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.
Termistorning qarshiligi haroratga qarab o'zgaradi va bu o'zgarish -chiziqli emas, buni Shteynxart-Xart formulasi ko'rsatadi. Haroratni o'lchashda chiziqli bo'lmagan javobga ega bo'lgan ekvivalent kuchlanishni yaratish uchun termistor orqali mos yozuvlar oqimini o'tkazish kerak. Mikrokontrollerda berilgan mos yozuvlar jadvalidan foydalanib, termistorning chiziqli bo'lmagan - javobini kompensatsiya qilishga urinib ko'rishingiz mumkin. Agar siz mikrokontroller proshivkasida bunday algoritmni ishga tushirishingiz mumkin bo'lsa ham, haddan tashqari harorat mavjud bo'lganda ma'lumotlarni olish uchun sizga yuqori aniqlikdagi konvertor kerak bo'ladi.
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 ).
1-rasm, iltimos, yuqori harorat mintaqasi 1-rasmda hal qilinmaganligini unutmang. Ammo agar bu harorat qiymatlarida PGA daromadi oshirilsa, PGA chiqish signali ADC ishonchli signalni ta'minlay oladigan diapazonda boshqarilishi mumkin. termistorning haroratini aniqlash uchun konversiyalar.
Mikrokontroller proshivkasining haroratni sezish algoritmi 10-bitli aniqlikdagi ADC raqamli qiymatini o'qiydi va uni PGA histerezis dasturiga o'tkazadi. PGA histerezisi tartibi PGA daromad sozlamasini tekshiradi va ADC raqamli qiymatini 1-rasmda ko'rsatilgan kuchlanish tugunining qiymati bilan solishtiradi. Agar ADC chiqishi kuchlanish tugunining qiymatidan oshsa, mikrokontroller PGA daromadini keyingi yuqoriroqqa o'rnatadi. yoki pastroq daromad sozlamasi. Agar kerak bo'lsa, mikrokontroller yana yangi ADC qiymatini oladi. Keyin PGA daromadi va ADC qiymatlari mikrokontrollerga qismli chiziqli interpolyatsiya tartibiga o'tkaziladi.
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 .
