NTC self-heating problem.Infrared sensing module

　　Since the thermistor is a resistor, a certain amount of heat will be generated when the current flows through it, so the circuit designer should ensure that the pull-up resistor is large enough to prevent the thermistor from overheating, otherwise the system measures the output from the thermistor heat, not the ambient temperature.

　　The effect of the energy dissipated by the thermistor on temperature is expressed by the dissipation constant, which refers to the number of milliwatts required to raise the thermistor temperature by 1°C above the ambient temperature. The dissipation constant varies depending on the thermistor package, pin size, encapsulation material, and other factors.

　　The allowable self-heating and current-limiting resistance of the system are determined by the measurement accuracy. A measurement system with a measurement accuracy of ±5°C can withstand more self-heating of the thermistor than a measurement system with an accuracy of ±1°C.

　　It should be noted that the value of the pull-up resistor must be calculated to limit the self-heating power dissipation over the entire measurement temperature range. After the resistance value is given, the dissipated power varies at different temperatures due to the change in the resistance value of the thermistor.

　　Sometimes it is necessary to calibrate the input of the thermistor in order to obtain appropriate temperature resolution. Figure 3 is a circuit that extends the temperature range of 10 to 40°C to the entire 0 to 5V input range of the ADC.

　　Once the thermistor input has been calibrated, the actual resistance versus temperature can be graphed. Since the thermistor is non-linear, it needs to be represented by a graph. The system needs to know what the value of the ADC corresponds to each temperature. The accuracy of the table is specifically 1°C increments or 5°C increments, depending on the specific application. Certainly.

　　cumulative error

　　When measuring temperature with a thermistor, select sensors and other components in the input circuit to match the required accuracy. Some applications require resistors with 1% accuracy, while others may require resistors with 0.1% accuracy. In any case, a table is used to calculate the effect of the cumulative error of all components, including the resistor, the reference voltage, and the thermistor itself, on the measurement accuracy.

　　If high accuracy is required and you want to spend less money, you need to calibrate the system after it is built, which is generally not recommended since the circuit board and thermistor must be replaced in the field. In the case that the equipment cannot be replaced on site or the engineer has other methods to monitor the temperature, the software can also create a table of the temperature corresponding to the ADC change. At this time, other tools are needed to measure the actual temperature value before the software can create the corresponding table. For some systems that must replace the thermistor in the field, the component to be replaced (sensor or the entire analog front end) can be calibrated at the factory, and the calibration results are saved on disk or other storage media. Of course, after the component is replaced, the software It must be known to use the calibrated data.

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