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handbook of modern sensors

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16.1 Thermoresistive Sensors 465<br />

Fig. 16.4. Resistivity and number <strong>of</strong> free charge carriers for n-doped silicon.<br />

where R 0 and T 0 are the resistance () and temperature (K) respectively, at a reference<br />

point. For instance, for the KTY-81 <strong>sensors</strong> operating in the range from −55 ◦ Cto<br />

+150 ◦ C, the coefficients are A = 0.007874 K −1 and B = 1.874 × 10 −5 K −2 .Atypical<br />

transfer function <strong>of</strong> the sensor is shown in Fig. 16.5.<br />

16.1.3 Thermistors<br />

The term thermistor is a contraction <strong>of</strong> the words thermal and resistor. The name<br />

is usually applied to metal-oxide <strong>sensors</strong> fabricated in the form <strong>of</strong> droplets, bars,<br />

cylinders, rectangular flakes, and thick films. A thermistor belongs to the class <strong>of</strong><br />

absolute-temperature <strong>sensors</strong>; that is, it can measure temperature that is referenced<br />

to an absolute-temperature scale. All thermistors are divided into two groups: NTC<br />

(negative temperature coefficient) and PTC (positive temperature coefficient). Only<br />

the NTC thermistors are useful for precision temperature measurements.<br />

16.1.3.1 NTC Thermistors<br />

A conventional metal-oxide thermistor has a NTC; that is, its resistance decreases<br />

with the increase in temperature. The NTC thermistor’s resistance, as <strong>of</strong> any resistor,

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