logo
Navigation
Home
Jens
python code
Mea mockup
B25-85
online calc
NTC + selfheating
articles
test of Steinhart

Thermistors - (focus on NTC)

A thermistor is a resistor where the resistance is strongly dependent on temperature.

So we can use this for place a thermistor at a location where we want to measure temperature.

Measure the resistance for the thermistor and map it to the temperature.

NTC and PTC

We do have two types

  • PTC : Positive Temperatur Coefficient thermistor

    • increasing temperature = increasing resistance

  • NTC : Negative Temperatur Coefficient thermistor

    • increasing temperature = decreasing resistance

PTC

As we can see there is no uniq mapping between temperature and resistance. Its not trivial to formulate a transfer function which can go from resistance to temperature - not to say impossible.

The PTC is not usefull for temperature measurements and is only mentioned here for info.

 

NTC

An picture says it all. There is a non linear monotone relation between temperature and resistance.

 

NTC resistance temperature relationship II

  1. Resistance is falling when temperature is increasing

  2. This relation is NOT linear - see fig on left

  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)

So by proper modelling of the temperature-resistance relation we can use it to measure temperature as depicted on the figure below.


Steinhart-Hart did formulate a third order approximation.

 

(The most general form of the applied equation contains a ln(R)^2 term, but this is frequently neglected because it is typically much smaller than the other coefficients, and is therefore not shown above.)


We do have three unknown A,B and C. So by measure 3 experimental dataset (temperature and resistance) hopefully covering the intended temperature range we can find the A,B,C coefficients for the model approximation.

See more here

See mockup for measurements

comment: you need to know or set Vs and R

The 4th order approximination formula

FIY we will not use it

There is also a 4th order Steinhart-HArt approximation whichis not so well suited for DIY estimatino of the parameters.

 

  1. Steinhart Hart formula for calculating Temp from resistance

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

  3. See datasheet for values

From supplier Vishay here is an annotated datasheet It requires a NTC from Vishay. As you can see the B parameter vary as nominal resistance vary.

It might or should be a little more accurate due to higher order model. We not tested that.

Here is a NTC

 

A small NTC of value 47 kOhm (color coded)

  • NTC can be of various physical size

  • heavy NTC reacts slow

  • small react fast ..

  • small Ohm values and relaively large current adds self heating

    • P = U I = I^2 / R

    • So take caution

Howto

Just go to here for intro and howto estimate Steinhart-Hart 3rd orders parameters - including a pythonscript for it

Litt

Jens