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Part Name  Description 
N_09S00223 Datasheet(PDF) 6 Page  AVX Corporation 

6 page 5 2.2.5. Voltage – Current curves V (l) These curves describe the behavior of the voltage drop V measured across the NTC as the current l through the NTC is increased. They describe the state of equilibrium between power resulting from Joule effect and dissipated power in the surroundings. (Figure 4) Figure 4 – Voltage – current curve V (l) Several zones can be identified: – low current zone dissipated energy only produces negligible heating and the curve V (l) is almost linear. – nonlinear zone the curve V (l) displays a maximum voltage Vmax for a current lo.This maximum voltage Vmax and the temper ature Tmax reached by the NTC under these conditions can be determined by using the equations: P = V2/R = (T  Tamb) and R = Ramb • exp B (1/T  1/Tamb) therefore: Tmax = B/2  B2/4  BTamb ~ Tamb Vmax = (Tmax  Tamb ) • Ramb exp [B( 1  1 )] Tmax Tamb where is the dissipation factor and Tamb is the ambi ent temperature. – high current zone for higher currents, an increase in temperature of the NTC decreases the resistance and the voltage more rapidly than the increase of the current. Above a certain dissipated power, the temperature of the NTC exceeds the permissible value. 2.2.6. Current – Time curves l(t) When voltage is applied to a thermistor, a certain amount of time is necessary to reach the state of equilibrium described by the V(l) curves. This is the heating up time of the thermistor which depends on the voltage and the resistance on one side and the heat capacity and dissipation on the other. The curves l(t) are of particular interest in timing applications. 2.2.7. Thermal time constant When a thermistor is selfheated to a temperature T above ambient temperature Tamb, and allowed to cool under zero power resistance, this will show a transient situation. At any time interval dt, dissipation of the thermistor ( (T – Tamb)dt) generates a temperature decrease –HdT, resulting in the equation: 1 dT =  dt (T  Tamb)H The solution to this equation for any value of t, measured from t = 0, is: n (T  Tamb) =  t (To  Tamb)H We can define a thermal time constant as: = H/ expressed in seconds. Where the time t = : (T  Tamb) / (To  Tamb) = exp  1 = 0.368 expressing that for t = , the thermistor cools to 63.2% of the temperature difference between the initial To and Tamb (see Figure 5). According to IEC 539 our technical data indicates mea sured with To = 85°C, Tamb = 25°C and consequently T = 47.1°C. Figure 5 – Temperature – time curve T(t) 2.2.8. Response time More generally, it is possible to define a response time as the time the thermistor needs to reach 63.2% of the total temperature difference when submitted to a change in the thermal equilibrium (for example from 60°C to 25°C in silicone oil 47V20 Rhodorsil). T ( °C) 85 47.1 25 t t (s) V Vmax Io I NTC Thermistors General Characteristics ( ) 1+Tamb B 
