Use math for NTC temperature calculations

Per Svensson · Joined: 03 Oct 2004 Posts: 235 Location: Gothenburg, Sweden

Using tables for calculating temperature from thermistor readings is not very flexible when Alfa and Beta parameters varies

A more general method is to use the formula Temp(Celcius) =Ro*EXP(Beta/(273.15+$A32)-Beta/(273.15+25))
See attached Excel sheet where you can simulate all variants as long as Alfa and Beta are known.

If there is an interest I can also upload a bascom implementation so you don't have to figure out the math coding

/Per

Per Svensson · Joined: 03 Oct 2004 Posts: 235 Location: Gothenburg, Sweden

… And here is the code

'************************************************************************************
' A thermistor is connected from Vref (+5V) to GND via a resistor R1.
' The voltage to the ADC is a nonlinear representation of the temperature.
' NTC resistance is nominally Res25 ohm at 25degC and R1 is fixed and can be any value. But..
' the best accuracy when R1 is selected to equal NTC's resistance at the temp where best accuracy is desired.
' See Excel spreadsheet "NTC beräkningar.xls"

' /---> To ADC
' |
'Vref o---NTC---o---R1-----|

'Farnell 732-138
Const Beta = 3620 'Thermistors Beta value
Const Res25 = 10000 'Thermistors resistance at 25 deg C
Const Res1 = 10000 'Fixed resistor to ground (Ususally close to Res25)

Function Adc_to_temp(byval Adcval As Word) As Single
'****************************************************************
' Convert from ADC reading to temperature Celsius
' T = 1/Beta(ln(R1/R25((1024/ADCVAL)-1))+1/(273.15+25)) - 273.15
'****************************************************************
Local X As Single

X = Adcval 'Avarage voltage expressed in ADC-units (0-1023)

'convert ADC-output to temperature using a logarithmic relation:
X = 1024 / X 'Ratio of max value
X = X - 1
X = X * Res1 'Resistor connector to ground
'X is now the resistance of the thermistor. Compute the temperature this represents
X = X / Res25 'NTC resistance at 25 deg C
X = Log(x)
X = X / Beta
X = X + 0.003354
X = 1 / X
X = X - 273.15 'Kelvin -> Celsius
'T is now the true temperature in degrees Celsius

Adc_to_temp = X + Temp_offset 'adjust sample with offset and Return result

End Function