Installation
4.2 Power budget
S7-1200 Programmable controller
56 System Manual, V4.2, 09/2016, A5E02486680-AK
Your CPU has an internal power supply that provides power for the CPU, the signal
modules, signal board and communication modules and for other 24 V DC user power
requirements.
Refer to the technical specifications (Page 1359) for information about the 5 V DC logic
budget supplied by your CPU and the 5 V DC power requirements of the signal modules,
signal boards, and communication modules. Refer to "Calculating a power budget"
(Page 1555) to determine how much power (or current) the CPU can provide for your
configuration.
The CPU provides a 24 V DC sensor supply that can supply 24 V DC for input points, for
relay coil power on the signal modules, or for other requirements. If your 24 V DC power
requirements exceed the budget of the sensor supply, then you must add an external 24 V
DC power supply to your system. Refer to the technical specifications (Page 1359) for the
24 V DC sensor supply power budget for your particular CPU.
If you require an external 24 V DC power supply, ensure that the power supply is not
connected in parallel with the sensor supply of the CPU. For improved electrical noise
protection, it is recommended that the commons (M) of the different power supplies be
connected.
Connecting an external 24 V DC power supply in parallel with the 24 V DC sensor supply
can result in a conflict between the two supplies as each seeks to establish its own
preferred output voltage level
The result of this conflict can be shortened lifetime or immediate failure of one or both
power supplies, with consequent unpredictable operation of the PLC system. Unpredictable
operation could result in death, severe personal injury and/or property damage.
The DC sensor supply and any external power supply should provide power to different
points.
Some of the 24 V DC power input ports in the S7-1200 system are interconnected, with a
common logic circuit connecting multiple M terminals. For example, the following circuits are
interconnected when designated as "not isolated" in the data sheets: the 24 V DC power
supply of the CPU, the power input for the relay coil of an SM, or the power supply for a non-
isolated analog input. All non-isolated M terminals must connect to the same external
reference potential.
Connecting non-isolated M terminals to different reference potentials will cause unintended
current flows that may cause damage or unpredictable operation in the PLC and any
connected equipment.
Failure to comply with these guidelines could cause damage or unpredictable operation
which could result in death or severe personal injury and/or property damage.
Always ensure that all non-isolated M terminals in an S7-1200 system are connected to the
same reference potential.
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