com.raspoid.additionalcomponents

Class Thermistor

java.lang.Object

com.raspoid.AnalogComponent

com.raspoid.additionalcomponents.Thermistor

All Implemented Interfaces:

Component

Direct Known Subclasses:

ThermistorNTCLE203E3103SB0

public class Thermistor
extends AnalogComponent

An NTC (negative temperature coefficient) thermistor changes its effective resistance with regard to temperature. To measure a temperature, we then need to measure the value of this resistance.

To do this, we use a an ADC (Anolog to Digital Converter) to measure the voltage value around the thermistor. We then calculate the value of the thermistor resistance, using the voltage divider in place with a resistor in serie with the thermistor.

We can then convert the resistance value with the Steinhart-Hart equation.

1/T = a + b ln(R) + c ln(R)^3

As we don't know all the requested parameters (A, B and C), we will use the B-parameter equation, which is essentially the Steinhart-Hart equation with

  a = (1/T0) - (1/B) ln(R0)
  b = 1/B
  c = 0
  

Note: with our observations, we can observe an error of about 1.4% in temperature measurements (using the Kelvin scale). A temperature of 24°C should correspond to a real temperature of 20°C. Different factors can be at the origin of this problem. It's important to keep this in mind, and use a thermistor for appropriate use. As an example, a thermistor can be used as a trigger to track a temperature exceeding a fixed threshold: alert when the tap water is too hot. It is less appropriate to precisely define the room temperature.

Formulas from: https://en.wikipedia.org/wiki/Thermistor

Field Summary

Fields

Modifier and Type	Field and Description
static double	K_TO_C Temperature in °C is temperature in K - K_TO_C.
static double	T0 T0 coefficient from the B-parameter equalition, in K.

Fields inherited from class com.raspoid.AnalogComponent

adc

Constructor Summary

Constructors

Constructor and Description
Thermistor(ADC adc, ADCChannel thermistorChannel, double serieResistor, double r0, double b) Constructor for a thermistor using a specific ADC instance, a specific channel on this ADC, and specific parameters regarding the thermistor characteristics.

Method Summary

Modifier and Type	Method and Description
double	getTemperature() Return the measured temperature, in °C.

Methods inherited from class com.raspoid.AnalogComponent

getADC, getType

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

T0

public static final double T0

T0 coefficient from the B-parameter equalition, in K.

298.15K = 25°C.

See Also:

Constant Field Values

K_TO_C

public static final double K_TO_C

Temperature in °C is temperature in K - K_TO_C.

See Also:

Constant Field Values

Constructor Detail

Thermistor

public Thermistor(ADC adc,
                  ADCChannel thermistorChannel,
                  double serieResistor,
                  double r0,
                  double b)

Constructor for a thermistor using a specific ADC instance, a specific channel on this ADC, and specific parameters regarding the thermistor characteristics.

Parameters:

adc - the ADC to use to decode analog voltage values around the thermistor.

thermistorChannel - the channel used on the ADC to read voltage values around the thermistor.

serieResistor - the value of the resistance placed in serie with the thermistor.

r0 - the resistance of the thermistor, at 25 degrees Celsius (cfr thermistor datasheet).

b - the B-parameter equation coefficient (cfr thermistor datasheet).

Method Detail

getTemperature

public double getTemperature()

Return the measured temperature, in °C.

Keep in mind the note presented in this class header: Thermistor

Returns:

the measured temperature, in °C.