Planning Analog Output Wiring; Signal Wire Carrying Current; Current Output Circuit - Siemens SIMATIC 505 PPX:505–6108A User Manual

2.2

Planning Analog Output Wiring

Signal Wire
Carrying Current
2-4 Installation
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Loop resistance for the output circuit is determined by the length and type
of wire and the device series resistance. This section gives instructions for
calculating the maximum length cable you can use. The circuit resistance
must be in the range of 10–600 Ω. If you use a separate power supply of
10 V, the minimum resistance increases to 600 Ω, and the maximum
resistance increases to 1000 Ω. To determine the resistance of a channel
output loop, use the following equation.
Loop resistance + (2
where
CL = cable length
RFT = conductor resistance (Ω/unit length)
TFL = sum of resistance for all field devices
The result of this equation must be less than 600 Ω. If the result is greater
than 600 Ω but less than 1000 Ω, insert a 10 V power supply into the loop to
maintain the accuracy of the module, or change one of the other loop
resistance factors. Any value over 1000 Ω prevents the module from
operating accurately. Figure 2-1 shows a schematic for wiring an output loop
with a resistance of less than 600 Ω. Also shown in Figure 2-1 is a schematic
for adding a power supply to allow loop resistance up to 1000 Ω.
Resistance: 10–600 Ohms
Module
0–20
mA
Resistance: 600–1000 Ohms
Module
0–20
mA
Figure 2-1 Current Output Circuit
CL RFT) ) TFL
User
Power
SHLD
RTN
IOUT
To Base Earth Ground
User
Power
10 V
SHLD
Power Supply
RTN + –
IOUT
To Base Earth Ground
Series 505 Analog I/O Modules User Manual
Field Device
–
Load
(Between 10
+ and 600 Ω)
Field Device
–
Load
(Between 600
+
and 1000 Ω)