Figure 2 1 illustrates the waveform of the rectified voltage with low respec
tively high voltage and current.
The ''Rectifying circuit" and filter operate as a coars e regulator. Its task
is to supply a rectified voltage to the s eries regulator which is a few volts
higher than the output voltage.
In
this way excessive power dissipation in the
series tra.rlsistors is avoided .
When low output voltage is desired only a low voltage has to b e supplied
to the "Series element" . Then the SCR 1s are fired late every half period.
Thus only the shadowed areas in figure 2 1A are fed to the filter input.
For high output voltage and current, more voltage has to be supplied to
the "Series element" . The the SCR's are fired earlier every half period, as
shown by the shadowed area in figure 2
lB .
By varying the firing point at each half period, the mean value of the voltage
to the filter and consequently the DC voltage after the filter is varied.
This regulation is achieved with "negligible power dissipation. After filtering
a voltage is obtained, being the mean value of the voltage supplied to the
filter.
D87 called freewheel diode, gives a path for the inductive current in Dr2
during the period when the thyristors are not fired.
D. F I L T E R
The " Controlled rectifying bridge" supplies a voltage, typically as in the figure
below.
F i g u r e 2 2
To convert these sine-wave pulses to a D C voltage, an LC low pass filter .
with cut off frequency far below the pulse f;requency is used.
Fi g u r e
2 3
26
