6) Thermal time constant (IEC 60539-1)

The thermal time constant is time for the temperature of the thermistor (no load applied) to change to 63.2% of the difference between their initial and final temperatures for a sudden change temperature.
When the temperature around the thermistor changes from T1 to T2, the elapsed time t and the thermistor's temperature T can then be expressed by the following equation:



The constant τ is the heat dissipation constant.
If t= τ, the equation is: (T-T1) / (T2-T1) = 0.632

As above, the thermal time constant is the time it takes for the temperature of the thermistor to change by 63.2% of its initial temperatrue difference.
The rate of change of the thermistor temperature versus time is shown in table 1.




Measuring conditions for all parts in this catalog are as follows:
1) Part is moved from a 50˚C environment to a still air 25˚C environment until the temperature of the thermistor reaches 34.2˚C.
2) Axial and radial leaded parts are measured as shipped.
The thermal dissipation and thermal time constant vary according to environment and mounting.

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