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Chapter 2 Installation Series 780S Instruction Manual
2-8 IM-78S-B
4-20 mA Output Wiring
The 4-20 mA current loop output can be self-powered (non-isolated)
or externally powered (isolated). To use the 4-20 mA isolated output,
an external 12 to 36 VDC power supply is required. The maximum
loop resistance (load) for both types of current loop outputs are de-
pendent upon the supply voltage and are given in Figure 2-10.
R
load
is the total resistance in the loop, including the wiring resis-
tance. To calculate R
max
, the maximum R
load
for the loop, use the
maximum loop current, 20 mA. The voltage drop in the loop due to
resistance is 20 mA times R
load
and this drop is subtracted from the
input voltage. Thus:
R
max
the maximum load resistance = 50
*
(V
supply
– 7.5V)
To use an external power supply for an isolated 4-20 mA output, con-
nect as shown in Figure 2-11 or Figure 2-13. For an internally pow-
ered, non-isolated 4-20 mA output, connect as shown in Figure 2-12
or Figure 2-14.
Figure 2-10. Load Resistance Versus Input Voltage
V
supply
R
max
(Volts) (Ohms)
11 (min) 175
12 225
15 375
18 525
24 825
30 1,125
36 (max) 1,425