A thermistor (THERMally sensitive resISTOR) is an electrical resistance whose value changes with the temperature.
Thermistors are divided into two groups, hot conductors which have a negative temperature coefficient (NTC), conduct electrically in a hot state better than in the cold state and cold conductors, which have a positive temperature coefficient (PTC) and conduct better electrically when cold. Metals, semiconducting metal oxides (ceramic materials) or silicon are used as resistance material, using both positive and negative temperature coefficients, depending on the application.
Thermistors with negative temperature coefficients (NTC) have a high dependence on flaws, such as the doping of the raw materials, due to the underlying semiconductor effect. Processing such as mixing, grinding, pressing, sintering has a major influence on properties and long-term stability. As a result, NTCs have long been only able to produce with highly scattering characteristics, and in their early days, NTCs have earned a reputation for being unsuitable for precise temperature measurement. Their nonlinear behaviour is described by nonlinear equations. The preferred representation of the dependency is given under Resistance Thermometer. Another common representation of the nonlinear relationship is the Steinhart-Hart equation. NTC Thermistors are also used to limit inrush currents to allow a smooth start-up. A hot conductor in the supply line of an electrical device is cold before switching on, conducts at the moment of activation still poorly and limits the inrush current.
10K Ohm resistance NTC thermistors in bead design with wire are commonly used for temperature measurement. They are also used in medical fields to measure body temperature. The DS18B20 (by MAXIM) IC works well for most of the temperature applications. The latest DS18B20 1 wire digital temperature sensor is good for many temperature control projects. 10K Ohm resistance NTC thermistors are a cheap and good thing to try.
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This is the schematic of a 10K Ohm resistance NTC thermistor with a 10K Ohm resistor :
Actually, you need the datasheet to code perfectly :
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 | //Thermometer with thermistor /*thermistor parameters: * RT0: 10 000 Ω * B: 3977 K +- 0.75% * T0: 25 C * +- 5% */ //These values are in the datasheet #define RT0 10000 // Ω #define B 3977 // K //-------------------------------------- #define VCC 5 //Supply voltage #define R 10000 //R=10KΩ //Variables float RT, VR, ln, TX, T0, VRT; void setup() { Serial.begin(9600); T0 = 25 + 273.15; //Temperature T0 from datasheet, conversion from Celsius to kelvin } void loop() { VRT = analogRead(A0); //Acquisition analog value of VRT VRT = (5.00 / 1023.00) * VRT; //Conversion to voltage VR = VCC - VRT; RT = VRT / (VR / R); //Resistance of RT ln = log(RT / RT0); TX = (1 / ((ln / B) + (1 / T0))); //Temperature from thermistor TX = TX - 273.15; //Conversion to Celsius Serial.print("Temperature:"); Serial.print("\t"); Serial.print(TX); Serial.print("C\t\t"); Serial.print(TX + 273.15); //Conversion to Kelvin Serial.print("K\t\t"); Serial.print((TX * 1.8) + 32); //Conversion to Fahrenheit Serial.println("F"); delay(500); } |
