45 lines
1.3 KiB
C
45 lines
1.3 KiB
C
/* Copyright (c) 2025 Rodrigo Arias Mallo <rodarima@gmail.com>
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* SPDX-License-Identifier: GPL-3.0-or-later */
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#include <math.h>
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/* Steinhart-Hart Thermistor Coefficients, used to convert resistance into
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* temperature.
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*
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* The current NTC sensor has 102kOhm at 24C but I don't know the specific
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* model, so the coefficients are computed for this NTC sensor instead:
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* https://www.tme.eu/Document/f9d2f5e38227fc1c7d979e546ff51768/NTCM-100K-B3950.pdf
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*
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* The table seems to match what I would expect. Their R2 resistor is 6.8 kOhm,
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* which yields a cuttof temperature of around 98.5 C at exactly half voltage
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* (where the ADC would have more precision).
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*
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* In any case, we can calibrate the original NTC sensor by taking three
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* temperature points. See:
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* https://www.thinksrs.com/downloads/programs/therm%20calc/ntccalibrator/ntccalculator.html
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*/
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#define C1 0.7740577674e-3
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#define C2 2.073449619e-4
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#define C3 1.263502259e-7
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/* Return the temperature in celsisus */
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float
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ntc_temp(float R)
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{
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/* Computing the log is slow, we may want to build a table */
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float logR = log(R);
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float T = (1.0 / (C1 + C2*logR + C3*logR*logR*logR));
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float Tc = T - 273.15;
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return Tc;
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}
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/* Return resistance in Ohms */
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float
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ntc_resistance(int Vo)
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{
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float R1 = 10.0e3; /* Resistor for voltage divider */
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float R2 = R1 * (1023.0 / (float)Vo - 1.0);
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return R2;
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}
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