Your question provides many many solutions. You have to fit data to a specific curve. You can interpolate the points manually or use a software (excel, calc, MagicPlot, etc).Your curve has a very simple shape, and a good formula for your data is this (exponential fit):
29.86234611 * exp(-0.0399180326029 * x)
But there are many different models.RegardsGiovanni Di Maria
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#3 21683522 23 Nov 2019 03:09

John Curtin John Curtin

Anonymous

Post #3
21683522 23 Nov 2019 03:09

Yeahh. Fantastic. You have open my mind.Thank you very much.JC
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#4 21683523 23 Nov 2019 13:10

David Ashton David Ashton

Anonymous

Post #4
21683523 23 Nov 2019 13:10

John - interesting problem. I got what maybe a slightly easier way than Giovannis's.In the following graphSeries 1 is the resistance of your NTCSeries 2 is 10 x the current thought the NTC with 10V across itSeries 3 is 10 x the current when you put a 1K resistor in series with it - you could use this resistor to ground and then measure the voltage across it with the ADC in your MCU.The dotted grey line is the best fit line for series 3 and you will see that apart from right at the bottom it is a pretty good fit.
You could try different values of resistor in series to see if you get a better fit.I did this in excel, can send you the details if you wish.I presume you're trying to use the NTC to measure temperature with your MCU?Can I gently suggest you look at a sensor like the DS18B20 rather? One wire interface, 0.5 degrees accuracy, covers well over the range you are looking at, and really easy to use.
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#5 21683524 23 Nov 2019 14:36

John Curtin John Curtin

Anonymous

Post #5
21683524 23 Nov 2019 14:36

David Thank you very much.Ok, i have understood very well your concept. Great!!!In effect, now i must convert these data to voltage, to put to an analog port of the MCU.Now i know how i can move. Thank you to all for your help.John
#6 21683525 23 Nov 2019 23:18

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #6
21683525 23 Nov 2019 23:18

Since your NTC goes from 31 Kohm to 3 Kohm, you can build a resistive partitor with the grounded and fixed resistor of 15 Kohm. From the central node you get a voltage proportional to the value of NTC and, certainly, between 0 V e 5 V. The MCU can process this signal thank to its analog input. Remember that NTC is not a "linear" component, to linearize it you can connect a further resistor in parallel to the NTC. Then you can measure the voltage from the output partitor at different temperatures and "fit" them to a new equation.
#7 21683526 26 Nov 2019 12:35

Richard Gabric Richard Gabric

Anonymous

Post #7
21683526 26 Nov 2019 12:35

Here is an interesting article, worth a look at, from adafruitCheers,Richard
#8 21683527 27 Nov 2019 18:21

Jane Smith Jane Smith

Anonymous

Post #8
21683527 27 Nov 2019 18:21

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Topic summary

The discussion addresses fitting resistance versus temperature data from an NTC thermistor to a formula suitable for microcontroller (MCU) use. The original data shows resistance values at various Celsius temperatures. A recommended approach is to fit the data to an exponential curve, for example, using a formula like R = 29.8623 * exp(-0.0399 * T), where R is resistance and T is temperature. Data fitting can be performed manually or with software tools such as Excel or MagicPlot. Another practical method involves creating a voltage divider with the NTC and a fixed resistor (e.g., 15 kΩ) to convert resistance changes into measurable voltage for the MCU ADC input. Adding a parallel resistor can help linearize the sensor response. Alternative sensor suggestions include the DS18B20 digital temperature sensor, which offers a simple one-wire interface and good accuracy. The discussion also highlights the importance of selecting appropriate series resistor values to optimize measurement linearity and fit. Additional resources such as an Adafruit article were recommended for further guidance.
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NTC Thermistor: How to Fit Resistance vs Temperature Data to a Formula for MCU Use

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Topic summary