Thermistor

A common temperature sensor is a Thermistor, which is a device with negative temperature coefficient (NTC), i.e., the Resistance decreases with temperature, or ${\partial R \over \partial T}<0$. Therefore, thermistors are sometime referenced as NTC sensors.

Their resistances can be well described by a Steinhardt-Hart equation, which relates resistance R and absolute temperature T:

${\frac {1}{T}}=A+B\ln R+C(\ln R)^{3}$

Usually, the coefficients A,B,C are not specified in a data sheet of a device, but three things are quoted:

• Reference temperature, typically 25 Celsius; sometimes this is not even specified
• Resistance R_0 at the reference temperature (typically 25 Celsius). Often, R_0=10kΩ
• Characteristic of the exponential, the constant B in the above expression. Typically around 4000 Kelvin.

These parameters can be used with a simplified Steinhart-Hart equation, which assumes C=0 in the expression above. Then, the equation becomes

${\frac {1}{T}}=A+B\ln R$

or

$R=R_{0}e^{B({1 \over T}-{1 \over T_{0}})}$.