1. Thermistor sensor temperature measurement
Thermistor sensors used to measure temperature are generally simple in structure and inexpensive. Thermistors without an outer protective layer can only be used in dry places; Hermetically sealed thermistors are not afraid of moisture and can be used in harsh environments. Due to the large resistance value of the thermistor sensor, the resistance and contact resistance of the connecting wire can be ignored, so the thermistor sensor can be used in the long-distance measurement temperature measurement of up to several kilometers, and the measurement circuit mostly adopts a bridge. Using its principle, it can also be used as other temperature measurement and temperature control circuits.
2. The thermistor sensor is used for temperature compensation
Thermistor sensors compensate for the humidity of certain components over a certain temperature range. For example, the moving coil in the moving coil meter head is made of copper wire. As the temperature increases, the resistance increases, causing temperature errors. Therefore, the thermistor with negative temperature coefficient can be connected in parallel with the manganese-copper wire resistor in the loop of the moving coil and then connected in series with the compensated component, so as to offset the error caused by the internal temperature change. In transistor circuits and logarithmic amplifiers, thermistors are also commonly used to form compensation circuits. Compensates for drift errors due to temperature.
3. Real-time output of temperature information
The amount of digital data acquired is clearly linearly related to the real surface temperature of the device, and the specific sensitivity depends on the model and performance of the PT100 and the choice of reference voltage. When the system is determined, it is easy to determine the functional relationship between the temperature signal and the voltage signal, and the scale transformation is realized in the microcontroller through the software to obtain the value of the temperature, which is converted into 8421BCD code. The temperature data is output, and the temperature data is displayed on the common yang digital tube in the form of dynamic scanning by the external decoding chip, and this signal can also be transmitted to the control station at the work site for manual monitoring.
4. Overheat protection of thermistor sensor
Overheating protection is divided into direct protection and indirect protection. For small current occasions, the thermistor sensor can be directly connected to the load to prevent overheating damage to protect the device, and for high current occasions, it can be used for the protection of relays, transistor circuits, etc. In either case, the thermistor is tightly bound to the device being protected, allowing for sufficient heat exchange between the two, and in the event of overheating, the thermistor acts as a protector. For example, a mutant thermistor sensor is embedded in the stator winding of an electric motor and connected in series with a relay. When the motor is overloaded, the stator current increases, causing heating. When the temperature is greater than the abrupt change point, the current in the circuit can be abruptly changed from a few tenths of a milliampere to tens of milliamps, so the relay acts, so as to achieve overheating protection.
5. The thermistor sensor is used for the measurement of the liquid level
When an NTC thermistor sensor is applied a certain heating current, its surface temperature will be higher than the ambient air temperature, and its resistance will be smaller. When the liquid is higher than its installation height, the liquid will take away its heat, causing its temperature to drop and the resistance to rise. Judging the change in its resistance value, you can know whether the liquid level is lower than the set value. The fuel level alarm sensor in the automobile fuel tank is made using the above principle. Thermistors are also used in automobiles to measure oil temperature, cooling water mixing, etc.
6. Digitize the electrical signal converted by the temperature signal
The output signal of the temperature measurement circuit is analog, and in order to identify and process the obtained temperature signal, it must also go through an A/D conversion process. There are many types of A/D chips, including counter A/D converters, double integral A/D converters, successive approach A/D converters, and parallel A/D converters. Here we use successive approximation A/D converters, which are widely used in computers. ADC0809 is an 8-bit successive approximation A/D converter with an enable terminal that can be connected to a microcomputer. The ADC0809 consists of 8 analog switches, address latches and decoders, comparators, 256 resistor ladders, tree switches, successive approximation registers SAR, control circuits and three-state output latches. The interface between the microcontroller and the ADC will not be repeated here. Here it is enough to perform the task of data conversion and delivery to the microcontroller.
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