Series 651T/652T/653T Isolator User’s Manual Process Current Two-Wire Transmitters
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IMPORTANT:
Note that input and output circuits are isolated
from each other allowing the output circuit to operate with
common-mode voltages up to 250VAC, or 354VDC off
ground, on a continuous basis. Channels are also isolated
from each other.
2.
Output/Power Connections (Each Output)
: Connect a DC
power supply and the load in a two-wire configuration as
shown in Electrical Connections Drawing 4501-727. For
supply connections, use No. 14-24 AWG wire rated for at
least 90
°
Celsius. Loop current is input to the OUT+ lead and
returned via the OUT- lead. Optionally, a DVM may be
placed between the “SNS+” terminal and the “OUT-“
terminals to monitor the loop current via the voltage drop
measured across a built-in, precision 10
Ω
resistor connected
in series with the “OUT-“ lead. These transmitters operate
from DC supplies only. Power supply voltage is not critical
and should be from 12-36V DC. The supply voltage must not
exceed 60V, even momentarily, or damage to the unit may
result. Further, the power supply voltage must be adequate
to supply full-scale current to the load(s), plus 12V to the
transmitter terminals, plus any transmission line drop.
Variations in power supply voltage or load resistance within
rated limits has negligible effect on transmitter accuracy.
Ripple & Noise:
Power supply ripple at 60Hz/120Hz is
reduced at the load by the transmitter. The ripple at the load
will generally be less than 0.02% of span per volt peak-to-
peak of power supply ripple. The unit may have up to 0.25%
of additional internally generated ripple. In this case, connect
an external 1uF capacitor across the load to reduce this
ripple to less than 0.1% if desired.
3.
Grounding:
See Electrical Connections Drawing 4501-727.
The module housing is plastic and does not require an earth
ground connection. If the isolator is mounted in a metal
housing, a ground wire connection is typically required.
Connect the metal enclosure’s ground terminal (green screw)
to earth ground using suitable wire and per applicable codes.
WARNING:
For compliance to applicable safety and
performance standards, the use of twisted pair wiring is
recommended as shown in Drawing 4501-727. Failure to
adhere to sound wiring and grounding practices as
instructed may compromise safety and performance.
3.0 CALIBRATION
Units are calibrated at the factory and further adjustment is
not normally required unless the circuit has aged or periodic
calibration is needed to meet your company’s maintenance
requirements.
Calibration is performed as an iterative process of zero and
span adjustment for each output channel. The zero and span
adjustment potentiometers are accessible from the front panel of
the unit. The screwdriver blade used to adjust the potentiometers
should not be more than 0.1 inch (2.54mm) wide.
TRANSMITTER ADJUSTMENT PROCEDURE
Equipment Required
•
An accurate input current source adjustable for 4-20mA DC.
This source should be accurate beyond the
±
0.1% module
specification for best results.
•
An accurate current or voltage meter is also required to
monitor the output level. This meter must be accurate
beyond the module specifications for best results.
•
A precision load resistance will be needed if you are
monitoring output voltage to complete calibration.
Adjustment Procedure
Note:
Zero and Span pots are turned clockwise to increase the
output signal, and counter-clockwise to reduce the output signal.
1. Connect the input signal and output power/load as shown in
Electrical Connections Drawing 4501-727 for the channel of
interest. Two power supplies and loads will be required to
calibrate 653T units.
2. Adjust the input signal to precisely 4.000mA DC. Adjust the
zero pot until the output reads 4.000mA
±
0.016mA DC.
Repeat this procedure for the second zero pot and output of
653T units.
3. Adjust the input signal to precisely 20.000mA DC. Adjust the
span pot until the output reads 20.000mA
±
0.016mA DC.
Repeat this procedure for the second span pot and output of
653T units.
4. Repeat steps 2 & 3 until the readings converge.
5. As a check of calibration, adjust the input to precisely
12.000mA and verify the output measurement to within
±
0.1% of span. Check both outputs of 653T units. If the
measured error is greater than
±
0.1%, then perform steps 2
and 3 again until rated accuracy is achieved.
6. Repeat the above procedure for the second channel of 652T
units.
4.0 THEORY OF OPERATION
Refer to Simplified Schematic 4501-726 to gain a better
understanding of the circuit. These isolators use a center-tapped
transformer to isolate from output to input. Half of the output loop
current is routed to a 2:1 transformer (output current:input
current) while being driven in an alternating push-pull fashion to
modulate the input current. The other half of the output current
drives an output circuit supply and is pulled through a current
steering circuit that regulates the input current (via the
transformer) and an adjustment current (zero and span circuit).
The input circuit utilizes a unique mosfet to limit the input voltage
drop to low V
GS
threshold levels. The input circuit also includes
overvoltage transient suppression circuitry, noise filtering, and a
reverse polarity protection diode. Transient voltage suppressors
and other filtering devices are also included at the output for
effective noise filtering and transient suppression. The output
“OUT-“ lead includes a precision series-connected 10
Ω
resistor to
facilitate the connection of a DVM across the output “SNS” and
“OUT-“ terminals for monitoring the output loop current without
breaking the loop.
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