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PICList Thread
'Thermistor Questions'
1998\07\07@112230 by ean-Francois ou Alain Joly

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Hello everyone! I'm currently in the need of a PIC16F84 circuit that
would analyse the temperature of an ice cube I have in my freezer. :) I
was thinking of a thermistor allong with a 10 or 12 bit A/D converter.
Would that make sense?

The problem is, do you think I could place the thermistor in the water?
Most of the time the water will be in a solid form but it could possibly
turn liquid depending on the temperture conditions. So would the
thermistor still work? Will it do the job? Or...?

Also, where can I get more information on interfacing the '84 with an
A/D?

Thank you!
Jean-Francois

1998\07\08@023235 by James Cameron

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Jean-Francois ou Alain Joly wrote:
> I was thinking of a thermistor allong with a 10 or 12 bit A/D
> converter.  Would that make sense?

Not to me.  I would use the cheapest design in terms of actual hardware
connections, which for me would  be a PIC connected to a DS1820
temperature sensor chip.

--
James Cameron                              (spam_OUTjames.cameronTakeThisOuTspamdigital.com)
Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800

1998\07\08@031120 by Dr. Imre Bartfai

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Hi,
good news for you.

There is the AN512 (maybe AN513) describing how to measure resistance with
a PIC w/o A/D converter. Depending on speed, you can increase resolution.
I tried it and it works fine!

The thermistor wires you can put in a plastic thin tube after making a
soldered connection, then pour some arteficial resin (I hope it can be
understood; I translated it from German: "gie§ etwas kŸnsliches Harz in
die Schlauche"). It should give enough protection.

Imre

1998\07\08@033035 by Mark Willis

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Epoxy potting the thermistor is good <G>  (That's what the Dr. was
trying to say, I think.)  Leave the tip of the thermistor close to the
surface of the epoxy, or even out of the epoxy...

 One project I was in, we ended up cutting slits into a polyurethane
wire bundle (16 or 20 pairs, one slit per pair to measure temp's at
different depths), fishing out the individual twisted pair of wires, and
soldering a thermistor across each pair, then epoxy potting the whole
mess inside of a 35mm film container to seal it.  Worked pretty well!

 Mark, .....mwillisKILLspamspam@spam@nwlink.com

Dr. Imre Bartfai wrote:
{Quote hidden}

1998\07\08@094319 by Andy Kunz

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>The thermistor wires you can put in a plastic thin tube after making a
>soldered connection, then pour some arteficial resin (I hope it can be
>understood; I translated it from German: "gie§ etwas kŸnsliches Harz in
>die Schlauche"). It should give enough protection.

Good enough translation!  Basically, use a thin-walled tubing of a
thermally-conductive nature, fill with a non-thermal-insulating potting
compound (or ceramic).  The idea is to keep the wires from breaking but
also to transmit the heat.

Andy

==================================================================
Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
==================================================================

1998\07\08@134537 by lilel

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Jean-Francois ou Alain Joly wrote:

> Hello everyone! I'm currently in the need of a PIC16F84 circuit that
> would analyse the temperature of an ice cube I have in my freezer.

This is similar to the oven controllers I design.  I use a thin piece
of teflon heat shrink tubing to isolate the theristor from the
outside world.  You could probably use standard heat shrink at these
low temperatures.


> :) I was thinking of a thermistor allong with a 10 or 12 bit A/D
> converter. Would that make sense?


How much resolution do you need?  I use the A/D inputs on a 16C620,
only good to 4 bits, to do a whole oven controller.  Other PIC parts
will give you 8 bit resolution.  How much range and how small an
increment do you want to measure?


-- Lawrence Lile

    "An Engineer is simply a machine for
     turning coffee into assembler code."

Download AutoCad blocks for electrical drafting at:
http://members.sockets.net/~llile/index.htm

1998\07\08@171229 by Reginald Neale

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>>The thermistor wires you can put in a plastic thin tube after making a
>>soldered connection, then pour some arteficial resin (I hope it can be
>>understood; I translated it from German: "gie§ etwas kŸnsliches Harz in
>>die Schlauche"). It should give enough protection.
>
>Good enough translation!  Basically, use a thin-walled tubing of a
>thermally-conductive nature, fill with a non-thermal-insulating potting
>compound (or ceramic).  The idea is to keep the wires from breaking but
>also to transmit the heat.
>

If you can tolerate a little more thermal inertia, use the sort of shrink
tubing that has an adhesive liner. This will seal the thermistor and wires
into a monolithic assembly in one easy step.

Reg Neale

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