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'[PICLIST] [EE] Thermistor resistance calculation'
2001\01\08@091730 by NDuckworth

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I'm using an NTC thermistor to detect an overheat condition within my project,
the device is specified as;

10k Ohms (+/-2%) at 25 degrees C
Beta value 25 to 85 degrees C = 3977K(+/-0.75%)

My question is - how do I calculate the resistance of the thermistor for a
given temperature (say 75 degrees)?

I'm sure I should know this but it's been a long time since College!

Thanks in advance.

Nigel

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2001\01\08@102334 by Spehro Pefhany

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At 02:05 PM 1/8/01 -0000, you wrote:
>I'm using an NTC thermistor to detect an overheat condition within my
project,
>the device is specified as;
>
>10k Ohms (+/-2%) at 25 degrees C
>Beta value 25 to 85 degrees C = 3977K(+/-0.75%)
>
>My question is - how do I calculate the resistance of the thermistor for a
>given temperature (say 75 degrees)?
>
>I'm sure I should know this but it's been a long time since College!

I never learned anything as practical as this in University!

Here is the basic approximation:-
R = R0 * exp(B/T0 - B/T) , where B is assumed to be constant, T0 is the
reference temperature for R0. All temperatures are absolute, so add
273 to the temperatures in degrees C, the units of Beta are Kelvins.

So, at 75 degrees C,

R = 10K / exp(3977/(25+273) - 3977/(75+273)) = 1470 ohms.

This is probably not accurate enough for your purposes, so we can
use something like the Steinhart and Hart equation:-

R = R25 * exp( a + b/T + c/T^2 + d/T^3), the  manufacturer should
supply the coefficients a..d, or you can curve fit it given a table
of values vs. temperature.

Best regards, (& sorry about the extra copy, Nigel)





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