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HP 6268B Operating And Service Manual

HP 6268B
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4-14
The
turn-on
control
circuit
is
a
long-time-constant
network
that
gives
the
supply
a
gradual
turn-on
characteristic.
The
slow
turn-on
feature
protects
the
preregulator
triac
and
the
series
regulator
from
damage
that
might
occur
when
ac
power
is
first
applied
to
the
unit.
At
turn-on,
the
control
circuit
sends
inhibiting
voltages
to
the
preregulator
control
and
the
series
regulator
(via
the
error
and
driver
amplifiers).
After
a
brief
delay,
the
inhibiting
voltages
are
removed
and
the
circuit
has
no
further
control
over
the
operation
of
the
supply.
4-1
5
The
reference
supply
provides
stable
reference
voltages
used
by
the
constant-voltage
and
current-compara¬
tors.
Less
critical
operating
voltages
are
obtained
from
an
unregulated
bias
supply.
4-16
DETAILED
CIRCUIT
ANALYSIS
4-17
Preregulator
Control
Circuit
4-18
The
preregulator
minimizes
the
power
dissipated
by
the
series
regulating
transistors
during
input
line
voltage
or
dc
output
voltage
variations.
Preregulation
is
accom¬
plished
by
a
phase
control
circuit
using
triac
A2CR1as
the
switching
element.
4-19
The
triac
is
a
bi-directional
device;
it
can
conduct
current
in
either
direction.
Hence,
it
fires
whenever
it
receives
a
gating
pulse
regardless
of
the
polarity
of
the
ac
voltage
applied
to
it.
The
triac
is
fired
once
during
each
half-cycle
of
the
ac
input
(see
Figure
4-3).
When
the
triac
is
fired
early
in
the
half-cycle,
the
ac
level
applied
to
the
power
transformer
is
relatively
high.
When
the
triac
is
fired
later
in
the
half-cycle,
the
ac
level
is
relatively
low.
4-20
Normally
the
ac
input
must
be
above
a
certain
minimum
potential
before
a
triac
will
conduct.
However,
A2C1
and
C2
and
A2R1
and
R2
provide
a
holding
current
that
allows
the
triac
to
conduct
at
any
time
during
the
ac
input
cycle.
RFI
choke
A2
LI
slows
down
the
turn-on
of
the
triac
in
order
to
minimize
spikes
at
the
output
of
the
supply.
The
components
of
the
A2RFI
Filter
Assembly
are
all
mounted
inside
a
shielded
box
to
minimize
radiated
and
reflected
RFI,
Further
RFI
suppression
is
provided
by
line
bypass
capacitors.
4-21
The
preregulator
control
circuit
samples
the
input
line
voltage,
the
dc
input
to
the
series
regulator,
and
the
volt¬
age
across
the
series
regulator
transistors.
It
generates
firing
pulses
that
fire
the
triac
and
adjusts
the
phase
of
these
pulses
so
that
the
dc
voltage
across
the
series
regulator
remains
constant.
4-22
The
inputs
to
the
control
circuit
are
algebraically
summed
across
capacitor
C70.
All
inputs
affect
the
time
required
to
charge
C70.
The
input
line
voltage
is
rectified
by
CR81,
CR82,
CR83,
and
CR84,
attenuated
by
voltage
divider
R83
and
R86,
and
applied
to
the
summing
point
at
the
collector
of
Q71
(TP80)
via
capacitor
C70.
Capacitor
C73
is
used
for
smoothing
purposes.
4-23
Transistor
Q71,
connected
in
a
common
base
con¬
figuration,
provides
a
charging
current
to
the
summing
capacitor
which
varies
with
the
input
signals
applied
to
its
emitter.
Resistor
R78,
connected
between
the
negative
output
line
and
the
emitter
of
Q71,
furnishes
a
signal
pro¬
portional
to
the
output
voltage.
Resistors
R75
and
R76
sample
the
voltage
across,
and
the
current
through,
the
series
regulator.
Resistors
R70
and
R80
are
the
source
of
a
constant
offset
current
to
the
summing
point,
which
ensures
that
the
triac
will
fire
at
low
output
voltages.
Capacitor
C72
and
resistor
R82
stabilize
the
entire
pre¬
regulator
feedback
loop.
4-24
The
summation
of
these
input
signals
results
in
a
voltage
waveform
at
TP80
similar
to
that
shown
in
wave¬
form
(A)
of
Figure
4-4.
When
the
linear
ramp
portion
of
the
waveform
reaches
a
certain
negative
threshold,
diodes
CR74
and
CR75
become
forward
biased.
These
diodes
couple
a
negative
voltage
to
the
base
of
transistor
Q72.
Transistors
Q72
and
Q73
form
a
bistable
circuit
similar
to
a
Schmitt
trigger.
Prior
to
firing
time,
Q72
is
conducting
due
to
the
positive
bias
supplied
to
its
base
throughR84.
Transistor
Q73
is
cut
off
because
its
base
is
driven
negative
by
the
collector
of
Q72.
4-3

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HP 6268B Specifications

General IconGeneral
BrandHP
Model6268B
CategoryPower Supply
LanguageEnglish

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