Power and control current to simulate a transmitter on an unpowered
loop
Figure 4-13:
Note
The Trex unit is connected to a 4-wire terminal block. Consult the host documentation for more
information.
A. Analog input
4.6 Power a 2-wire transmitter
The Trex unit can provide up to 16 volts (at 22.5 mA) to power a HART device. You do not
need to add an external resistor. The Trex unit has a 167 Ohm internal resistor in the HART
+ pwr terminals that is used when the Trex unit provides power to the transmitter.
If you try to apply power from the Trex unit when the device is externally-powered, the
Loop Diagnostics application prevents the Trex unit from supplying power. This protects
the Trex unit from damage.
CAUTION!
Remove the USB cable from the Trex unit before connecting to a device.
CAUTION!
Do not add any external power to the device when the Trex unit is powering the device. This
can blow a fuse inside the Trex unit. The repair/replacement will need to be completed at an
authorized service center. Ensure the device is disconnected from the loop/segment and no
other wires are connected to the device before providing power from the Trex unit.
Note
Do not insert a resistor in parallel with either the HART + pwr or HART terminals on the Trex unit, or in
parallel with the device's communication terminals. This will interfere with the current
measurements and may prevent communication.
Loop Diagnostics application
136 User Guide
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