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NTC

NTC - Negative Temperature Coefficient resistors is used in many places for measuering … temperature.

 

NTC resistance temperature relationship

  1. Resistance is falling when temperature is increasing

  2. This relation is NOT linear

  3. NTCs are specificed by nominal resistance which is the resistance at 25 C

  4. So - by inspection - this figure is for a 10 kOhm NTC(read resistance at 25C)

The 4th order approximination formula

 

  1. Steinhart Hart formula for calculating Temp from resistance

  2. A1, B1,… are specific constants for a givre NTC.

  3. See datasheet for values

Normal way of operation

The schematics below shows a standard setup.

By use of a voltage divider its easy to calculate the resisatcne of the NTC when knowing supply voltage

 

  1. Calcuate resistance for Termistor(NTC)

  2. Calculate temperature from NTC resistance by use of an approximation formula

  3. See an example in annotated datasheet for 10 kOhm NTC

About proper selection of resistor in series with the NTC

Take a look at the figure below for at 10 kOhm NTC

 

Resistance for an NTC for temperature interval -50C to 200C and 0C to 50C

  • So measuring -25C it may be wise to use a series R around 150 kOhm…

  • You can calculate the figure yourself 

Schematic  -  from above
---------------

Rldr  == resistance of LDR


Vo                  =    Vcc  * ( Rldr / (Rldr + R) )

Vo * (Rldr + R)     =    Vcc * Lldr

Rldr * ( Vcc - Vo ) =    R * Vo

Rldr                =    R  * ( Vo /( Vcc - Vo) )

QED

comment: you need to know Vs and R
 

A small NTC of value 47 kOhm (color coded)

  • NTC can be of various physical size

  • heavy NTC reacts slow

  • small eract fast ..

  • small Ohm values and relaively large current adds self heating

    • P = U I = I^2 / R

    • So take caution

Add info

In the datasheet just below you will find the approximation for calculation of temperature from measured resistance.

Its a 4th order approximation

Arduino code

See

  • ntc.zip for a library and a small demo

A movie