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4-3 7.
SCR REGULATOR
4-38.
GENERAL. The
SCR regulator
(CR17
and
CR18)
control
s the
ac
input voltage
and
current to main power transformer Tl
in
response
to
the voltage
and
current error
signals.
In
constant
voltage
operation, the ac input voltage
to Tl
is adjusted
so
that
the
output voltage
remains constant
with changing loads.
In
constant
current
operation, the ac
input
current
to Tl is
adjusted
so that the
output current
remains
constant
with
changing loads and the output voltage is allowed to vary.
4-39.
GATING. Each half
cycle
of
the ac
input,
either
CR17 or CR18 is forward
biased
.
The pulse
induced in
secondary windings 5-6 and
7-8
of
T3 by the SCR
control,
turns
on the SCR
that is forward
biased
when
the pulse occurs
.
The
other
SCR
is not
affected
by
the gate
pulse because
it
is
reverse
biased.
A
gate pulse
occurs
each half cycle
of the ac
input, unless the
output
is open.
The
timing of
the gate pulse with respect
to the ac
input is determined by the
error
in
the
de
out-
put
via the loop
action.
4-40.
AC
INPUT CONTROL. When
an SCR
is gated
on,
it
conducts
until its anode-
to-
cathode
voltage
goes to
approximately
zero.
Thus, the
earlier
an SCR
is gated
on,
the greater the portion
of the
ac
input that
will be
applied
to Tl. Because
of
the leakage inductance
of Tl,
the conduction of an
SCR
may extend into the next
half
cycle. The conduction period may be
shortened at
high output by the voltage
across
capacitor
C
13
through C
16 being
reflected back
into the primary. By con-
trolling
the
ac input
to Tl
each
half
cycle, the average value of
the
voltage or
current at
the
output of
bridge rectifier
CR19
through
CR2
l is
adjusted so
that de
output
voltage
or
current is maintained
constant.
4-41.
PROTECTION
.
Diodes CR50
and CR5
l prevent
anode
induced reverse gate
currents
from being fed back to
the
control
circuit. Resistors
R54
and
R55 limit
current
in
the SCR
gates.
4-42.
BIAS
AND REFERENCE CIRCUIT
4-43.
GENERAL. The bias
and
reference
circuit supplies
three voltages (+35,
+6.
0,
and -19
.
5
vdc)
for
internal
power supply operation, and
maintains the pro-
gramming currents
constant
.
The
+35 vdc is
not
regulated.
The -19
.
5 vdc, +6. 0
vdc,
and
the programming
currents
are regulated.
4.:...44.
+35
AND
+6.0
VDC. The output of secondary
winding 5-6
of transformer
T2
is
full
-
wave
rectified
by
CR30 and CR31. Capacitors C20
and C21
each
charge to
the
peak rectified voltage (voltage
doubling).
The
+6. 0
vdc (with respect to -S) is
maintained by
diodes
CR6 and CR14 and by zener diode VR4.
The
+35 vdc
includes
includes the +6
.
0
vdc
and the
voltage
across C2 l
.
The +6
.
0
vdc
and the
negative
voltage across C20
provide
the unregulated input to the -19. 5 vdc regulator.
4-45
.
-19.5 VDC. For the -19
.
5
vdc, transistor
QlO
is
the error
detector/
amplifier.
Zener diode
VR3 and
diode
CR2 7 provide a reference
voltage
at
the
emitter of QlO. Voltage
divider
R35-R36 supplies an error voltage to the base of
4-6
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