Thermistors are used as the sensor low temperature controllers, linearizers for resistance circuits, moisture detectors, and air flow detectors. The resistance of a thermistor can vary from 50 โอห์ม to 2 Mโอห์ม at 25 C. Thermistors are available with negative temperature coefficients(NTC) and positive temperature coefficients (PTC) with those possessing an NTC being much more prevelant in the commercial market.
Figure 14.26 is a block diagram of the method used to convert a thermistor output to a digital display of temperature. The microprocessor can be replaced by an EPROM. In this case the output from the A/D converter is the address of the code in the EPROM that corresponds to a particular temperature. This code is directly output to the display driver, which is simply a buffer or inverter, depending upon whether the display is common cathode, common anode, or LCD matrix.
Special linearized thermistor networks can be built and are available commercially in single-package form (e.g., from Yellow Springs Instrument Co.). Such products