Calculating A Power Budget - Siemens Simatic S7-200 System Manual

Calculating a Power Budget

The S7-200 CPU has an internal power supply that provides power for the CPU itself, for any expansion
modules, and for other 24 VDC user power requirements. Use the following information as a guide for
determining how much power (or current) the S7-200 CPU can provide for your configuration.
Power Requirements
Each S7-200 CPU supplies both 5 VDC and 24 VDC power:
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Each CPU has a 24 VDC sensor supply that can supply 24 VDC for local input points or for relay
coils on the expansion modules. If the power requirement for 24 VDC exceeds the power budget of
the CPU, you can add an external 24 VDC power supply to provide 24 VDC to the expansion
modules. You must manually connect the 24 VDC supply to the input points or relay coils.
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The CPU also provides 5 VDC power for the expansion modules when an expansion module is
connected. If the 5 VDC power requirements for expansion modules exceeds the power budget of
the CPU, you must remove expansion modules until the requirement is within the power budget.
The specifications in Appendix A provide information about the power budgets of the CPUs and the power
requirements of the expansion modules.
Warning
Connecting an external 24 VDC power supply in parallel with the S7-200 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 or serious injury to personnel, and/or damage to equipment.
The S7-200 DC Sensor Supply and any external power supply should provide power to different points.
A single connection of the commons is allowed.
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