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4-32.
AC
INPUT.
The ac input to
transformer
T2 is stepped-down
and
full-wave
rectified by bridge rectifier
CR39
through CR43.
The output of
the
bridge rectifier
is
a
negative-going pulsating de (120
cps). Voltage
divider
R50-R5
l
supplies
a
portion
of this pulsating
de through
C2 7 to
the
base of 07; thus, the base is reverse
biased.
4-33.
FIRING. A point is
reached
during each
cycle
of
the
120-cps
pulsating
de
(each
half
cycle
of
the 60-cps
ac
input) when
the reverse bias on the base and the
forward
bias
(capacitor C25)
on
the
emitter
of
07
a
re
equal,
a
nd
therefore 07 has
zero
bias.
As the ramp voltage across
C25 goes more negative than the base
volt-
age, the base-emitter junction of 07
begins to become forward
biased
.
When
the
emitter is more negative than
the
base by
approximately 0. 5
volts,
07 conducts.
The firing point
of
07 is
therefore
determined
by both
the
de output error and the
line
voltage
change. Because 07
saturates
when
it
conducts,
the collector voltage
approximates
a
rec
tangular
wave with
a
negative
going pulse
width
of approximately
20 microseconds (determined by C27
and
R5 l}.
The
conduction of 07
charges C25
in the positive direction (clamped
by CR49).
Wh
en
07
stops conducting,
the
ramp
across
C25
begins
again. However,
07
is held cut-off by
the charge
on
C2 7.
4
-
34.
INITIAL
CONDITIONS. At
the beginning
of each cycle of
the 120-cps
pulsat-
ing de, certain
initial
conditions
must
be established on
capacitors C25
and C2 7.
When the negative-:-going
pulsating
de is
at
the end
of its cycle (C2 7 negatively
charged earlier in
the
cycle by
the feedback voltage), CR44 and CR45 become forward
biased
and
current flows from the
35-vdc
bias
through R52, CR46,
and
CR44 to
dis-
charge C27 to
approximately
zero volts
and
through R52, CR46,
and CR45
to charge
C25 to approximately
0
.
7 volts (clamped by
CR49).
This discharge
and
charge
occurs rapidly,
so
that it is
completed before the
next cycle
begins
and
07
can con-
duct
again.
Diode CR47 provides
another path for
the
current
through
CR44 so that
the voltage to which C2 7 discharges remains predictable.
As
the negative-going
pulsating de increases in
the
next cycle, CR44
a
nd CR45 become
reverse
biased.
4-35.
BRIDGE RECTIFIER.
At
the
zero cross-over
region
of
the
voltage waveform
on secondary winding 3-4
of
transformer T2,
the voltage is
insufficien~
to forward
bias the rectifiers in the bridge. In
order
to maintain definition between the
end
of
one cycle of the rectified
output
and
the beginning of the next cycle,
diode CR4
l
provides
approximately 0.
7 volts
at
the rectified output. The current
for CR4
l is
supplied through CR46. As the voltage
across the secondary
winding
moves away
from the zero cross-over
region
, CR41 becomes reverse
biased.
4
-
36.
TRANSIENTS, DECOUPLING
AND PROTECTION.
Transients
in the
pulsating
de
are
reduced by R56
and C28.
The base
of 07 is
decoupled by
C3. The voltage
spike in
the
collector of 07, induced
by
secondary winding
1-
2 of transformer
T3
when 07 cuts-off, is clamped by CR48. The
collector
is decoupled by R53
and C26.
4-5
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