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NTC Thermistors Application Notes R S The dissipation of a thermistor is significantly different in aliquid or in a gas, in a static fluid or in a stirred one. A liquid
level detector or a gas–flow measurement can be designed
using this property.In Figure 11, the output voltage measured on the thermistordepends upon the dissipation factor of its environment, and
can be illustrated by V-l curves (Figure 12).This voltage can be used to detect the presence (V V in V R NTC Figure 11
Figure 12
Figure 15 2 ) orabsence (V 1 ) of liquid around the thermistor or measure theflow speed.A good design should define a precise operating temperaturerange, where dissipation in the high dissipating medium athighest ambient temperature remains higher than the dissipa-
tion in low dissipating medium at lowest ambient temperature. Voltage VinV2V1Vin/RS A soft start of sensitive apparatus can be achieved by usingNTC thermistors as described in Figures 13 and 14.At turn-on, the NTC absorbs the surge current, limits the current across the equipment and protects it. Then, the thermistor heats, its resistance decreases and most of the
power becomes applied to the apparatus.In its design, the thermistor will be selected with a thermalcapacity higher than the surge energy to absorb.This property is useful when using NTC thermistor as inrushcurrent limiter for switching power supplies manufacturer (see on page 29). Currentk2k1 R NTCEquipment Figure 13 TIME DELAY Power The current-time characteristic of a thermistor is used in time
delay applications such as delaying energization of a relay
after application of power to an electrical circuit.The time delay, time necessary for the thermistor to heat upto the temperature where its resistance allows the current to
reach the switching value of the relay, is mainly defined with
the nominal resistance of the thermistor.The time delay is also strongly dependent upon the ambienttemperature, as shown in Figure 15. Unprotected equipmentProtected equipmentNTC absorbed power Time Figure 14 CurrentT = 50 ° CT = 40 ° CT = 25 ° CTime 7
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