Electronic Components Datasheet Search
 Selected language English  ▼

N_06S00473 Datasheet(PDF) 6 Page - AVX Corporation

 Part No. N_06S00473 Description NTC Thermistors Download 38 Pages Scroll/Zoom 100% Maker AVX [AVX Corporation] Homepage http://www.avx.com Logo

 6 page 52.2.5. Voltage – Current curves V (l)These curves describe the behavior of the voltage drop Vmeasured across the NTC as the current l through the NTCis increased.They describe the state of equilibrium between powerresulting from Joule effect and dissipated power in thesurroundings. (Figure 4)Figure 4 – Voltage – current curve V (l)Several zones can be identified:– low current zonedissipated energy only produces negligible heating andthe curve V (l) is almost linear.– non-linear zonethe curve V (l) displays a maximum voltage Vmax for acurrent lo.This maximum voltage Vmax and the temper-ature Tmax reached by the NTC under these conditionscan be determined by using the equations:P = V2/R =(T - Tamb)andR = Ramb • exp B (1/T - 1/Tamb)therefore:Tmax = B/2 -B2/4 - BTamb ~ TambVmax =(Tmax - Tamb ) • Ramb exp[B(1 - 1 )]Tmax Tambwhereis the dissipation factor and Tamb is the ambi-ent temperature.– high current zonefor higher currents, an increase in temperature of theNTC decreases the resistance and the voltage morerapidly than the increase of the current. Above a certaindissipated power, the temperature of the NTC exceedsthe permissible value.2.2.6. Current – Time curves l(t)When voltage is applied to a thermistor, a certain amount oftime is necessary to reach the state of equilibrium describedby the V(l) curves.This is the heating up time of the thermistor which dependson the voltage and the resistance on one side and the heatcapacity and dissipation on the other.The curves l(t) are of particular interest in timing applications.2.2.7. Thermal time constantWhen a thermistor is self-heated to a temperature T aboveambient temperature Tamb, and allowed to cool under zeropower 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:1dT = -dt(T - Tamb)HThe solution to this equation for any value of t, measuredfrom t = 0, is:n (T - Tamb) = -t(To - Tamb)HWe can define a thermal time constantas:= H/expressed in seconds.Where the time t =:(T - Tamb) / (To - Tamb) = exp - 1 = 0.368expressing that for t = , the thermistor cools to 63.2% of thetemperature difference between the initial To and Tamb (seeFigure 5).According to IEC 539 our technical data indicatesmea-sured with To = 85°C, Tamb = 25°C and consequentlyT = 47.1°C.Figure 5 – Temperature – time curve T(t)2.2.8. Response timeMore generally, it is possible to define a response time as thetime the thermistor needs to reach 63.2% of the totaltemperature difference when submitted to a change in thethermal equilibrium (for example from 60°C to 25°C insilicone oil 47V20 Rhodorsil).T (°C)8547.125tt (s)VVmaxIoINTC ThermistorsGeneral Characteristics()1+TambB