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3330B
its
base voltage
from
the
wiper
of
the
VOLTAGE
TRIP
VERNIER
control or Ladder.
When
the base voltage,
which
is
proportional
to the
output
voltage
of
the
instru-
ment, exceeds
the established
level
at
the base
of Ql,
the
transistor
conducts and
biases
Q2
into
saturation.
The conduction
of
Q2
clamps
the voltage
at
terminal
10 to nearly zero
volts
and
de-energizes the associated
relays.
This action
trips
the instrument
to the
STANDBY
mode.
Transistor
Q8
is
also
turned off
by
a
corresponding
decrease
in
negative
base
voltage,
and
the
OPR
lamp
is
extinguished.
The
instrument
is
returned
to
the
OPR
mode
by
first
selecting the
STDBY/RESET
mode,
which
turns
off
Q2
by
applying
a circuit
common
to terminal
10 through the contacts of
A4K5,
and then
selecting
the
OPR
mode.
3-62.
The
overcurrent
trip
feature
is
a
protective
circuit
that
functions
in
the event
of
a
current
limit
malfunction.
When
the current
through
the sensing
resistors
of
R22
and
R29
exceeds
200
milliamperes,
Q4
conducts
and
biases
Q2
into saturation,
which
activates
the
trip
circuit
discussed
in
the
preceding paragraph.
3-63.
METER
MULTIPLIERS.
Resistors
R1
through.
R6
provide
a
current
through
the
front
panel
meter
that
is
proportional
to either
the
output
voltage or current
of
the
instrument.
When
the
METER
READING
VOLTAGE
switch
is
pressed,
R3
through
R6
and
A4R1
through
R3
are
connected
to the
meter and produce
a reading
propor-
tional to
the
output
voltage of the instrument.
Variable
resistor
R5
allows
calibration
of
the
voltage
reading.
When
the
METER
READING CURRENT
switch
is
pressed,
Ri
and
R2
are
connected
to the
meter and produce
a
reading proportional to the
output
current
of
the
instru-
ment. Variable
resistor
R2
allows
calibration
of
the current
reading.
3-64.
Series Pass
(A4A5)
3-65.
Th,e Series
Pass contains the
series-pass transistors
used
to
control the
output
current or voltage
from
the
instrument.
Additional
circuitry consists
of
the pre-regu-
lator
VCO
used
to control the
amount
of
voltage across
the
series
pass
transistors,
and
an automatic
"crowbar"
driver
used
tO'
limit
the
maximum
voltage across the
series-pass
elements.
A
+150
volt
power
supply
is
also
in-
cluded to provide operating
voltages for the
series-pass
and
VCO
circuitry.
The +150
volt
power
supply
consists
of
the bridge
rectifier
CRl
througli
CR4,
and
the
series
pass
elements
are
Ql
through Q8.
The
VCO
consists
of
the
relaxation
oscillator
Q9
and
the associated
timing
networks.
Transistor
QIO
is
the
automatic
"crowbar"
driver.
3-66.
+150
VOLT POWER
SUPPLY.
AC
voltage
at
terminals 8
and
9
is
rectified
by
diodes
CRl
through
CR4
to
produce an unfdtered
positive voltage.
This
voltage
is
isolated
by
CR5
and
filtered
by
C2
to
provide
a
+150
volt
dc
operating voltage
for
the
series
pass
transistors.
The
voltage
divider
of
Ri
through
R3
and
zener diode
CR6
produces
a
clipped full-wave
rectified
six
volt
synchron-
izing
signal
for the
VCO,
Q9.
3-67.
SERIES PASS
TRANSISTORS
AND
AUTOMATIC
CROWBAR.
The
series-pass transistor
of
Ql
through
Q7
are
normally
saturated,
and
transistor
Q8
is
absorbing
the
total
voltage required
to establish
the
output of
the
instru-
ment. However,
when
the
output
level
or load
is
changed
and
the
voltage across
Q8
exceeds
150
volts,
Ql
through
Q7
absorb
the additional
voltage.
The
pre-regulator
circuitry
then reduces
the
output of
the
high voltage supply
and
the
voltage
across the
series
pass
transistors.
As
soon
as
the
voltage
drop
across
Q8
decreases
below 150
volts,
Ql
through
Q7
again saturate,
and
Q8
absorbs
the
total
regu-
lation voltage.
Load
or
output changes
that increase the
voltage
across the
series
pass transistors to
above 225
volts
result
in
the
conduction of
the
"crowbar"
driver,
QIO.
Conduction
of
QIO
energizes relay
K1
in
the
A4A6
High
Voltage
assembly,
which
places
a
load
across the high
vol-
tage
rectifier.
This action
limits
the voltage
across the
series
pass
transistors
and
provides
a
quick
discharge
path
for
the
high
voltage
filter
capacitors.
3-68.
PREREGULATOR
VCO.
Unijunction
transistor
Q9
and
associated
timing elements
comprise
a voltage-
controlled
oscillator
which
furnishes a turn-off pulse to
the
pre-regulator.
A
clipped,
six
volt pulse,
synchronous
with
the
60
Hz
line,
is
applied to base
2 of
Q9
(see
Figure
3-5).
The
potential
at
the emitter
of
Q9
depends
upon
the
charge
on
capacitors
C4
and
C5.
The
charge
on
C5
depends
upon
the voltage across the
series
pass
control
transistor
Q8.
When
the
charge
on
C4
and C5
equals
a
critical
level,
determined
by
the bias
on Q9,
Q9
will
conduct,
delivering a pulse
of
current
through
transformer
T1
to the
pre-regulator.
The
oscillator
will
be enabled
only during
interval
ti
and
the point
at
which
oscilla-
tion
will
occur
is
determined
by
the charge
on
C5.
If
the
Charge
across
C5
increases,
the
initial
output
pulse
of
the
oscillator
will
occur
earlier
in
each
half cycle.
During
each
half cycle, the
initial
pulse
from
the
oscilla-
tor
will
switch the pre-regulator off
to
control the
amount
of
line
power
supplied
to
the
high voltage transformer.
Thus,
if
the
voltage across
Q8
increases, the pre-regulator
is
switched
off
earlier in
each
half cycle.
This,
in
turn,
reduces
the voltage across
Q8
to the
proper
working
value.
11/72
3-8
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