Voltage And Current Measurements In Loop Diagnostics - Emerson AMS Trex User Manual

Power a positioner
The Trex unit can power one positioner and set the current to 4 mA, 20 mA, or another
value between 3.5 and 22.5 mA. This allows you to set the position of the valve. The Trex
unit begins providing power and current when you tap OK.
When the Trex unit provides power to a positioner, the Trex Unit Current option is also
enabled. This allows you to control the current to the device by using the quick buttons,
the pencil icon, or the up/down arrow keys.
Simulate a transmitter with 4 mA
The Loop Diagnostics application lets you simulate a transmitter to verify an analog input
loop before installing a device. This option is useful to perform a loop check on an analog
input that will be wired to a 4-wire transmitter, or on an unpowered loop that has 2-wire
devices. The Trex unit provides power and 4 mA to the current loop.
Disable power to a device
To disable power, tap Trex Unit Power to set it to OFF. The Trex unit also stops providing
power if you tap Exit or remove the lead set from the device or the Trex unit.
If the Trex unit is powering a positioner, it is recommended that you disable power by
tapping Trex Unit Power. This allows you to move the positioner to a safe position without
causing any unexpected moves from the valve when the Trex unit stops providing power.
You can select 4 mA, 20 mA, or the present value. After the desired current level is
reached, the Trex unit stops providing power.
Note
The Loop Diagnostics application has a Duration of Current Change option that lets you set how quickly
the device will reach the set current level. If the Duration of Current Change option is not set to zero, the
current changes gradually to the selected current. This is intended to protect large equipment from
potential damage from abrupt movement.
4.4 Voltage and current measurements in Loop
Diagnostics
The Trex unit can read voltage and current in several ways.
The voltage is measured from the HART terminals on the Trex unit when Trex Unit Power is
off, or from the HART + pwr terminals on the Trex unit when the Trex Unit Power is on.
The current measurements are taken at one of the following:
1.
2.
3.
User Guide
The mA terminals when the Trex Unit Power is off and Trex Unit Current is off. If a device
has test terminals, you can measure current using the mA terminals on the Trex unit.
The HART terminals when the Trex Unit Power is off and Trex Unit Current is on.
The HART + pwr terminals when the Trex Unit Power is on and the Trex unit is
powering a transmitter.
Loop Diagnostics application
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