Calculating temperature using NTC thermistor

1. Feb 5, 2008

KaK's_SLiM

Hello,
i was doing a practical experiment using NTC thermistor. I recorded the resistance at different voltages (1v, 2v, 3v, etc) and to find the temperature i need to use the temperature coefficient formula "The Steinhart-Hart equation":

R(T) = R(T0) * [ 1 + a(T - T0) ]

where
T0 = reference temperature (deg Celsius)
T = temperature of interest (deg Celsius)
R(T0) = resistance at reference temperature (ohm)
R(T) = resistance at temperature of interest (ohm)
a = temperature coefficient of resistivity (1/deg Celsius)

i am confused to how calculate the temperature.
i know: T0 = room temperature (20 degrees C)
a = for copper wire is 0.004041
T = is what im trying to find
R(T0) = ?? my thermistor says "resistance value to be 2kΩ" is this the value of R(T0)?
R(T) = this is the resistance value i get when passing current through copper wire

in some books i've seen the values of T to be in kelvin and sometimes in celcius. which is the correct way in this case? and instead of resistance some books replace it with "rho" for resistivity.

does anyone know where i can find some example worked questions to understand this.

Slim

2. Feb 5, 2008

stewartcs

$$R(T) = R(T_0) \cdot [1 + a(T - T_0)]$$

Where,

$$R(T)$$ = resistance at temperature of interest
$$R(T_0)$$ = resistance at reference temperature
$$a$$ = temperature coefficient of resistivity
$$T$$ = temperature of interest
$$T_0$$ = reference temperature

This equation is for predicting the resistance at another temperature based on a resistance at a reference temperature. The resultant resistance is due to the temperature increase at the temperature of interest.

Example:

$$R(55) = R(20) \cdot [1 + 0.004041(55 - 20)$$

$$= 223.75 \cdot 1.14143$$

$$= 255.396$$ $$\Omega$$

The temperature should be in Celsius.

I'm not 100% sure if there is a way to relate your measured voltages to temperature using this equation so I'll let an EE chime in.

CS

3. Mar 2, 2008

meereck

I suggest you to take a look at the following application notes:
Analog Devices AN 709 (right now can be found here http://www.analog.com/UploadedFiles/Application_Notes/2001119207465975025AN709_0.pdf )
Microchip AN685 - http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011704

Anyway, I dont understand what you are trying to achieve.
The formula you posted is NOT the Steinhart-Hart formula. The proper one is here http://en.wikipedia.org/wiki/Steinhart-Hart_equation

If you want to measure temperature using the NTC thermistor, check the app. notes above. Basically, you connect a resistor and NTC thermistor in series and measure a voltage drop on one of these components.
It is easy to get the NTC resistance then. If you know coefficients of Steinhart-Hart equation of your particular NTC, it is easy to work out the temperature of the NTC.

good luck
meereck

4. Mar 2, 2008