Thermistor - Revision history

Corecode at 11:42, 15 July 2012

2012-07-15T11:42:31Z

Corecode at 21:25, 31 August 2011

2011-08-31T21:25:25Z

Corecode at 19:37, 31 August 2011

2011-08-31T19:37:56Z

Corecode at 19:33, 31 August 2011

2011-08-31T19:33:48Z

Corecode at 19:32, 31 August 2011

2011-08-31T19:32:42Z

Corecode: Created page with "== Objective == How to read a NTC thermistor via ADC to get the current temperature. == Problem == * NTCs have a negative exponential behavior [1,2,3]. * When trying to read ..."

2011-08-31T19:30:19Z

Created page with "== Objective == How to read a NTC thermistor via ADC to get the current temperature. == Problem == * NTCs have a negative exponential behavior [1,2,3]. * When trying to read ..."

New page

== Objective ==

How to read a NTC thermistor via ADC to get the current temperature.

== Problem ==

* NTCs have a negative exponential behavior [1,2,3].

* When trying to read with the ADC on an AVR, you somehow need to fit
the voltage into AVcc, for example using the internal reference at
2.56V.

== Solution ==

[2] suggests combining a resistor with the NTC in parallel to create
an approximate linear response for a given temperature range.

I decided to do the math first, and it turns out a voltage divider
works like a charm. Set the ADC reference voltage to 2.56V. Then wire
the divider up this way:

<pre>
5V ----------+
|
NTC 10k@25C
|
+----------o ADC
|
3.6k
|
__|__
___
-
</pre>

The 3.6k value can be calculated from [2] and a bit of
experimentation.

When you read the ADC (say, 8bit ADC), you'll have to post-process the
reading. BUT! It is already linear. That means we can easily compute T
(in degC):

T = (ADC - 40) >> 2;

Of course, you can decide not to shift and instead have .25 degC
resolution.

The resulting temperature reading can be seen in [4] (analysis).

[1] https://secure.wikimedia.org/wikipedia/en/wiki/Thermistor
[2] https://www.avx.com/docs/Catalogs/ntc-general.pdf
[3] http://imgur.com/6efmA.png
[4] http://imgur.com/YlRCY.png

@@ Line 5: / Line 5: @@
 == Problem ==
-* NTCs have a negative exponential behavior <ref name="WP">{{cite web |url=https://secure.wikimedia.org/wikipedia/en/wiki/Thermistor}}</ref>
+* NTCs have a negative exponential behavior <ref name="wikipedia thermistor">{{cite web
 [2,3].

@@ Line 5: / Line 5: @@
 == Problem ==
-* NTCs have a negative exponential behavior <ref name="WP">{{cite web |url="https://secure.wikimedia.org/wikipedia/en/wiki/Thermistor"}}</ref>
+* NTCs have a negative exponential behavior <ref name="WP">{{cite web |url=https://secure.wikimedia.org/wikipedia/en/wiki/Thermistor}}</ref>
 [2,3].

@@ Line 7: / Line 7: @@
 * NTCs have a negative exponential behavior [1,2,3].
-* When trying to read with the ADC on an AVR, you somehow need to fit
+* When trying to read with the ADC on an AVR, you somehow need to fit the voltage into AVcc, for example using the internal reference at 2.56V.
 the voltage into AVcc, for example using the internal reference at
 .56V.
 == Solution ==
-[2] suggests combining a resistor with the NTC in parallel to create
+[2] suggests combining a resistor with the NTC in parallel to create an approximate linear response for a given temperature range.
 an approximate linear response for a given temperature range.
-I decided to do the math first, and it turns out a voltage divider
+I decided to do the math first, and it turns out a voltage divider works like a charm. Set the ADC reference voltage to 2.56V. Then wire the divider up this way:
 works like a charm. Set the ADC reference voltage to 2.56V. Then wire
 the divider up this way:
 <pre>
@@ Line 34: / Line 29: @@
 </pre>
-The 3.6k value can be calculated from [2] and a bit of
+The 3.6k value can be calculated from [2] and a bit of experimentation.
 experimentation.
-When you read the ADC (say, 8bit ADC), you'll have to post-process the
+When you read the ADC (say, 8bit ADC), you'll have to post-process the reading.  BUT! It is already linear.  That means we can easily compute T (in degC):
 reading.  BUT! It is already linear.  That means we can easily compute T
 (in degC):
 T = (ADC - 40) >> 2;
-Of course, you can decide not to shift and instead have .25 degC
+Of course, you can decide not to shift and instead have .25 degC resolution.
 resolution.
 The resulting temperature reading can be seen in [4] (analysis).
 [1] https://secure.wikimedia.org/wikipedia/en/wiki/Thermistor
 [2] https://www.avx.com/docs/Catalogs/ntc-general.pdf
 [3] http://imgur.com/6efmA.png
 [4] http://imgur.com/YlRCY.png