Loop Power Supply Mode (789 Only) - Fluke 789 User Manual

789/787B
Users Manual

Loop Power Supply Mode (789 only)

The Loop Power Supply Mode can be used for powering
up a process instrument (transmitter). While in Loop
Power Mode, the meter acts like a battery. The process
instrument regulates the current. At the same time, the
meter measures the current that the process instrument is
drawing.
The meter supplies loop power at a nominal 24 V dc. An
internal series resistance of 250 Ω can be switched in for
communication with HART and other smart devices by
pressing J (Blue). See Figure 3. Pressing J (Blue)
again switches out this internal resistance.
When loop power is enabled, the meter is configured to
measure mA and >24 V dc is sourced between the mA
and A jacks. The mA jack is the common and the A jack is
at >24 V dc. Connect the meter in series with the
instrument current loop. See Figure 4.
28
32
30
28
26
24
22
20
0 4 8 12
Current (mA)
Loop Voltage w/o 250 Ω
Figure 3. Loop Power Voltage vs. Current
16 20 24
Loop Voltage w/ 250 Ω
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