[SOLVED] calculate thermistor resistance

[JohnJohn20]

I have a NJ28 NTC thermistor with these characteristics:
Accuracy ±1%
Length 2.8mm
Maximum Operating Temperature +150°C
Minimum Operating Temperature -55°C
Resistance @ 25 °C 10000Ω
Temperature Coefficient Type NTC
Thermal Coefficient -4.6%/°C
Thermal Time Constant 8s
Tolerance ±1%

How would I calculate the expected resistance over the range of 0C to 100C?

I plan to connect these to a PIC to control a pump in a solar water heating system.

Do you just work it out step by step? That is:
25 C 10000Ω
26C (10000-4.6%*10000) = 9540Ω
27C (9540-4.6%*9540) = 9101.16Ω
etc??

That would be easy enough if it is accurate.

Thanks.

 

[IanP]

I have a NJ28 NTC thermistor with these characteristics:
Accuracy ±1%
Length 2.8mm
Maximum Operating Temperature +150°C
Minimum Operating Temperature -55°C
Resistance @ 25 °C 10000Ω
Temperature Coefficient Type NTC
Thermal Coefficient -4.6%/°C
Thermal Time Constant 8s
Tolerance ±1%

How would I calculate the expected resistance over the range of 0C to 100C?

I plan to connect these to a PIC to control a pump in a solar water heating system.

Do you just work it out step by step? That is:
25 C 10000Ω
26C (10000-4.6%*10000) = 9540Ω
27C (9540-4.6%*9540) = 9101.16Ω
etc??

That would be easy enough if it is accurate.

Thanks.

You have to use tables for the right material, in your case it's NA4100, and then approximate values (every 5deg C) in-between ...
Here is the link to tables Resistance vs Temperature:
http://www.pdfdoc.ru/passives/resis...c-thermistor-2-8mm-10k-1-nj28na0103f-697-4578


:wink:
IanP

 

[sabouras]

you could find an example for equations you need in that application note
**broken link removed**

 

[JohnJohn20]

OK. Thank you IanP. If I look at the thermistor data sheet there is a table, of which one row has the values for alpha = -4.6 @25C and R = 10,000 Ohms @ 25C. This row also has a B(K) value of 4100 +- 1%. Is that where you got the material code NA4100 from?

The table you linked to actually the material code of NE4100. But that would be the same right? Also the alpha@25C is -4.5. Is that significant?

Thank you too sabouras. It looks like you can use the equations to calculate a resistance value between two known values. Without the data table (as linked to by IanP) I only know 1 value which is 10000 ohms at 25C.

So this leads me to think that you can not calculate resistance values for a thermistor. Rather you have to physically measure them. Does that sound right?

 

[sabouras]

This article may help you
https://www.embedded.com/design/pro...ent/4023910/Temperature-Measurement-Technique

 

[JohnJohn20]

Yes sabouras. The other alternative I considered to using a thermistor was the LM335 IC wich has a linear output (and so would be easier to program for) but they require calibration otherwise they can have an error of up to 6 C. All in all the thermistor works out easier.