Power Budget - Siemens SIMATIC S7 System Manual

3.2

Power budget

Your CPU has an internal power supply that provides power for the CPU, the expansion
modules, signal boards, and other 24 V DC user power requirements. Use the following
information as a guide for determining how much power (or current) the CPU can provide for
your configuration. The new compact CPUs (CRs) do not support expansion modules or
signal boards.
Refer to the technical specifications for your particular CPU to determine the 24 V DC sensor
supply power budget, the 5 V DC logic budget supplied by your CPU and the 5 V DC power
requirements of the expansion modules and signal boards. Refer to the Calculating a power
budget (Page 787) to determine how much power (or current) the CPU can provide for your
configuration.
The standard CPU provides the 5 V DC logic power needed for any expansion in your
system. Pay careful attention to your system configuration to ensure that the CPU can
supply the 5 V DC power required by your selected expansion modules. If your configuration
requires more power than the CPU can supply, you must remove a module.
Note
If the CPU power budget is exceeded, you may not be able to connect the maximum number
of modules allowed for your CPU.
The standard CPU also provides a 24 V DC sensor supply that can supply 24 V DC for input
points, for relay coil power on the expansion modules, or for other requirements. If your
power requirements exceed the budget of the sensor supply, then you must add an external
24 V DC power supply to your system. You must manually connect the 24 V DC supply to
the input points or relay coils.
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 power supplies safely
Connecting an external 24 V DC power supply in parallel with the 24 V DC sensor supply of
the CPU 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 or serious injury to personnel, and/or damage to equipment.
The DC sensor supply of the CPU and any external power supply should provide power to
different points. A single connection of the commons is allowed.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
WARNING
Installation
3.2 Power budget
49