17
Model 764 Differential Pressure Transmitter
Section 3
Example 2 : (Maximum loop resistance for 4-20 mA system):
V
DC
= 53 Vdc
T
VDC
= 15 Vdc
I
DC
= 20 mA R
T
= (53-15) / 0.02 = 1900 Ohms
Example 3 : (Calculation to determine maximum loop resistance with power
supply ≥ 15 Vdc, but ≤ 53 Vdc for 10-50 mA systems):
V
DC
= 40 Vdc
T
VDC
= 15 Vdc
I
DC
= 50 mA R
T
= (40-15) / 0.05 = 500 Ohms
Minimum loop resistance is determined based on maximum power supply volt
-
age, maximum transmitter current, and maximum no load transmitter voltage.
Example 4 : (Minimum loop resistance for 10-50 mA system):
V
DC
= 53 Vdc
T
VDC
= 19 Vdc
I
DC
= 50 mA R
T
= (53-19) / 0.05= 680 Ohms
For 10-50 mA systems with a 53V power supply, if R
Load
= 200 Ohms (Ex
-
ample 1, maximum loop resistance and Example 4, minimum loop resis
-
tance), then R
Ext
+ R
Line
must be ≥ 480 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, minimum loop resistance is determined using Example 5.
Example 5 : (Minimum loop resistance for 4-20 mA system):
V
DC
= 53 Vdc
T
VDC
= 47.2 Vdc
I
DC
= 20 mA R
T
= (53-47.2) / 0.02 = 290 Ohms
Care must be exercised when calculating the power supply output voltage. A
power supply specified as 50Vdc ±1 volt must be considered a 49Vdc source
to ensure the minimum required voltage at the transmitter. Use the actual
value when available; otherwise, use "worst case" value.
Use Figure 3.2 as a reference to determine if the maximum calculated value
of R
T
= R
Line
+ R
Load
+ R
Ext
is correct.
