10 Kohm Thermistor
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Posts: 46,084
Hi , I have spent the last few days on the web looking for the best way to
monitor standard industrial (10k ohm @ 77deg. F / 25deg.C ) thermistors. I
have found plenty of reference material but failed to find details. Like a
formula to compensate for the non-linier curve of the thermistor. Is it
better to use a RC circuit with RCTIME or is it better to use a AD
converter.
I'm using a BSIIp24. Thanks in advance for any help. Kirt
[noparse][[/noparse]Non-text portions of this message have been removed]
monitor standard industrial (10k ohm @ 77deg. F / 25deg.C ) thermistors. I
have found plenty of reference material but failed to find details. Like a
formula to compensate for the non-linier curve of the thermistor. Is it
better to use a RC circuit with RCTIME or is it better to use a AD
converter.
I'm using a BSIIp24. Thanks in advance for any help. Kirt
[noparse][[/noparse]Non-text portions of this message have been removed]
Comments
RCTIME is limited somewhat in the range of resistance it can handle. If you
have a small range of temperature that you want to monitor, then you can
probably use RCTIME, even if you have to add resistance in series or
parallel to get into the proper range. There are some techniques for
"linearizing" the T vs R curve over limited temp ranges.
And you can use table lookup and linear (or other) interpolation to get
the temperature.
Even if you use an external A/D converter, you still have to convert the
voltage you get to temperature, again using table lookup and interpolation.
I finally came to the conclusion that using a thermistor to measure
temperature over a wide range was a non starter, and switched to using
chips line the LM34/35 that give a linear voltage vs temperature driving an
A/D converter. There are, of course, chips that give you digital values.
Larry
At 10:15 AM 3/25/2003 -0800, you wrote:
>Hi , I have spent the last few days on the web looking for the best way to
>monitor standard industrial (10k ohm @ 77deg. F / 25deg.C ) thermistors. I
>have found plenty of reference material but failed to find details. Like a
>formula to compensate for the non-linier curve of the thermistor. Is it
>better to use a RC circuit with RCTIME or is it better to use a AD
>converter.
> I'm using a BSIIp24. Thanks in advance for any help. Kirt
>
Larry Bradley
Orleans (Ottawa), Ontario, CANADA
using the Steinhart-Hart equation so I guess that I will have to determine
the resolution and range I want and make the table and sacrifice a little
memory. Thanks Again
Original Message
From: Larry Bradley [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=rmRVroXqye1SULYHUwYvWeF0hakek5fTPIDRgi5b5j-FycRUNUG64HjwZUBGy_h_55_-LCsVYMkXuWGq]lhbradley@i...[/url
Sent: Tuesday, March 25, 2003 10:50 AM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] 10 Kohm Thermistor
The answer, I'm afraid, is "it depends".
RCTIME is limited somewhat in the range of resistance it can handle. If you
have a small range of temperature that you want to monitor, then you can
probably use RCTIME, even if you have to add resistance in series or
parallel to get into the proper range. There are some techniques for
"linearizing" the T vs R curve over limited temp ranges.
And you can use table lookup and linear (or other) interpolation to get
the temperature.
Even if you use an external A/D converter, you still have to convert the
voltage you get to temperature, again using table lookup and interpolation.
I finally came to the conclusion that using a thermistor to measure
temperature over a wide range was a non starter, and switched to using
chips line the LM34/35 that give a linear voltage vs temperature driving an
A/D converter. There are, of course, chips that give you digital values.
Larry
At 10:15 AM 3/25/2003 -0800, you wrote:
>Hi , I have spent the last few days on the web looking for the best way
>to monitor standard industrial (10k ohm @ 77deg. F / 25deg.C )
>thermistors. I have found plenty of reference material but failed to
>find details. Like a formula to compensate for the non-linier curve of
>the thermistor. Is it better to use a RC circuit with RCTIME or is it
>better to use a AD converter.
> I'm using a BSIIp24. Thanks in advance for any help. Kirt
>
Larry Bradley
Orleans (Ottawa), Ontario, CANADA
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to construct stamp-friendly polynomials - Takes a bit of fiddling
but may do what you need.
The general subject is called curve fitting, btw.
regards, Jack
Kirt Boynton wrote:
>
> Hi , I have spent the last few days on the web looking for the best way to
> monitor standard industrial (10k ohm @ 77deg. F / 25deg.C ) thermistors. I
> have found plenty of reference material but failed to find details. Like a
> formula to compensate for the non-linier curve of the thermistor. Is it
> better to use a RC circuit with RCTIME or is it better to use a AD
> converter.
> I'm using a BSIIp24. Thanks in advance for any help. Kirt