SIEMENS
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Siemens KT 10.1 · 2016
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Technical information and configuration
Power supplies general
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Overview
Power supplies
In plant building or mechanical equipment manufacture, or in
any other situations in which electrical controls are used, a safe
and reliable power supply is needed to supply the process with
power.
The operational reliability of electronic controls and associated
reliable operation of automated plants is extremely closely linked
to the resistance of the load current supply to failure. Final con-
trol elements as well as input and output modules will only
respond to command signals if the power supply is operating
reliably.
In addition to requirements such as safety, particular demands
are placed on the electromagnetic compatibility (EMC) of the
power supply with reference to the tolerance range of the output
voltage, as well as its ripple.
Important factors that determine problem-free implementation
are, in particular:
•
An input current with a low harmonic content
•
Low emitted interference
•
Adequate immunity (noise immunity) to interference
Selected interference phenomena
General notes on DC power supplies
The DC power supply is a static device with one or more inputs
and one or more outputs that converts a system of AC voltage
and AC current and/or DC voltage and DC current to a system
with different DC voltage and DC current values by means of
electromagnetic induction for the purpose of transmitting electri-
cal energy.
The type of construction of a DC power supply is primarily
decided by its intended use.
Non-stabilized DC power supplies
The AC mains voltage is transformed using 50 Hz/60 Hz safety
transformers to a protective extra-low voltage and smoothed
with down-circuit rectification and capacitor filtering.
In the case of non-stabilized DC power supplies, the DC output
voltage is not stabilized at a specific value, but the value is var-
ied in accordance with the variation in (mains) input voltage and
the loading.
The ripple is in the Volt range and is dependent on the loading.
The value for the ripple is usually specified as a percentage of
the DC output voltage level. Non-stabilized DC power supplies
are characterized by their rugged, uncomplicated design that is
limited to the important factors and focused on a long service
life.
Block diagram of a non-stabilized power supply
Stabilized DC power supplies
Stabilized DC power supplies have electronic control circuits
that maintain the DC voltage at the output at a specific value with
as little variation as possible. Effects such as variation in input
voltage or changes in load at the output are electrically compen-
sated in the specified function area.
The ripple in the output voltage for stabilized DC power supplies
lies in the millivolt range and is mainly dependent on the loading
at the outputs.
Stabilized DC power supplies can be implemented on different
functional principles. The most common types of circuit are:
•
Linear stabilized power supplies
•
Magnetic voltage stabilizers
•
Secondary pulsed switched-mode power supplies
•
Primary pulsed switched-mode power supplies
The most suitable principle for a particular application case will
depend mainly on the application. The objective is to generate a
DC voltage to supply the specific load as inexpensively and as
accurately as possible.
EMC
Interference phenomena
Emission (emitted interference)
Interference caused by television
and radio reception
Interference coupling on data lines
or power supply cables
Noise immunity (immunity to interfer-
ence)
Faults on the power cable due to
switching non-resistive loads such
as motors or contactors
Static discharge due to lightning
strikes
Electrostatic discharge through
the human body
Conducted interference induced
by radio frequencies
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