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