Overview of NTC Thermistor
A thermistor is a variable resistance device whose resistance changes with temperature. For Positive Temperature Coefficient (PTC) thermistors, resistance increases as temperature rises. Conversely, for Negative Temperature Coefficient (NTC) thermistors, resistance decreases with an increase in temperature. Thermistors are commonly used in applications such as current limiters, temperature sensors, and overcurrent protectors.
The change in a thermistor’s resistance provides a way to measure temperature. By connecting the thermistor in series with a fixed resistor to form a simple voltage divider network, the resulting voltage varies with temperature changes. This occurs because temperature affects the thermistor’s resistance, which in turn alters the voltage output of the divider.
For more details about thermistors and their usage, refer to the topic NTC Thermistor in the Sensors and Modules section. For information on utilizing the ADC in the PIC18F4550 microcontroller, refer to the topic ADC in PIC18F4550 in the PIC Inside section.
Connection Diagram of NTC Thermistor with PIC18F4550
NTC Thermistor Interfacing with PIC18F4550
NTC Thermistor Code for PIC18F4550
/*
Thermistor Interfacing with PIC18F4550
http://www.electronicwings.com
*/
#include <pic18f4550.h>
#include <string.h>
#include <stdio.h>
#include "LCD_16x2_8-bit_Header_File.h"
#include "PIC18F4550_ADC_Header_File.h"
#include <math.h>
#define ohm 0xf4
#define B_coefficient 3950.00 /* B coefficient of NTC Thermistor*/
#define Room_temperature 25.00
#define Series_Resistance 10000.00
float Get_Temperature(int);
long NTC_Resistance;
void main(void)
{
float Temperature;
OSCCON =0x72;
LCD_Init(); /* initialize LCD16x2 */
LCD_Clear(); /* clear LCD */
ADC_Init(); /* initialize ADC */
char Temperature_buffer[20],Resistance[20];
int Analog_Input;
while(1)
{
Analog_Input = ADC_Read(0); /* store the analog data on a variable */
Temperature = Get_Temperature(Analog_Input);
sprintf(Temperature_buffer,"Temp: %.2f%cC ",Temperature,0xdf); /* convert integer to ASCII string */
LCD_String_xy(0, 0, Temperature_buffer);
sprintf(Resistance,"Res: %ld %c ",NTC_Resistance,ohm);
LCD_String_xy(1,0,Resistance);
MSdelay(1000); /* wait for 1 second */
}
}
float Get_Temperature(int analog_value)
{
float Thermistor_temperature;
analog_value = ADC_Read(0); /* store adc value on val register */
/* calculate the NTC resistance */
NTC_Resistance = ((1023*Series_Resistance/analog_value) - Series_Resistance);
Thermistor_temperature = log(NTC_Resistance); /* calculate natural log of resistance */
/* Calculate Temperature using B parameter Equation */
/* 1/T = 1/T0 + ((1/B_coefficient)*log(NTC_Resistance/Series_Resistance)) */
Thermistor_temperature = (1.0/(Room_temperature + 273.15))+
(1.0/B_coefficient)*log(NTC_Resistance/Series_Resistance));
Thermistor_temperature = (1/Thermistor_temperature) - 273.15; /* convert kelvin to °C */
return Thermistor_temperature;
}