Vaisala WAT12, User Manual

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The following figure shows 4-wire wiring for long distance with a 48 V DC power supply.
A source resistance, over 1000 Ω, must be used for dropping VIN to
an acceptable level, because if VIN exceeds 28 V DC, the transmitter may be
damaged permanently.
CAUTION!
Figure 12 Long distance wiring with 4 wires
3.4.3 Powering
WAT12 accepts a wide range of input power 12 ... 28 V DC. When the 5 mA loop current is
selected, the total current consumption is less than 40 mA including the sensors and the loop
current. The low current consumption enables to remotely supply the operating power from a
distance of several kilometers, even through private or leased telephone lines.
The maximum power line resistance depends on the supply voltage and the loop current
selection. For an example with an 24 V DC power supply, see Figure 9 (page 17) .
Table 5 Power line resistance with 24 V DC power supply
Loop current, mA Power line resistance, Ω
5 360
10 310
20 250
For other powering options, see Figure 10 (page 17) , Figure 11 (page 17) , and Figure 12
(page 18)
The power line is protected against the wrong polarity. Transient protection is accomplished
with VDRs, series inductors and resistors, and transient zener diodes on both the power line
and each I/O line.
WAT12 User Guide M210309EN-B
18