Apogee TS-100 Instrukcja obsługi - Strona 13
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Drukuj stronęConversion of Thermistor Resistance to Temperature
The thermistor is a resistive element, where resistance changes with temperature. Thermistor resistance (R
is measured with a half-bridge measurement, requiring a known excitation voltage input (V
of output voltage (V
):
OUT
V
R
24900
T
V
where 24900 is the resistance of the bridge resistor in Ω. From resistance, temperature (T
with the Steinhart-Hart equation and thermistor specific coefficients:
T
K
A
B
ln(
R
where A = 1.129241 x 10
measured temperature in Kelvin can be converted to Celsius (T
T
T
273
.
15
C
K
Fan Operation
Under some environmental conditions (e.g., high wind speed, low solar radiation), accurate measurements can be
made without running the fan at full speed. Fan speed and power consumption can be decreased using a pulse
width modulation (PWM) signal. The PWM signal should have a frequency of approximately 20 kHz and a duty
cycle of 50 to 100 %, where a duty cycle of 100 % is full power and 50 % is low power. Running the fan in a low
power mode reduces the current draw from 80 mA to approximately 25 mA. The only way to completely stop the
fan is to turn the power off.
EX
1
(1)
OUT
1
3
)
C
(ln(
R
))
(2)
T
T
-3
-4
, B = 2.341077 x 10
, and C = 8.775468 x 10
.
(3)
Red: 12 V DC (positive fan lead)
Black: Analog Ground (negative fan lead)
Clear: Analog Ground
Green: Control port (controls fan speed using pulse width
modulation)
Blue: Voltage Source (power input for tachometer; must be less
than 30 V)
White: Pulse Counter or Control Port (tachometer output)
-8
(Steinhart-Hart coefficients). If desired,
):
C
, in Ω)
T
) and a measurement
EX
, in Kelvin) is calculated
K