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.
Otpor termistora se mijenja s temperaturom, a ova promjena je ne-linearna, kao što pokazuje Steinhartova-Hartova formula. Prilikom mjerenja temperature, referentnu struju treba provući kroz termistor kako bi se stvorio ekvivalentni napon koji ima ne-linearni odgovor. Možete pokušati kompenzirati ne-linearni odziv termistora koristeći referentnu tabelu koja se nalazi na mikrokontroleru. Čak i kada biste mogli da pokrenete takav algoritam na firmveru mikrokontrolera, i dalje bi vam bio potreban konvertor visoke preciznosti za hvatanje podataka u prisustvu ekstremnih temperatura.
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 ).
Na slici 1, imajte na umu da područje visoke temperature nije riješeno na slici 1. Ali ako se pojačanje PGA poveća na ovim temperaturnim vrijednostima, izlazni signal PGA može se kontrolisati unutar raspona unutar kojeg ADC može pružiti pouzdan konverzije za identifikaciju temperature termistora.
Algoritam senzora temperature firmvera mikrokontrolera čita 10-bitnu preciznu digitalnu vrijednost ADC-a i prenosi je u softversku rutinu PGA histereze. PGA histerezna rutina provjerava postavku PGA pojačanja i upoređuje digitalnu vrijednost ADC-a sa vrijednošću naponskog čvora prikazanog na slici 1. Ako izlaz ADC-a premašuje vrijednost naponskog čvora, mikrokontroler će postaviti PGA pojačanje na sljedeći viši ili niže podešavanje pojačanja. Ako je potrebno, mikrokontroler ponovo dobija novu vrednost ADC. Vrijednosti PGA pojačanja i ADC-a se zatim prosljeđuju rutini podjele linearne interpolacije mikrokontrolera.
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 .
