Designing a thermistor temperature sensing device can be challenging if you plan to use it over its entire temperature range. A thermistor is typically a high-impedance, resistive device, so it can simplify one of the interface issues when you need to convert the thermistor's resistance to a voltage value. A more challenging interface issue, however, is how to capture the thermistor's nonlinear behavior digitally with a linear ADC.
The term "thermistor" comes from a generalization of the description "heat{{0}}sensitive resistor". Thermistors include two basic types, positive temperature coefficient thermistors and negative temperature coefficient thermistors. Negative temperature coefficient thermistors are ideal for high-precision temperature measurement. To determine the temperature around the thermistor, you can do it with the help of the Steinhart-Hart formula: T=1/(A0 plus A1(lnRT) plus A3(lnRT3)). Among them, T is the temperature in Kelvin; RT is the resistance value of the thermistor at temperature T; and A0, A1 and A3 are constants provided by the thermistor manufacturer.
Rintangan termistor berubah mengikut suhu, dan perubahan ini tidak-linear, seperti yang ditunjukkan oleh formula Steinhart-Hart. Apabila membuat pengukuran suhu, arus rujukan perlu dipacu melalui termistor untuk mencipta voltan setara yang mempunyai tindak balas bukan-linear. Anda boleh cuba mengimbangi respons bukan{3}}linear termistor menggunakan jadual rujukan yang disediakan pada mikropengawal. Walaupun anda boleh menjalankan algoritma sedemikian pada perisian tegar mikropengawal, anda masih memerlukan penukar ketepatan tinggi untuk menangkap data dalam keadaan suhu yang melampau.
Alternatively, you can use a "hardware linearization" technique and a lower precision ADC before digitizing. (Figure 1) One technique is to place a resistor RSER in series with the thermistor RTHERM and a reference voltage or power supply (see Figure 1). The PGA (Programmable Gain Amplifier) is set to 1V/V, but in such a circuit, a 10-bit precision ADC can only sense a very limited temperature range (about ±25 degree ).
Rajah 1, sila ambil perhatian bahawa kawasan suhu tinggi tidak diselesaikan dalam Rajah 1. Tetapi jika keuntungan PGA meningkat pada nilai suhu ini, isyarat keluaran PGA boleh dikawal dalam julat di mana ADC boleh memberikan yang boleh dipercayai. penukaran untuk mengenal pasti suhu termistor.
Algoritma pengesan suhu perisian tegar mikropengawal membaca nilai digital ADC ketepatan 10-bit dan memindahkannya ke rutin perisian histeresis PGA. Rutin histerisis PGA mengesahkan tetapan keuntungan PGA dan membandingkan nilai digital ADC dengan nilai nod voltan yang ditunjukkan dalam Rajah 1. Jika output ADC melebihi nilai nod voltan, mikropengawal akan menetapkan keuntungan PGA ke lebih tinggi seterusnya atau tetapan keuntungan yang lebih rendah. Jika perlu, mikropengawal mendapat nilai ADC baharu sekali lagi. Keuntungan PGA dan nilai ADC kemudiannya dihantar ke rutin interpolasi linear sekeping mikropengawal.
Getting data from a nonlinear thermistor is sometimes seen as an "impossible task". You can use a series resistor, a microcontroller, a 10-bit ADC, and a PGA to solve the measurement problems of non-linear thermistors beyond ±25 degree .
