I am a complete noob to temperature measurement. I initially tried to measure temperature using LM35 but i got negative values at room temperature and the values were fluctuating a lot.
I want to try to measure temp between 0-100 degrees Celsius using thermistor and found out that need to connect one end of the thermistor to a resistor and then to ground for voltage divider rule. How do i calculate this resistance. i randomly tried 10K ohms and am getting room temperature values, but it is not measuring any higher. i also tried 100k and 3.3k but i am getting extremely small or negative values even at room temperature.I hope you guys can help me out.
Electrical – How to calculate resistance to use along with NTC 103 thermistor to measure temperatures between 0-100 degrees celcius
temperaturethermistor
Related Solutions
Looks like there's some simple issue with the code.
To assist with getting the Steinhart-Hart equation parameters right, you can refer to the part specifications.
That refers to the "M" curve.
From the website, we get:
Which gives you the resistance values for various temperatures (multiply by 1000 for the values of your part in ohms).
Then, you can calculate the Steinhart-Hart parameters directly (from the Wikipedia page)
Or you can try this online calculator (I have not verified it is accurate)
In either case, optimize the thermistor linearization by picking three temperatures well spaced apart covering your range of interest.
It appears that the reading has already been scaled by the software, since the 315 ADC reading you are seeing is very close to the 312 you'd expect with a room temperature of 31.2°C (which, in turn, seems about 10°C too hot for comfort, but that's just my opinion).
As you've found, you only need to divide the number (which is the mV) by 10.0 (mV/°C) to get the temperature in Celsius.
Best Answer
Thermistors are harder to use than the LM35- they are usually very nonlinear.
You need the parameters for the model of thermistor you have, either the \$\beta\$ (and the associated \$T_0\$) or the Steinhart–Hart equation parameters.
Once you have that, you can estimate the temperature as a function of the resistance. For example, the simplified equation:
\$\frac{1}{T} = \frac{1}{T_0} + \frac{1}{\beta}\ln \left(\frac{R}{R_0}\right)\$
One common set of parameters for 10K thermistors is 25/85 3977.
Of course you find the resistance from the ADC reading and a series resistance set up as a voltage divider. You would typically want to maximize the resolution near the temperature where it will be most used, so the series resistor can be chosen to be the same as the thermistor resistance at that temperature. For example, if the thermistor is ~5K at 43°C (and that is near where it will be used) you can use a series 4K99 resistor, and the voltage will be Vin/Vref = R/(R+4K99)
Rx = \$ \frac{4990}{\frac{Vref}{Vin}-1)}\$
You would also want to move to a higher resistance thermistor and series resistor if the self-heating is excessive for your application.
Rather than use transcendental functions of the S-H equation in your MCU you can do the calculations once and use a lookup table to do the conversion from ADC reading to temperature.