overview of math with thermistors and diodes.

added link to the NTC Thermistor guide to the useful addresses.


git-svn-id: http://lm-sensors.org/svn/lm-sensors/trunk@317 7894878c-1315-0410-8ee3-d5d059ff63e0

doc/temperature-sensors

@@ -0,0 +1,69 @@
+Termistor and Transistors as Temperature Sensors
+------------------------------------------------
+Temperature measuring chips that use external sensors
+generally are designed to use either thermistors or
+transistors.
+
+Exceptions are the Winbond W83782D and W83783S which claim
+to be able to use both, however it isn't clear how
+to configure these to use transistors.
+
+
+
+Thermistors
+-----------
+
+(summarized from http://www.thermometrics.com/htmldocs/ntcapp.htm  -
+ most thermistor companies don't have much on their web sites but
+ Thermometrics has a comprehensive guide)
+
+NTC (negative temperature coefficient) thermistors
+have a parameter B (beta), units are K (kelvin).
+B is the "material constant" of the thermistor and
+is a measure of the change of resistance with respect to temperature.
+It is the slope of the line graphing (ln R) vs. (1/T).
+
+B values for three common termistor materials are
+approximately 3400K, 3900K, and 4300K.
+
+The other measure of thermistors is the temperature coefficient 'a' (alpha).
+This is the percentage change in resistance for a given change
+in temperature. This varies with temperature.
+
+T = temp in Kelvin (C + 273)
+
+A = (dR/dT) / R
+
+A = - B / T**2
+
+So by the last equation, a thermistor with a Beta of 3900 will change
+resistance about 4.4% for 1 degree change at 25C (298K).
+For a beta of 3423 as recommended in the 782D data sheet the change is 3.8%.
+
+For thermistors, resistance (= voltage for a constant current)
+is exponentially related to temperature.
+
+
+
+Transistors
+--------------
+To use a transistor as a sensor, hook the base of an NPN transistor
+(such as the 3904) to the collector.
+
+This is from Richard Dorf, "Electrical Engineering Handbook".
+
+For a diode, the voltage Vd = Vt ln(Id/Is).
+Vt = kT/q where k = Boltzman's constant 1.38 e-23;
+                q = elementary charge 1.6 e-19;
+                T = temp in kelvin.
+		Is is reverse saturation current.
+
+(more math and greek letters I don't understand completely)
+
+dVd/dT works out to, at room temperature, about 2.1 mV/degree
+or a temperature coefficient of about 0.32%/degree.
+
+For diodes, voltage is linear with temperature.
+
+
+

doc/useful_addresses.html

@@ -93,6 +93,10 @@ Make considered Harmful</A>
 <LI><A HREF="http://www.3dhardware.net/features/thermistor/">Building a
 temperature sensor using a thermistor</A>
 An article on the <A HREF="http://www.3dhardware.net">3D Hardware</A> site.
+<LI><A HREF="http://www.thermometrics.com/htmldocs/ntcapp.htm"> 
+NTC Thermistor Guide</A>
+<BR>A comprehensive description of the physics and mathematics
+of NTC Thermistors</A>
 </UL>
 </UL>
 </BODY>