mirror of
https://github.com/lm-sensors/lm-sensors
synced 2025-09-06 01:05:32 +00:00
1798a1cf9b
git-svn-id: http://lm-sensors.org/svn/lm-sensors/trunk@1103 7894878c-1315-0410-8ee3-d5d059ff63e0
108 lines
3.7 KiB
Plaintext
108 lines
3.7 KiB
Plaintext
Thermistors and Transistors as Temperature Sensors
|
|
--------------------------------------------------
|
|
Temperature measuring chips that use external sensors
|
|
generally are designed to use either thermistors or
|
|
transistors. Check the driver documentation in doc/chips
|
|
or the actual data sheets to determine which.
|
|
|
|
Exceptions are the Winbond W83782D and W83783S which can use either.
|
|
These chips default to termistors but can be configured to use
|
|
Pentium II / Celeron diodes or 3904 transistors.
|
|
|
|
Thermistors are about 10 times more sensitive than
|
|
diodes or transistors at room temperature.
|
|
|
|
|
|
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 decrease their
|
|
resistance with an increase in temperature.
|
|
They commonly 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 3435 as recommended in the
|
|
Winbond W83782D data sheet the change is 3.8%.
|
|
|
|
For thermistors, resistance (= voltage for a constant current)
|
|
is exponentially related to temperature.
|
|
|
|
PTC (positive temperature coefficient) thermistors increase
|
|
in resistance with an increase in temperature. They are not
|
|
commonly used for accurate temperature sensing.
|
|
|
|
|
|
Transistors / Diodes
|
|
--------------------
|
|
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.
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
------------------
|
|
Copyright (c) 1999, 2001 Mark D. Studebaker <mdsxyz123@yahoo.com>
|