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9.
Switched-mode power supply
The input voltage (mains voltage) supplied to the switched-mode power supply (Fig.21) may vary
between 115VAC and 240VAC.
The output voltage of the switched-mode power supply is about 24VDC and it is independent of
the input voltage as long as this is between 115VAC and 240VAC.
9.1
Operation of switched-mode power supply
The input voltage (mains voltage) is an alternating voltage, which is rectified and smoothed by the
switched-mode power supply.
This rectified voltage is supplied to the primary coil of the transformer by means of a high-
frequency pulsating electronic circuit.
The transformation ratio of this transformer determines the output voltage and the voltage coming
from the secondary coil will be a (lower) alternating voltage.
This (lower) alternating voltage is then rectified and smoothed again, making the output voltage of
the switched-mode power supply a rectified voltage.
However, by measuring the output voltage of the switched-mode power supply this can be
adjusted if necessary.
This adjustment is done by varying the high-frequency pulsating electronic circuit at the primary
coil of the transformer.
Due to this, it does not matter if the input voltage is 115VAC or 240VAC or somewhere in
between. The switched-mode power supply has been externally secured with two 4A fuses.
Fig.21 Switched-mode power supply
16
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