HP 6296A - Page 29

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4-29
Stage
Q200A
is
connected
to
+S
through
im-
pedance
equalizing
resistor
R203.
Instantaneous
changes
in
output
current
on the
positive
line
are
felt
at
the
current
summing point (terminal
A7)
and,
hence,
the
base
of
Q200B. Stage
Q200B
varies
its
conduction
in
accordance
with
the
polarity
of
the
change
at
the
summing point. The
error
voltage
is
taken
from
the
collector
Q200B
and
ultimately
varies
the
conduction
of
the
series
regulator.
4-30
Resistor
R304,
in
conjunction
with
C300,
helps
stabilize
the
feedback
loop.
Resistor
R808,
shunting
the
pullout
resistor,
serves
as
a trimming
adjustment
for
the
programming
current
flowing
through
R809
and
R810.
4-31
GATING
CIRCUIT
4-32
The
gating
circuit
(see
schematic)
consists
of
gating
amplifiers
Q301
and
Q300
and
associated
OR-gate
diodes,
CRJOO
and
CR301.
The
gating
circuit
provides
sharp
crossover
between
constant
voltage
and
constant
current
operation. During
steady
state
conditions,
one
transistor
is
saturat-
ed
while
the
other
is
conducting in
its
linear
region.
The
cathodes
of
the
OR-gate
diodes
are
always
at
a more
positive
potential
than
the
satu-
ation
potential
of
0300
or
Q301.
Thus,
the
diode
associated
with
the
saturated
transistor
is
reverse
biased
while
the
diode
associated
with
the
other
transistor
is
forward
biased.
In
the
constant
volt-
age
mode Q301
is
operating
in
its
linear
region
and
Q300
is
saturated
due
to
the
positive
collector
voltage
of Q200B. OR-gate
diode CR301
is
there-
fore
reverse
biased
while
OR-gate
diode
CR300
is
forward
biased,
coupling
the
constant
voltage
feedback
si9nal
to
the
error
amplifier.
Opposite
conditions
prevail
during
constant
current
opera-
tion.
4-33
Capacitor
C302
is
a commutating
capacitor
which
improves
the
transient
response
of
the
unit.
Resistor
R300
is
the
biasing
resistor
for
the
OR-
gate
diodes.
4-34
ERROR
AMPLIFIERS
4-35
The
error
amplifiers
Q302
and
QJOJ,
ampli-
fy
the
feedback.
si9nal
from the
constant
voltage
or
constant
current
input
circuit
to
a
level
suffi-
cient
to
drive
the
series
regulator
transistor.
Transistor
Q303
serves
as
the
driver
and
Q302
the
predriver.
for
the
series
regulator. The
RC
net-
work,
composed
of
C301
and
R307,
is
an
equaliz-
ing
network
which
provides for high
frequency
roll-
off
in
the
loop
gain
response
in
order
to
stabilize
the
feedback
loop
.
4-7
4-36
Capacitor
C900,
diode
CR901,
and
resistor
R902
form a
long
time
constant
network
which
achieves
a
slow
tum-on
characteristic.
When
the
unit
is
first
turned
on,
C900
provides
a
positive
voltage
to
the
base
of
Q302
keeping
the
series
regulator
from
conducting
initially.
As
C900
charges
up,
the
restrictive
bias
becomes
less
positive
enabling
the
regulator
to
conduct.
Diode
CR902
provides
a low
resistance
discharge
path
for
C900
when
the
unit
is
turned
off.
4-37
FAULT
INPUT CIRCUIT
4-38
The
fault
input
circuit
(see
schematic
at
rear)
protects
the
power
supply
against
overvolt-
age
and
overcurrent
conditions.
Transistor
0800,
and
associated
components,
comprise
the
over-
voltage
detector.
With
normal
output
voltages
QSOO
is
cut
off
due
to
the
+15.4V
reference
voltage
connected
to
the
top
of
voltage
divider
R823
and
R821.
If
the
output
voltage
exceeds
a
certain
limit
(about
20%
above
the
maximum
rated
output
voltage)
transistor
QSOO
is
driven
into
conduction.
Current
is
then
conducted
away
from the summing
point
and
in
opposition
to
the
charge
path
of
C700
in
the
SCR
control
circuit.
As
a
result,
the
SCR'
s
are
fired
at
a
later
time,
reducing
the
series
reg-
ulator
input
voltage
to
a
safe
value.
4-39
A
full
wave
rectified
voltage,
obtained
from
the
SCR
control
circuit,
excercises
a
stabilizing
influence
on
transistor
QBOO.
This
signal
tends
to
synchronize
the
conduction
of
QBOO
at
a 120
Hz
rate
preventing
random firing of
the
SCR'
s.
4-40
Transistors
Q801
and
Q802
provide
overcur-
rent
and
short
circuit
protection
for
the
unit.
Overcurrent
protection
is
accomplished
by
Q802
which
is
activated
only
1f
the
constant
current
input
circuit
should
fall.
Q802
monitors
the
volt-
age
drop
across
the
current
sampling
resistor
and
conducts
if
this
drop
exceeds
a
certain
level.
The
output
of
Q802
is
fed
to
gating
amplifier
QJOl
via
R815
and
ultimately
reduces
the
conduction
of
the
series
regulator.
4-41
Short
circuit
protection
is
provided
by
tran-
sistors
Q801
and
Q802.
Transistor
Q801,
normal-
ly
biased
below
cutoff,
monitors
the
voltage
drop
across
the
series
regulator.
Under
short
circuit
conditions,
the
·increased
voltage
across
the
reg-
ulator
drives
Q801
into
saturation.
The
positive
going
emitter
voltage
of
Q801
also
drives
Q802
into
conduction.
The
output
of
Q802
limits
the
current
flow through
the
series
regulator
to
a
pre-
scribed
leve
1.