NTC calculator

Negative temperature coefficient resistors (NTCs) are widely used as temperature sensors for their low cost, this availability in many different shapes and nominal resistance value (from 1 Ω to 10 MΩ).

Picture of a NTC

The downside is that their resistance is not a linear function of the temperature as one can see in the two plots below (which refers to an NTC specified R25 = 6.8 kΩ and B25/100 = 4200 K). Even by restricting the temperature range, still the function can hardly be approximated by a line. In an NTC the resistance decreases as the temperature increases.

Resistance as a function of temperature Resistance as a function of temperature (zoom 0-50 °C)

As one can see in the plots below, the resistance varies as the exponential of the inverse of the absolute temperature (same NTC as above).

Logarithm of resistance as a function of temperature Logarithm of resistance as a function of the inverse of the absolute temperature

NTCs have two major parameters: the nominal resistance R25, which is their resistance at the standard temperature of 25 °C (T25 = 25 °C = 298.15 K) and their constant B25/100.

With these parameters the resistance (or the temperature) can be calculated as follows:

R(T) = R_25 * e ^ ( B_25_100 * ( ( 1/T ) - ( 1 / T_25 ) ) )          T(R) = 1 / ( ( ln( R / R_25 ) / B_25_100 ) + ( 1 / T_25 ) )
T_25 = 298.15 K

Temperature and resistance:
R = Ω
T = °C
NTC parameters:
B25/100 = K  
R25 = Ω  

By using an NTC as temperature sensor, one should be careful in not running to much current though it, since the current will heat the NTC and introduce a measurement error.

Home Electronics 02562 Last update: 11/01/09