15
Model 763A Gage Pressure Transmitter
Section 3
Minimum loop resistance is determined based on maximum power supply
voltage, maximum transmitter current, and maximum transmitter voltage.
Example 4: (Minimum loop resistance for 10-50 mA system):
VDC = 53 Vdc TVDC = 30.5 Vdc
IDC = 50 mA RT =
= 450 Ohms
0.05
53-30.5
For 10-50 mA systems, if R
Load
= 200 Ohms (Example 1, loop resistance and
Example 4, minimum loop resistance), then R
Ext
+ R
Line
must be ≥ 250 Ohms,
but ≤ 560 Ohms to satisfy both the maximum and minimum loop resistance
values.
For 4-20 mA systems, maximum loop resistance is determined using Example
2. No minimum loop resistance is required with a 53 Vdc power supply.
Care must be exercised when calculating the power supply output voltage. A
e
to ensure the minimum required voltage at the transmitter. Use the actual
value when available; otherwise, use “worst case” value.
Use Figure 3.3 as a reference to determine if the maximum calculated value
of R
T
= R
Line
+ R
Load
+ R
Ext
is correct.
EMI/RFI Shielding
IMPORTANT: The 763A transmitter has no integral electronic interference suppression
features. If an instrument is to be installed in an area containing EMI/RFI
sources and this interference cannot be tolerated, take precautions to
protect the transmitter signal.
The following precautions are recommended to limit EMI/RFI interference:
1. Run signal wires in solid conduit or use high quality shielded cable to
connect the transmitter to the power equipment.
2. The transmitter leads should be housed in solid conduit up to the junction
box where the shielded cable is connected to the leads.
3. Ground the electronic transmitter, junction box (including the cover),
conduit, and cable shield.
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