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.