The sensor circuit design of digital thermometer
The temperature sensing element in the digital display (electronic) temperature is mainly P-N type semiconductors. PT100 platinum resistance and NTC type thermistor from the temperature measurement principle, they have a common characteristic:
That is, the resistance of its own material can change with temperature (the volt-ampere characteristic of the P-N junction has -2.2mv/℃ characteristic).
These temperature sensing elements have their own characteristics, such as easy integration of P-N junction semiconductors, good linearity of platinum resistance, large temperature coefficient of NTC thermistor, and so on. In the past ten years, digital (electronic) display thermometers have been widely used. Among them, the digital display thermometer quickly replaced the glass mercury thermometer because of its intuitive, accurate, convenient, safe use, and pollution-free reading.
The outstanding advantages of NTC thermistor as a temperature sensing element are small size, large temperature coefficient of resistance, rapid response, low price and high cost performance. The basic working principle of the digital (electronic) display thermometer using the NTC thermistor as the temperature data acquisition system is shown in Figure (10).
NTC thermistor is used as the front-end input for temperature data collection. After entering the amplifying circuit for sufficient amplifying processing, it enters the A/D conversion circuit in the CPU. The high-level double integral A/D circuit converts the collected temperature value (analog) into a fine digital bit signal. Then give the CPU a series of operation transformation processing to drive the display, and realize the real-time display of the temperature of the measured object.
(2) Application method of NTC thermistor:
According to the requirements of design accuracy, resolution and temperature range, select the NTC thermistor with higher material constant (B value), better linearity in the temperature measurement interval, and response speed, and the package size meets the requirements;
Due to the convenience of portability and the use of button batteries in a small size, the bias voltage on the circuit is very small (usually 2V), which often conflicts with the accuracy and resolution required by the design. Therefore, special attention must be paid to minimize the impact of system errors, and the pre-amplification must be large enough. Pay attention to the specific design:
(1) The self-heating effect of NTC thermistor should be as small as possible. That is, the bias current of the NTC thermistor is as small as possible (uA~mA level).
(2) The rated resistance of NTC thermistor should be as accurate as possible. That is, generally select varieties with an accuracy of> 5%.
(3) Use the R25 value of the selected NTC thermistor to correct the reference temperature of the entire system.
(4) Do a good job of non-linear correction.
Figure (11) is to use NTC thermistor as the sensitive element to collect temperature data, and then use an operational amplifier to amplify the temperature data to 0~2V. Of course, it is also easy to implement with ordinary single-chip microcomputers, and can also be implemented with dedicated ICL7136 series chip circuits.
The NTC thermistor for MF51E electronic thermometer produced by YAXUN Electronic Technology Co., Ltd. has excellent performance and reliable quality. The product was approved by the Science and Technology Innovation Fund of the Ministry of Science and Technology: Project Code: 7C26213210427.
Approval number: Guokefajizi [2007] No. 335;
High-tech product of thermistor: No. 150115G0375N;
The key promotion and application of thermistor: No.201802096;
The key promotion and application of thermistor: No.2018NRC19166;
Compared with its peers, this NTC thermistor produced by YAXUN Electronics has a low annual resistance drift rate (≤0.04%/year). Small size, fast response speed, high temperature measurement accuracy (within the temperature range of 30°C to 43°C, the temperature measurement accuracy reaches ±0.012°C) and other outstanding advantages. The digital display clinical thermometer designed and manufactured with it can be easily realized:
The measuring range is 30℃~43℃;
The display accuracy is up to 0.1℃;
The temperature resolution reaches 0.02℃, and the response speed is ≤2.8 seconds. After 10 years of use, the temperature error due to the resistance drift of the thermistor is less than 0.1°C.
That is, the resistance of its own material can change with temperature (the volt-ampere characteristic of the P-N junction has -2.2mv/℃ characteristic).
These temperature sensing elements have their own characteristics, such as easy integration of P-N junction semiconductors, good linearity of platinum resistance, large temperature coefficient of NTC thermistor, and so on. In the past ten years, digital (electronic) display thermometers have been widely used. Among them, the digital display thermometer quickly replaced the glass mercury thermometer because of its intuitive, accurate, convenient, safe use, and pollution-free reading.
The outstanding advantages of NTC thermistor as a temperature sensing element are small size, large temperature coefficient of resistance, rapid response, low price and high cost performance. The basic working principle of the digital (electronic) display thermometer using the NTC thermistor as the temperature data acquisition system is shown in Figure (10).
NTC thermistor is used as the front-end input for temperature data collection. After entering the amplifying circuit for sufficient amplifying processing, it enters the A/D conversion circuit in the CPU. The high-level double integral A/D circuit converts the collected temperature value (analog) into a fine digital bit signal. Then give the CPU a series of operation transformation processing to drive the display, and realize the real-time display of the temperature of the measured object.
(2) Application method of NTC thermistor:
According to the requirements of design accuracy, resolution and temperature range, select the NTC thermistor with higher material constant (B value), better linearity in the temperature measurement interval, and response speed, and the package size meets the requirements;
Due to the convenience of portability and the use of button batteries in a small size, the bias voltage on the circuit is very small (usually 2V), which often conflicts with the accuracy and resolution required by the design. Therefore, special attention must be paid to minimize the impact of system errors, and the pre-amplification must be large enough. Pay attention to the specific design:
(1) The self-heating effect of NTC thermistor should be as small as possible. That is, the bias current of the NTC thermistor is as small as possible (uA~mA level).
(2) The rated resistance of NTC thermistor should be as accurate as possible. That is, generally select varieties with an accuracy of> 5%.
(3) Use the R25 value of the selected NTC thermistor to correct the reference temperature of the entire system.
(4) Do a good job of non-linear correction.
Figure (11) is to use NTC thermistor as the sensitive element to collect temperature data, and then use an operational amplifier to amplify the temperature data to 0~2V. Of course, it is also easy to implement with ordinary single-chip microcomputers, and can also be implemented with dedicated ICL7136 series chip circuits.
The NTC thermistor for MF51E electronic thermometer produced by YAXUN Electronic Technology Co., Ltd. has excellent performance and reliable quality. The product was approved by the Science and Technology Innovation Fund of the Ministry of Science and Technology: Project Code: 7C26213210427.
Approval number: Guokefajizi [2007] No. 335;
High-tech product of thermistor: No. 150115G0375N;
The key promotion and application of thermistor: No.201802096;
The key promotion and application of thermistor: No.2018NRC19166;
Compared with its peers, this NTC thermistor produced by YAXUN Electronics has a low annual resistance drift rate (≤0.04%/year). Small size, fast response speed, high temperature measurement accuracy (within the temperature range of 30°C to 43°C, the temperature measurement accuracy reaches ±0.012°C) and other outstanding advantages. The digital display clinical thermometer designed and manufactured with it can be easily realized:
The measuring range is 30℃~43℃;
The display accuracy is up to 0.1℃;
The temperature resolution reaches 0.02℃, and the response speed is ≤2.8 seconds. After 10 years of use, the temperature error due to the resistance drift of the thermistor is less than 0.1°C.
