NTC Power Thermistor
Description
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NTC POWER THERMISTOR, SCK SERIES
Meritek
Thermal dissipation coefficient( δ )
The thermal dissipation coefficient is the ratio, normally expressed in milliwatts per degree C (mW/oC), at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change.
(δ =VXI/∆T)
Surge energy
Surge energy is the maximum energy of pulses.
The thermistor is capable of tolerating surge energy more than 1000 times with the resistance changing rates within ± 10%.
This energy varies with voltage and capacitance.
Thermal time constant( t )
The thermal time constant is the time required for a thermistor to change 63.
2 percent of total difference between its initial and final body temperature when subjected to a step function change in ambient temperature under zeropower condition and is normally expressed in seconds.
Storage temperature range:- 40 to +125 o C o Operating temperature range:- 30 to +125 C
DERATING CURVE OF SURGE CURRENT LIMITING THERMISTOR
The maximum power of a thermistor will decrease with the change of ambient temperature.
(See Fig.
G)
Material constant(B)
The material constant of a NTC thermistor is a measure of its resistance at one temperature compared to its resistance at a different temperature.
Its value may be calculated by the formula shown below and is expressed in degrees kelvin (oK ).
The reference temperature used in this formula for determining material constant rating of Meritek’s thermistor is 298.
15'oK and 323.
15 oK B=Ln(R1/R 2)/( 1/T1-1/T2 )
Temperature coefficient of resistance (α )
The temperature coefficient of resistance is the ratio at a specified temperature, T, of the rate of change of zero-power resistance with temperature to the zeropower resistance of the thermistor.
The temperature coefficient is commonly expressed in percent per degree C(%/oC).
α =I /R dR/dT
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