Configuring
S7-300 Programmable Controller Hardware and Installation
4-26 A5E00105492-01
4.9 Selecting the load power supply unit
Load power supply unit
The load power supply feeds input and output circuits (load circuits), as well as
sensors and actuators.
Characteristics
Below we have listed the properties of load power supply units required in specific
cases of application for the selection of load power supply units.
Table 4-10 Characteristics of load power supplies
Characteristics of the load
power supply
Mandatory for ... Remarks
Protective separation Modules requiring voltage
supplies of ≤ 60 VDC or ≤ 25
VAC.
24 VDC load circuits
These characteristics apply
to Siemens power supplies
of the series PS 307 and to
SITOP power (series 6EP1).
Output voltage tolerances:
20.4 V to 28.8 V
40.8 V to 57.6 V
51 V to 72 V
24 VDC load circuits
48 VDC load circuits
60 VDC load circuits
-
Requirements
The DC load power supply must meet following requirements:
It must provide an extra-low voltage of ≤ 60 VDC and be safely isolated from
mains. Safe isolation from mains can be realized, 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.
How to determine load current
The required load current is determined by the sum load current of all sensors and
actuators connected to the outputs.
In case of short-circuit the DC outputs are briefly loaded with twice to three times
the rated output current before the clocked electronic short-circuit protection comes
into effect. Thus, you must consider 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 - especially for low
output power (up to 20 A) - you must ensure that the supply can handle this excess
current.
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