Configuring
4.10 Selecting the load power supply
S7-300, CPU 31xC and CPU 31x: Installation
4-26 Operating Instructions, Edition 08/2004, A5E00105492-05
4.10 Selecting the load power supply
Task of the load power supply
The load power supply feeds the input and output circuits (load circuits), and the sensors
and actuators.
Features of load power supply units
You will have to adapt the load power supply unit to your specific application. The table
below shows a comparison of the various load power supply units and their features to help
you make your choice:
Table 4-12 Features of load power supply units
Necessary for ... Feature of the load power
supply
Remarks
Modules requiring voltage
supplies ≤ 60 VDC or ≤ 25 VAC.
24 VDC load circuits
Safety isolation This is a common feature of the
Siemens power supply series
PS 307 and SITOP power
series 6EP1.
24 VDC load circuits
48 VDC load circuits
60 VDC load circuits
Output voltage tolerances:
20.4 V to 28.8 V
40.8 V to 57.6 V
51 V to 72 V
-
Load power supply requirements
Only an extra-low voltage of ≤ 60 VDC which is safely isolated from mains may be used as
load voltage. Safe isolation from mains can be achieved, for example, in accordance with
VDE 0100 Part 410 / HD 384-4-41 / IEC 364-4-41 (as functional extra-low voltage with safe
isolation) or VDE 0805 / EN 60950 / IEC 950 (as safety extra-low voltage SELV) or VDE
0106 Part 101.
Load current determination
The required load current is determined by the accumulated load current of all sensors and
actuators connected to the outputs.
A short-circuit induces a surge current at the DC outputs which is 2 to 3 times higher than
the rated output current, until the clocked electronic short-circuit protection comes into effect.
Make allowances for this increased short-circuit current when selecting your load power
supply unit. Uncontrolled load power supplies usually provide this excess current. With
controlled load power supplies, and particularly for low output power up to 20 A, always
ensure that the supply can handle this excess current.
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