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update. Clarify that w83782d/783s support both thermistors

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and diodes but must be configured for one or the other.


git-svn-id: http://lm-sensors.org/svn/lm-sensors/trunk@425 7894878c-1315-0410-8ee3-d5d059ff63e0
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Mark D. Studebaker 1999-04-25 23:52:37 +00:00
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@ -2,17 +2,19 @@ Termistor and Transistors as Temperature Sensors
------------------------------------------------ ------------------------------------------------
Temperature measuring chips that use external sensors Temperature measuring chips that use external sensors
generally are designed to use either thermistors or generally are designed to use either thermistors or
transistors. transistors. Check the driver documentation in doc/chips
or the actual data sheets to determine which.
Exceptions are the Winbond W83782D and W83783S which claim Exceptions are the Winbond W83782D and W83783S which can use either.
to be able to use both, however it isn't clear how These chips default to termistors but can be configured to use
to configure these to use transistors. Pentium II / Celeron diodes or 3904 transistors.
Thermistors are about 10 times more sensitive than
diodes or transistors at room temperature.
Thermistors Thermistors
----------- -----------
(summarized from http://www.thermometrics.com/htmldocs/ntcapp.htm - (summarized from http://www.thermometrics.com/htmldocs/ntcapp.htm -
most thermistor companies don't have much on their web sites but most thermistor companies don't have much on their web sites but
Thermometrics has a comprehensive guide) Thermometrics has a comprehensive guide)
@ -38,15 +40,16 @@ A = - B / T**2
So by the last equation, a thermistor with a Beta of 3900 will change 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). 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 a beta of 3435 as recommended in the
Winbond W83782D data sheet the change is 3.8%.
For thermistors, resistance (= voltage for a constant current) For thermistors, resistance (= voltage for a constant current)
is exponentially related to temperature. is exponentially related to temperature.
Transistors Transistors / Diodes
-------------- --------------------
To use a transistor as a sensor, hook the base of an NPN transistor To use a transistor as a sensor, hook the base of an NPN transistor
(such as the 3904) to the collector. (such as the 3904) to the collector.
@ -67,3 +70,30 @@ For diodes, voltage is linear with temperature.
Winbond W83782D and W83783S
---------------------------
These chips default to thermistors with Beta = 3435.
Each sensor on the chip can be individually set to be
a diode, transistor, or thermistor.
If the temperature reading changes much less than expected,
the sensor type probably needs to be changed to a diode.
See doc/chips/w83781d for details.
Summary
-------
Thermistors are about 10 times more sensitive than
diodes or transistors at room temperature.
Thermistors change resistance exponentially with respect to temperature.
Diodes and transistors change voltage linearly with respect to temperature.
Sensors built into chips (including Pentium II / Celeron processors)
are always diodes/transistors because that's what's on a chip
already - transistors.
External, discrete sensors can be either diodes/transistors or
thermistors. Generally, sensor chips are designed to be connected
to one or the other and cannot be configured. The Winbond
W83782D/W83783S chips are exceptions, they can be
configured for either.