Temperature Readings

[Andrew Ewert]

Greetings,

The current Regulator board schematics call for a 100k Thermistor
(490-4803-1-ND - Digikey). I was able to get the microcontroller on
the board to successfully read information from the thermistor,
however so far I have been unable to convert the resistance values
provided into a usable measurement system such as Celsius.

I got values of around 800 for room temperature (72 degrees
Fahrenheit).

We are using a PIC16F88.

I committed my most recent changes to the FreeBMS SVN code repository
(http://code.google.com/p/freebms/).

If anyone has any suggestions or comments about this, please feel free
to post. Here are some relevant links:

DigiKey Datasheet for Thermistor:
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=490-4803-1-ND

SVN Repository for Microcontroller Code:
http://freebms.googlecode.com/svn/trunk/Bypass_Regulators/src/

Thanks!
-Andrew

[Forbes Bagatelle-Black]

Hi Andrew,

I'm not so good with the code you linked, but I have
worked with these thermistors in LabVIEW.

Have you seen this page already?

http://search.murata.co.jp/Ceramy/CatalogframeAction.do?sDirnm=A11X&sFilnm=NTHCG143&sType=0&sLang=en&sNHinnm=NCP15WF104F03RC&sCapt=Resistance_VS._Temperature

or

http://tinyurl.com/yras2p

At AeroVironment, we used lots and lots of this very
thermistor. They are rock solid and work very well.

I'm way down on the learning curve here, so pardon me
if I am repeating the obvious.

Yours,

Forbes Bagatelle-Black

- Forbes Bagatelle-Black, Santa Clarita, CA

650B Tires, Leather Saddles, Classic Roadster Bikes and Parts at:

http://cyclesvalhalla.com/

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[Andrew Ewert]

Thank you Forbes for your response. It was very helpful because it
allowed us to test our results.

Unfortunately we are still having difficulties turning the voltage
that we are receiving from the thermistor through the PIC16F88 into
something usable.

The values that we are getting from the thermistor start around 771 at
room temperature and increase and decrease with temperature. We are
using the formula on http://en.wikipedia.org/wiki/Voltage_divider and
solving for R1 where R2 is a constant (39k), Vin is the voltage in
(5v) and Vout is the voltage from the thermistor (771). So far we
haven't figured out how to make this produce valid results.

Does anyone have any experience working with thermistors and PIC
microcontrollers that might be able to shed some light on this?

Thanks!
-Andrew

On Jan 1, 10:46 pm, Forbes Bagatelle-Black <diarma...@yahoo.com>
wrote:

> Hi Andrew,
>
> I'm not so good with the code you linked, but I have
> worked with these thermistors in LabVIEW.
>
> Have you seen this page already?
>

> http://search.murata.co.jp/Ceramy/CatalogframeAction.do?sDirnm=A11X&s...

>
> or
>
> http://tinyurl.com/yras2p
>
> At AeroVironment, we used lots and lots of this very
> thermistor. They are rock solid and work very well.
>
> I'm way down on the learning curve here, so pardon me
> if I am repeating the obvious.
>
> Yours,
>
> Forbes Bagatelle-Black
>

> --- Andrew Ewert <Andrew.J.Ew...@gmail.com> wrote:
>
>
>
>
>
> > Greetings,
>
> > The current Regulator board schematics call for a
> > 100k Thermistor
> > (490-4803-1-ND - Digikey). I was able to get the
> > microcontroller on
> > the board to successfully read information from the
> > thermistor,
> > however so far I have been unable to convert the
> > resistance values
> > provided into a usable measurement system such as
> > Celsius.
>
> > I got values of around 800 for room temperature (72
> > degrees
> > Fahrenheit).
>
> > We are using a PIC16F88.
>
> > I committed my most recent changes to the FreeBMS
> > SVN code repository
> > (http://code.google.com/p/freebms/).
>
> > If anyone has any suggestions or comments about
> > this, please feel free
> > to post. Here are some relevant links:
>
> > DigiKey Datasheet for Thermistor:
>

> http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=490-480...

[Chris Ewert]

Andrew Ewert wrote:
> Thank you Forbes for your response. It was very helpful because it
> allowed us to test our results.
>
> Unfortunately we are still having difficulties turning the voltage
> that we are receiving from the thermistor through the PIC16F88 into
> something usable.
>
> The values that we are getting from the thermistor start around 771 at
> room temperature and increase and decrease with temperature. We are
> using the formula on http://en.wikipedia.org/wiki/Voltage_divider and
> solving for R1 where R2 is a constant (39k), Vin is the voltage in
> (5v) and Vout is the voltage from the thermistor (771).

771 isn't a voltage, its an integer representation of a voltage by the
A/D converter. Assume your reference voltage is 5v (for this case).
The A/D Converter is 10 bit resolution, so:
5 volts / 1024 = 0.00488 volts

So each number that the A/D converter gives you represents 0.00488
volts. So in this case, 771 would be:
771 * 0.00488 = 3.76 volts

So 3.76 volts is what is present at the PIC's analog input pin.

The other way you can think of this is that the A/D converter is giving
you a percentage of the reference voltage. So:

771 / 1024 = 0.752, so multiply that percentage by the reference voltage:
0.752 * 5 volts = 3.76 volts

I'm not sure about the rest of the equation though.

Chris