Using NTC thermistors

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Using NTC thermistors

Post by AndrewA » Fri Sep 25, 2015 2:24 pm

You can use NTC thermistors to measure temperature.
Here are some basic connection schematics for the picolog 1000 and ADC-20/24.
Setup on Channel 1.
Picolog 1000 NTC.png
Picolog 1000 NTC
ADC-20_24 NTC.png
ADC-20_24 NTC
NOTE for the ADC20/24- If you are going to use more than one NTC you will need to use external 2v5 supply reference. (The ADC-20/24 onboard 2v5 rail is rated for 2mA)

NTC's have a non linear resistive response to temperature.

There two methods that can be used to calculate temperture-

-Use a lookup table in the form of a scaling file in picolog
-Or use equations with the use of the 'calculate parameters'

Example of using equations with the use of picologs 'calculate parameters'

Hardware setup (As above)-
Using the ADC-24, with 10K thermistors wired in series with 6.49K Ohm resistors, connected to the 2.5v reference.

Here's the method and equations, which uses the Steinhart-Hart equation with modifications to reduce the dynamic range of values required.
As channels calculates are limited to 8 digits and 5 decimal places.

Doing the linearisation for each thermistor uses three calculated parameters.

1. Compute natural log of thermistor resistance as first calculated parameter

Code: Select all

LnR1 = ln ( Rseries * ( A / ( 2500 - A ) ) )
where A is the voltage across the thermistor, expressed in millivolts
where Rseries is the series resistor in this case 6.49K Ohms.

2. Compute the inverse temperature times 10,000 as the second calculated parameter

Code: Select all

InvT1 = ( ( ( ( ( 8.863 * A * A ) / 100 ) + 233.9 ) / 100 ) * A ) + 11.303
where A is the natural log of the thermistor resistance (A = LnR1)

3. Compute the temperature as the third calculated parameter

Code: Select all

Temp1 = ( 10000 / A ) - 273.15
where A is the inverse temperature times 10,000 (A = InvT1)

You download the example settings file here-
Picolog ADC-20 NTC example
(5.55 KiB) Downloaded 351 times
This is setup for the ADC-20 with above circuit.
Regards Andrew
Technical Specialist

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