# NTC thermistance : how to compute B(25, X)

Discussion in 'Sensors and Actuators' started by @[email protected]@me, Nov 26, 2020.

1. ### @[email protected]@me

48
2
Dec 15, 2016
Hello all !
Long time no see here, but I'm still working on my home automation system. I'm slowly getting equipped with the necessary tools & knowledge to go forward.
ATM I'm trying to design a simple circuit around a single IC, which requires temperature control. The datasheet give me the necessary information to compute how to plug the NTC thermistor with a voltage divider circuit so I can set up min & max temperatures I want to support.
So I did some reading about thermistors, and especially the NTC kind. I found that 2 very important parameters are the resistance at a given temperature (25°C), and the B value (usually B(25,100)), used to compute the resistance of the component when it is at a given temperature.

For example, if I want to know the temperature at 80°C, I have to follow this equation (if I recall correctly):
B(25,80) = ((353.15 * 298.15) / (80 - 25)) * ln(R(25) / R(80)

353.15 & 298.15 are temperatures in K. I simplified in the denominator by removing the "+273.15" on each side of the subtraction.

However, something prevents me from doing the calculation here. As I need to compute R(80), I first need to know B(25,80). The datasheet usually only gives B(25,100). The question is simple : how do I compute B(25,100) ? Is that a simple linear interpolation ? Or more complex than that ?

11,276
2,583
Nov 17, 2011

3. ### @[email protected]@me

48
2
Dec 15, 2016
Yup, exactly. B is a constant, whatever the temperature we want to compute the thermistor resistance of ! I don't have A and C for the component I wish to use, so I guess I will stick to the "simplified" formulae ! Thanks a lot
I'll post on this forum with a (quite) finished schematic when I'll be done with calculating the missing resistors, as I'll have another question.

11,276
2,583
Nov 17, 2011
De rien.