Temperature Coefficient of Resistance α
The temperature coefficient of a thermistor denotes the rate of change of thermistor resistance per 1℃ and is commonly expressed in ％/℃.
The coefficient α is defined by
Here, the equation to approximate the relationship between resistance R and absolute temperature T (see the Section "Resistance") is differentiated for temperature T and substituted into the above equation.
The coefficient α of a thermistor with its B = 3400K and T = 293.15K (20℃), for example, can be determined as follows.
The negative sign of the coefficient α indicates that the thermistor resistance decreases with increasing temperature.
Metals and alloys, in general, raise their resistance as temperature rises. Their temperature coefficients of resistance, for example, are 0.4%/℃ (gold), 0.39%/℃ (platinum), and iron and nickel are relatively larger with 0.66%/℃ and 0.67%/℃, respectively. Thermistors, as compared with these metals, vary their resistance significantly with a small temperature change. Therefore, thermistors are suitable for precise temperature measurements and controlling the temperature by using slight differences in temperature.