DL205 User Manual, 4th Edition, Rev. D
2-16
Chapter 2: Installation, Wiring and Specifications
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Powering I/O Circuits Using Separate Supplies
In most applications it will be necessary to power the input devices from one power source, and
to power output loads from another source. Loads often require high-energy AC power, while
input sensors use low-energy DC. If a machine operator is likely to come in close contact with
input wiring, then safety reasons also require isolation from high-energy output circuits. It is
most convenient if the loads can use the same power source as the PLC, and the input sensors
can use the auxiliary supply, as shown to the left in the figure below.
If the loads cannot be powered from the PLC supply, then a separate supply must be used as
shown to the right in the figure below.
Some applications will use the PLC external power source to also power the input circuit. This
typically occurs on DC-powered PLCs, as shown in the drawing below to the left. The inputs
share the PLC power source supply, while the outputs have their own separate supply. A worst-
case scenario, from a cost and complexity viewpoint, is an application which requires separate
power sources for the PLC, input devices, and output loads. The example wiring diagram
below on the right shows how this can work, but also the auxiliary supply output is an unused
resource. You will want to avoid this situation if possible.
Input Module
Auxiliary
+24VDC
Supply
Power Input
DL205 PLC
Output Module
Loads
AC Power
+
–
Inputs Com. Outputs Com.
Input Module
Auxiliary
+24VDC
Supply
Power Input
DL205 PLC
Output Module
Loads
AC Power
+
–
Inputs Com. Outputs Com.
Load
Supply
Input Module
Power Input
DL205 PLC
Output Module
Loads
DC Power
+
–
+
–
Inputs Com. Outputs Com.
Load
Supply
Input Module
Auxiliary
+24VDC
Supply
Power Input
DL205 PLC
Output Module
Loads
AC Power
+
–
Inputs Com. Outputs Com.
Load
Supply
Input
Supply
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