How to convert ADC-Values from a temperatursensor?

Hi, I bought a temperature sensor with 2 electrical contacts. The sensor change its resistance in relation to the temperature and i can find the values in a table in the datasheet of the sensor. For example the resistance at 25°C and Icont (continuous sensor current)= 1 mA is between 1980 ohm and 2020 ohm.

The vendor lists the following other specs: Latency: ca. 2 s Tolerance: ± 1 % Typ: KTY 10-6 = KTY 81/210 Temperature range: -50 - +150 °C Base-Resistance: (R25) 2000 [ch937]

I connected the sensor exactly like in the following picture by using a 10kOhm Resistor.

At normal room temperatue (~25°C) i get a sensor reading of 853 from the analog pin the sensor is connected to.

How can i calculate the actual resistance of the sensor by using the value (i.e. 853) provided by the ADC?

thanks a lot, Sebastian

The ADC gives 1024 steps of reading over the range of 0 to 5V so each step is 5 / 1024 volts.

So 853 * (5 / 1024) is the voltage at the analogue input and measures the voltage across the 10K.

Using E = IR you can calculate the current through the 10K. This is the same current that is flowing through your thermistor.

So you know the current through the thermistor, you know the voltage across it (5V minus what is across the 10K) so you calculate the resistance.

853 is correct if you are using the circuit shown.

Basicly the circuit is a voltage divider.

Vout= 10k/(10k+thermistor resistance) *Vin
In this case the votlage going into the analog pin is:

Vout = 10k/12k*5V = 4.167V

Now since analog read gives you a number between 0-1024:
value = Vout/Vref *1024

In your case Vref is 5V, so:
value = 4.167V/5V*1024 = 853

Hope this explains it. Heres some more info: http://en.wikipedia.org/wiki/Voltage_divider

thank you for your quick help! No i can calculate the resistance of the thermistor. :)

kind regards, sebastian

I've used this temperature sensor. It may be a little more straight forward, although it's Fahrenheit.... http://www.allelectronics.com/index.php?page=item&id=LM34DZ