UMG70XV3-4EN 12.01.2009 71
7 Installation of FLUXUS G709
If the flowmeter has a passive current input, an external power supply is necessary. The
power supply U
H
must be able to
• supply sufficient power for the energy requirements of the passive current source
• cover the voltage drop over the measurement resistance (max. 1 V) and
• cover all other voltage drops (e.g. cable resistance) in the circuit.
A current of min. 20 mA has to be provided.
Connection of an Active Current Source
The terminals TxA and TxB are on the upper terminal row (x =1...4)
If the polarity of the current source is inversed, only the sign of the measured current will
change.
Fig. 7.10: Connection of an active current source
example: A passive current source (e.g. a pressure sensor) is connected to a passive
current input.
technical data of the pressure sensor:
auxiliary power supply: U
S
= 11...30 V DC
output signal: I
A
= 4...20 mA
The required auxiliary power supply of the pressure sensor will be calculat-
ed as follows:
U
H min
= U
S min
+ I
A max
· R
mea
+ I
A max
· R
cab
R
mea
- input impedance
R
cab
- cable resistance)
= 11 V + 20 mA · 50 + 20 mA · 2
= 12.04 V
U
H max
= 30 V (according to specification of the supplier)
Attention! Never connect an active current source to an active current input!
Attention! The terminals Txa and Txb must not be connected.
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