how to pick thermistor based on data points?

Discussion in 'Electronic Design' started by [email protected], Jul 6, 2007.

1. Guest

i have a temperature gauge which is designed for a NTC thermistor. I
have the thermistor it works with, and I have measured the gauges
indication to various resistances, as follows:

79.4F 1773 ohms
180F 192 ohms
240F 78 ohms

I need to buy lots more of the same thermistors but I dont know how to
find them based on just the above datapoints. Is there a way I can
come up with a thermistor spec so I can go thermistor shopping?

2. Tony WilliamsGuest

From the numbers above it looks like a thermistor
specified as R25= 1.9k and Beta = 4152 (average).

R25 is the resistance at 25C. The nearest standard
R25 value is 2k.

3. WinfieldGuest

Thermistors are usually specified for their resistance at 25C,
which is 77F, so you need to know your part's resistance at that
temperature. Common thermistors drop about 4% per degree C (and
this agrees with your 180F measurement), so your thermistor's
resistance should be about 1870 ohms at 25C. Hmm, that's not
near a common part value. For example, 2252 ohms at 25C is a
common value, which would show about 9F too low on your gauge.
2.0k is another common value, this would only be 3F low at 80F.
It's possible your gauge is no more accurate than that anyway.

4. TomGuest

Converting those to Celsius, that corresponds to something like:
R25=1900 ohm
Beta=4233

The (simplified) formula for the resistance of the NTC thermistor is
then: R=R25*exp(beta*(1/T0-1/T)), where all temperatures are in Kelvin
(Celsius + 273.15).

greetings,
Tom

5. Guest

You've already got a couple of good answers.

Note that negative temperature coefficinet thermistors can be
reasonably accurately modelled by the Steinhart-Hart three-paramter
formula

http://www.betatherm.com/stein.htm

Betatherm makes "interchangeable" thermistors and Farnell stock their
+/-0.2C parts, including one that offers a 2.252k (+/- 0.8%)
resistance at 25C. They aren't cheap. Cheaper parts come in at +/-5%
and 10% tolerance on resitance, and a temperature guage designed to
use one of them would include some means of calibrating out this
tolerance.

There are other sources for interchangeable thermistors -
Thermometrics and Yellow Springs Instruments (YSI) come to mind.
Newark used to stock a +/-0.5C YSI part (at ten times the price of the
Betatherm 0.2C units) and may still do.

6. GPGGuest

http://www.epcos.com/web/generator/Web/Sections/DesignTools/NTCThermistors/Page,locale=en.html