SunFounder 3in1 Kit - Page 134

SunFounder 3in1 Kit
Each thermistor has a normal resistance. Here it is 10k ohm, which is measured under 25 degree Celsius.
When the temperature gets higher, the resistance of the thermistor decreases. Then the voltage data is converted to
digital quantities by the A/D adapter.
The temperature in Celsius or Fahrenheit is output via programming.
Here is the relation between the resistance and temperature:
RT =RN expB(1/TK – 1/TN)
• RT is the resistance of the NTC thermistor when the temperature is TK.
• RN is the resistance of the NTC thermistor under the rated temperature TN. Here, the numerical
value of RN is 10k.
• TK is a Kelvin temperature and the unit is K. Here, the numerical value of TK is 273.15 + degree
Celsius.
• TN is a rated Kelvin temperature; the unit is K too. Here, the numerical value of TN is 273.15+25.
• And B(beta), the material constant of NTC thermistor, is also called heat sensitivity index with a
numerical value 3950.
• exp is the abbreviation of exponential, and the base number e is a natural number and equals 2.7
approximately.
Convert this formula TK=1/(ln(RT/RN)/B+1/TN) to get Kelvin temperature that minus 273.15 equals
degree Celsius.
This relation is an empirical formula. It is accurate only when the temperature and resistance are within
the effective range.
Wiring
130 Chapter 4. Basic Projects