Main parameters of NTC thermistor.Smart PIR switch

　　Thermal time constant (τ): Thermal time constant is a parameter that describes the thermal inertia of the thermistor. In still air, in the state of no power consumption, when the ambient temperature suddenly changes from a specific temperature to another specific temperature, The time it takes for the thermistor to change from its own temperature to 63.2% of the difference from the ambient temperature. That is, under the condition of zero power, when the temperature suddenly changes, the time required for the temperature of the thermistor to change by 63.2% of the temperature difference between the beginning and the end, the smaller the τ, the smaller the thermal inertia of the thermistor. The thermal capacity of the NTC thermistor is proportional to its dissipation factor

　　τ=C/δ

　　where τ is the thermal time constant, s; C is the thermal capacity of the NTC thermistor; δ is the dissipation coefficient of the NTC thermistor.

　　When the ambient temperature of the thermistor changes from T1 to T2, the following relationship exists between the elapsed time t and the temperature T of the thermistor

　　T=(t1-t2)exp(-t/τ)+T2

　　=(T2 -T1)[1-exp(t/τ)+T1

　　(TーT1)/(T2ーT1)=0.632

　　The thermal time constant τ values given in the NTC thermistor catalog are typical values under the following measurement conditions: In still air,

　　When the ambient temperature changes from 50 to 25 ℃, the time required for the temperature of the thermistor to change to 34.2 ℃, the NTC thermistor is of the axial lead radial lead type, which is measured in the factory state.

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