Thermistors and Transistors as Temperature Sensors
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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
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(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
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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
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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.
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Copyright (c) 1999, 2001 Mark D. Studebaker <mdsxyz123@yahoo.com>