# How to get A, B and C values for this thermistor?

I wonder if it's possible to extract A, B and C values for TTF-103 thermistor from datasheet. Those values needed to calculate resistance using Steinhart–Hart equation.

• It is possible to interpolate the graph accurately to get --25'C, +85'C for a given part at 25'C and compute A,B,C but standard practise is to use R25 and B(25/85) with B tolerance My quick read from graph for the 10K part is.. .} R-25 = 90000 } R+25 = 10000 } R+85 = 1300 Dec 21, 2012 at 0:39

The fourth column of the datasheet table, indicated as "B25/85 Value", is key: that is the Beta value measured between two temperature endpoints (25C and 85C). Second column has the resistance for the first temperature endpoint.

Given for device TTF-103: B25/85 = 3435, R25 = 10, R85 can be solved by using the formula:

R2 = R1 / (exp( B*(1/ T1 - 1/ T2) ))

Thus, R85 = 1.4513 and now you have two points. Beta can be assumed to be constant between the two test endpoints. Using the same formula and chosen temperature, you can establish the third point. Choosing T = 60, R60 = 2.9809 for example

• Visual validation of R at 60C using the graph in page 3 of the datasheet gives us R closer to 3 for device TTF-103 and is in agreement to formula results. Dec 20, 2012 at 23:23
• Great, but could you please help me to calculate A, B and C out of Beta and R25? I still have to use the SH function I have... Basically I give temperature to that function and it gives me R, with known A, B and C. Dec 20, 2012 at 23:25
• The C and D coefficients for the Steinhart-Hart function vary significantly between devices that manufacturers rarely provide these coefficients. Practically, these coefficients are established through calibration. The formula given in the answer assumes for practicality that Beta is constant which is true to a certain degree of error in reality (reason you have the 5th column in the datasheet). What do you really need the SH function for? Can you not use the formula in the answer? Dec 20, 2012 at 23:45
• @Pablo "Because equations (15) and (16) each have four unknown constants, a minimum of four calibration data points are required in order to determine the constants. The constants may be obtained from the solution of four simultaneous equations if only four data points are given, or, they may be obtained by polynomial regression analysis when more than four points are given." - me.psu.edu/rahn/me462/ntcnotes.pdf Dec 20, 2012 at 23:53
• Thermistor is hooked up to AVR ADC with 10k bias resistor. Using SH I am generating lookup table for ADC values 0..1023. Dec 21, 2012 at 0:09

Yes, you can pull the the resistance/temperature values from the figure on page 3.

Just pull any 3 distinct values and you have enough data to perform the calculations.

• Since picking values from figure may produce some errors due to resolution constraints of image, can I take 2 values from table and only one from figure? what are those 2 values? one is R25, but I don't know where is the second one... Dec 20, 2012 at 22:54

From Wikipedia (yeah, I know this thread is years old, but I was accidentally running into it): "NTC thermistors can also be characterised with the B (or β) parameter equation, which is essentially the Steinhart–Hart equation with a=1/T0−(1/B)lnR0, b=1/B and c=0."

You can convert the beta parameter to Steinhart-Hart coefficients (Instructions Here and here), but if you can use beta it will be more accurate to use the beta you were given, if its from anywhere near your temp range.

Clearly not. The datasheet is only giving you data for two operating points, which is enough information to determine approximations to A and B, but not C.

• What is B25/85 then? is it beta value? Dec 20, 2012 at 22:34
• There is a plot of R vs. T on page 3, should be able to pull the 3 points necessary for the calculation. Dec 20, 2012 at 22:48