Powering I/O Circuits Using Separate Supplies - Automationdirect.com DirectLOGIC DL205 Series User Manual

Chapter 2: Installation, Wiring and Specifications

Powering I/O Circuits Using Separate Supplies

1
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
2
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
3
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
4
shown to the right in the figure below.
5
6
Power Input
Auxiliary
7
+24VDC
Supply
8
–
+
9
10
Some applications will use the PLC external power source to also power the input circuit.
11
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
12
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
13
an unused resource. You will want to avoid this situation if possible.
14 +
+
– –
A
Power Input
B
C
D
2–16
DL205 User Manual, 4th Edition, Rev. B
AC Power
DL205 PLC
Input Module
Output Module
Inputs Com. Outputs Com.
Loads
DC Power
DL205 PLC
Input Module
Output Module
Inputs Com. Outputs Com.
Loads
AC Power
Power Input
Auxiliary
+24VDC
Supply
–
+
AC Power
Power Input
Auxiliary
+24VDC
Supply
–
+
Load
Supply
DL205 PLC
Input Module
Output Module
Inputs Com. Outputs Com.
Loads
DL205 PLC
Input Module Output Module
Inputs Com. Outputs Com.
Input Loads
Supply
Load
Supply
Load
Supply