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The pulsating DC from
the half wave rectifier
needs to be smoothed to
be useful.
This is done using a
RESERVOIR capacitor.
It is not called this
because of its
construction but because
of its application.
In the UK the weather is
very variable. Sometimes
we have periods of
drought and sometimes
periods of prolonged
rain. April is very
showery and occasionally
we have snow in Winter.
However, when I turn on
the shower, the water
always comes out at the
same steady rate.
This is because it has
been stored in a
reservoir and is
released as required.
It is the same with the
reservoir capacitor.
The capacitor is charged
up by the pulses.
It stores the pulses of
voltage and the load is
able to use this stored
power as if it were a
steady supply available
from a battery.
Looking at the first
waveform, the red line
indicates the voltage
across the capacitor.
When the first pulse
arrives the capacitor
charges up to the peak
voltage.
When this pulse has
ended, the capacitor
voltage starts to fall
as the load takes energy
from the capacitor.
However, the voltage
falls only slightly
before the next pulse
arrives to recharge the
capacitor to the peak
voltage.
The result is a DC
voltage with a
superimposed 50 Hz (in
the UK) ripple.
This is shown in the
second waveform.
At low frequencies the
capacitor is usually an
electrolytic type, but
at higher frequencies,
where smaller values are
required, other types
can be used.
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