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NTC

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

An NTC is a resistor which resistance is depended of temperature.

As name indicates resistance drops with increasing temperature

The relation is nonlinear as seen on the figure just below.

 

NTC resistance temperature relationship

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

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

  2. Resistance is falling when temperature is increasing

  3. This relation is NOT linear

The 4th order approximination formula

Steinhart and Hart went public with the model in 1968. (wikipedia

You might see variants where the second order part is omitted bq it is much smaller than the first an third order part.

 

  1. Steinhart Hart formula for calculating Temp from resistance

  2. A1, B1,… are specific constants for a given 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 resistance 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

  • and for 40C 5000 Ohm might be better

  • 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