This is a design circuit for discrete-component circuits for both continuous temperature measurement and over-temperature alarm indication have traditionally used a thermal-resistor (thermistor) for a sensor element. This circuit is based on LM75 from National Semiconductor. This is the figure of the circuit;
The most commonly used thermistor is a negative-temperature-coefficient (NTC) thermistor; as temperature increases, the resistance of the NTC thermistor decreases. For a system processor to utilize the temperature information from a thermistor, the temperature-to-resistance parameter is generally converted to a temperature-to-voltage signal. The analog temperature signal is derived directly from the voltage divider, producing a voltage level that is an analog of the thermistor's temperature (VTEMP). The RBIAS1 resistor is necessary to set the gain of the circuit and to keep the thermistor operating within its optimal power dissipation, which minimizes temperature-induced error in the resistance. The over-temperature alarm (TOVER) is generated by connecting the output of the thermistor to the input of the comparator in order to set the voltage (over-temperature level) at which the comparator output is to go active. A hysteresis feedback loop is included to keep the comparator from rapidly switching back and forth when VTEMP is equal to the VREF.
[Schematic circuit source: National Semiconductor Notes]
The most commonly used thermistor is a negative-temperature-coefficient (NTC) thermistor; as temperature increases, the resistance of the NTC thermistor decreases. For a system processor to utilize the temperature information from a thermistor, the temperature-to-resistance parameter is generally converted to a temperature-to-voltage signal. The analog temperature signal is derived directly from the voltage divider, producing a voltage level that is an analog of the thermistor's temperature (VTEMP). The RBIAS1 resistor is necessary to set the gain of the circuit and to keep the thermistor operating within its optimal power dissipation, which minimizes temperature-induced error in the resistance. The over-temperature alarm (TOVER) is generated by connecting the output of the thermistor to the input of the comparator in order to set the voltage (over-temperature level) at which the comparator output is to go active. A hysteresis feedback loop is included to keep the comparator from rapidly switching back and forth when VTEMP is equal to the VREF.
[Schematic circuit source: National Semiconductor Notes]
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