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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 ..."
== 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
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
