root/lm-sensors/trunk/doc/temperature-sensors

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1Thermistors and Transistors as Temperature Sensors
2--------------------------------------------------
3Temperature measuring chips that use external sensors
4generally are designed to use either thermistors or
5transistors. Check the driver documentation in doc/chips
6or the actual data sheets to determine which.
7
8Exceptions are the Winbond W83782D and W83783S which can use either.
9These chips default to termistors but can be configured to use
10Pentium II / Celeron diodes or 3904 transistors.
11
12Thermistors are about 10 times more sensitive than
13diodes or transistors at room temperature.
14
15
16Thermistors
17-----------
18(summarized from http://www.thermometrics.com/htmldocs/ntcapp.htm  -
19 most thermistor companies don't have much on their web sites but
20 Thermometrics has a comprehensive guide)
21
22NTC (negative temperature coefficient) thermistors decrease their
23resistance with an increase in temperature.
24They commonly have a parameter B (beta), units are K (kelvin).
25B is the "material constant" of the thermistor and
26is a measure of the change of resistance with respect to temperature.
27It is the slope of the line graphing (ln R) vs. (1/T).
28
29B values for three common termistor materials are
30approximately 3400K, 3900K, and 4300K.
31
32The other measure of thermistors is the temperature coefficient 'a' (alpha).
33This is the percentage change in resistance for a given change
34in temperature. This varies with temperature.
35
36T = temp in Kelvin (C + 273)
37
38A = (dR/dT) / R
39
40A = - B / T**2
41
42So by the last equation, a thermistor with a Beta of 3900 will change
43resistance about 4.4% for 1 degree change at 25C (298K).
44For a beta of 3435 as recommended in the
45Winbond W83782D data sheet the change is 3.8%.
46
47For thermistors, resistance (= voltage for a constant current)
48is exponentially related to temperature.
49
50PTC (positive temperature coefficient) thermistors increase
51in resistance with an increase in temperature. They are not
52commonly used for accurate temperature sensing.
53
54
55Transistors / Diodes
56--------------------
57To use a transistor as a sensor, hook the base of an NPN transistor
58(such as the 3904) to the collector.
59
60This is from Richard Dorf, "Electrical Engineering Handbook".
61
62For a diode, the voltage Vd = Vt ln(Id/Is).
63Vt = kT/q where k = Boltzman's constant 1.38 e-23;
64                q = elementary charge 1.6 e-19;
65                T = temp in kelvin.
66                Is is reverse saturation current.
67
68(more math and greek letters I don't understand completely)
69
70dVd/dT works out to, at room temperature, about 2.1 mV/degree
71or a temperature coefficient of about 0.32%/degree.
72
73For diodes, voltage is linear with temperature.
74
75
76
77Winbond W83782D and W83783S
78---------------------------
79These chips default to thermistors with Beta = 3435.
80Each sensor on the chip can be individually set to be
81a diode, transistor, or thermistor.
82If the temperature reading changes much less than expected,
83the sensor type probably needs to be changed to a diode.
84See doc/chips/w83781d for details.
85
86
87Summary
88-------
89Thermistors are about 10 times more sensitive than
90diodes or transistors at room temperature.
91
92Thermistors change resistance exponentially with respect to temperature.
93Diodes and transistors change voltage linearly with respect to temperature.
94
95Sensors built into chips (including Pentium II / Celeron processors)
96are always diodes/transistors because that's what's on a chip
97already - transistors.
98
99External, discrete sensors can be either diodes/transistors or
100thermistors. Generally, sensor chips are designed to be connected
101to one or the other and cannot be configured. The Winbond
102W83782D/W83783S chips are exceptions, they can be
103configured for either.
104
105
106------------------
107Copyright (c) 1999, 2001 Mark D. Studebaker <mdsxyz123@yahoo.com>
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