DIY NTC

Measure temperature with ntc

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

NTC resistance temperature relationship

Resistance is falling when temperature is increasing
This relation is NOT linear - see fig on left
NTCs are specificed by nominal resistance which is the resistance at 25 C
So - by inspection - this figure is for a 10 kOhm NTC(read resistance at 25C)

Mockup

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

Find resistance for the NTC (voltage divider formula) we do know the fixed resistor (here 10kOhm)
Calculate temperature from NTC resistance by use of an approximation formula
OR better estimate your own 3rd order approximation and …
Calculate temperature based on pt 3

From schematic  above
---------------
Vcc  : voltage supply  max 5V for Arduino , max 3.3V for esp32
Vo   : Measured voltage from the divider
R    : Series resistor - not a NTC !
Rntc : NTC which is the component that shall feel the temperature


Vo               =    Vcc  * ( Rntc / (Rntc + R) )
Vo * (Rntc + R)  =    Vcc * Lntc
R * ( Vcc - Vo ) =    R * Vo
Rntc             =    R  * ( Vo /( Vcc - Vo) )

About proper selection of resistor in serie=s 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

Test set

If you want to use the NTC within -10 to 100C or so then you should take three measurements with temperatures within this interval.

It could be

4 C - use water from the frige
or 8C - cold tap water
100C - boiling water
52C or 54 C (mix half and half boilign water and 4 or 8C water - be quick

You measure the divided voltage by our arduino/esp32 or a multineter or an oscilloscope

doing 3rd order Steinhart-Hart model

Now go to pythoncode.html for calculating 3rd Steinhart-Hartapproximation

Self heating, dynamics

Ith the value of your NTC is low you can be subejct for selfheating e.g. the NTC heat it self and will be (slightly) hotter than the item it is measuring.

More info here

A small NTC of value 47 kOhm (color coded)

NTC can be of various physical size
heavy(weight) NTC reacts slow
small react fast ..
small Ohm values and relatively large current adds self heating - not good
- P = U I = I^2 / R
- So take caution

Vishay NTC

See ntc10k-with-comments.pdf

Last update: 250922 19:22:01