6) Thermal time constant (IEC 60539-1)

A constant expressed as the time for the temperature at the element of a thermistor, with no load applied, to change to 63.2% of the difference between their initial and final temperatures, during a sudden change in the surrounding temperature.
When the surrounding temperature of the thermistor changes from T1 to T2, the relation between the elapsed time t and the thermistors temperature T can then be expressed by the following equation. by ambient temperature change.

 

 

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


In other words, the above definition states that 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 parts in this catalog are as follows:
1) Part is moved from a 50˚C envirconment 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 in their shipping form.
Please note, the thermal dissipation constant and thermal time constant will vary according to environment and mounting conditions

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