When a voltage is applied to a thermistor, self-heating occurs and eventually the thermistor reaches a temperature balanced with the ambient temperature. When excessive power is applied, the thermistor's characteristics can be damaged through excessive self-heating. Therefore, it is necessary to determine the maximum power that can be applied continuously. Maximum load power is limited by such factors as rated power, maximum permissible power, the thermistor's de-rating curve, and the maximum operating temperature.
Rated Power is the maximum power that can be applied continuously at an ambient temperature of 25°C.
Maximum Permissible Power is the maximum power that can be applied continuously at any temperature between 25°C and the maximum operating temperature. It is depicted by a de-rating curve or expressed as a function of temperature by the following equation:
Where:
PMAX is rated power [mW]
T is ambient temperature [°C]
P(T) is maximum permissible power [mW] at ambient temperature
TMAX is maximum operating temperature [°C]
A de-rating curve depicts the relationship between ambient temperature and maximum power and is expressed as a percentage of rated power - see Fig. 3
- Figure 3 - Power De-Rating Curve
