| High sensitivity and low cost make NTC thermistors the most common device used for temperature measurement. Non-linearity of the R -T curve generally leads to the use of a resistor network to linearize the signal. An example is given in Figure 6. More precise measurements and temperature display can also be achieved with simple electronic equipment as shown in Figure 7. The choice of the model will particularly take into account the small size (better response time) and the resistance tolerance. Mounting conditions (dissipation), and input voltage (self-heating) will also be carefully defined to avoid serious errors in temperature measurement. NTC thermistors can be used as a simple on-off control temperature system or temperature alarm system. Figure 8 gives an example of such a circuit. When the temperature increases to a defined value, the resistance of the thermistor decreases and the current becomes sufficiently high to energize the relay and provide temperature alarm or heating system turn-off. The high sensitivity of thermistors (about 4% resistance change for 1°C) allows the temperature to be controlled very precisely. TEMPERATURE COMPENSATION As many electronic components (integrated circuits, amplifiers,...) have a positive temperature coefficient of resistance, NTC thermistors represent a cheap and interesting solution to compensate for this effect and provide an improved temperature stability for electronic equipment. It is necessary to include the thermistor in a resistor network (Figure 10) calculated in such a manner that the network coefficient compensates exactly for the positive temperature coefficient of the other component (Figure 9). Common leaded discs or chip thermistors are well suited for this application. |