Decagon Devices MAS-1 Manuel de l'utilisateur - Page 10
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MAS-1 Soil Moisture Sensor
3. Integrating the MAS-1
the voltage drop over the R
, then the current can be cal-
Volt
culated as
I = V
/ R
(1)
measured
Volt
Where I (mA) is the 4-20 mA current, R
(ohms) is the
Volt
resistance of the pickoff resistor, and V
(mV) is the
measured
voltage drop over R
.
Volt
Figure 3 - Wiring connection for devices without current
inputs
The optional 100uF capacitor shown in parallel with the R
Volt
reduces measurement noise. It should have a voltage rating
higher than the largest supply voltage and be sure to observe
correct polarity.
The value of R
is limited by the amount of impedance
Volt
that the MAS-1 circuitry can drive current through. The val-
ues in Table 1 represent the maximum load that the MAS-1
can drive at the given supply voltage. Equation (2) was
derived from the values in Table 1, which were obtained
through direct measurement. V
represents the supply volt-
in
age to the MAS-1, which ranges from 10-32 VDC. R
Volt Max
7