microBlox® Transmitter/Alarm Carrier
Model uBTA-H-1MR/uBTA-H-1SR/uBTX-H-1
Acromag, Inc. Tel: 248-295-0880
- 16 -
http://www.acromag.com
- 16 -
https://www.acromag.com
Optional Output Current Trim Adjustment
IMPORTANT:
This adjustment has
already been done at the factory
using precision instruments and
should not be attempted in the
field unless fine accuracy can be
assured (precision load, DVM,
input source, etc.). Otherwise, you
could make the current output less
accurate.
This carrier includes both analog voltage (0-5V) and current output (0-20mA). The
voltage output is driven by the plug-in module directly, while the carrier includes a
voltage to current converter on-board to drive 0-20mA from the 0-5V provided by
the module. Normally, the current output will not require a trim adjustment,
except perhaps to precisely set its output to tighter accuracy or to correct for long-
term component aging. A 15-turn mechanical trim pot is included in this circuit to
fine tune the full-scale 20mA output with 5V full-scale from the module. It provides
roughly ±0.08mA of adjustment around 20mA and is accessible with a screw-driver
inserted through one of the vent openings on the bottom/input side of the module
as shown in the drawing below. If your full-scale current output is not within ±0.1%
of 20mA, you may utilize this trim pot to make fine adjustment to its current output
at full-scale (with 5V full-scale from module).
CAUTION:
You must be very careful when inserting a screwdriver into this vent to
adjust the trim pot. Most screwdrivers are conductive and it would be very easy to
inadvertently short circuit the enclosed circuit if the screwdriver blade did not align
with the trim pot screw and was inadvertently pushed too far into the circuit
enclosure. This could damage the live circuit and may be unsafe if isolation is
inadvertently violated.
Refer to the following figure to locate this adjustment screw. Turn the pot screw
clockwise to increase the output current at full-scale, and counter-clockwise to
reduce the full-scale current. To monitor the output current, it is best to use a
precision load resistor with greater accuracy than ±0.1%, and measure output
current as a function of the voltage drop across this load
resistor. A 250Ω precision
resistor will indicate 5V across it with precisely 20mA through it.
Note that you can optionally make this adjustment without plugging a module into
a powered carrier by simply connecting a precision 5V signal to the voltage output
terminals to mimic a module present (with no module plugged in, observe proper
polarity), then adjusting the current output to precisely 20mA by adjusting this pot
as required
. With a precision 250Ω load across the current output terminals, you
would adjust this pot until you read precisely 5V±5mV on a DVM placed across the
load resistor. Of course, doing it this way would not additionally compensate for
any potential module offset of up to ±0.05% or ±2.5mV between different
calibrated modules in the same carrier socket.