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HP 6960A - Page 17

HP 6960A
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Model
6960
A
Section
IV
,
,
R 3 and R 4
allow
a
current
flow
through
the
rectifier
bridges
even when no
output
current
is drawn from
the
instrument.
Series
regulators Q 1 and Q 2
act
as
variable
resistors
determining
output
voltage
and current. The
output
current
from
one
regulator
is
monitored
by the
resistor
combinations
R 39 B, R 9 and R
8,
while
R 39 A, R 9 and R 10
monitor
the
current
from
the
other
regulator.
The
voltage
drop
across
these
resistors
is
applied
to
the
emitter
of
Q
5.
When
this
potential
exceeds
a
dist
inet
value
determined
by R
39
A and B and
the
load current,
transistor
Q 5 is caused
to
clip. From then on
Q 5 holds the base
voltage
and
hence
the
emitter
voltage
of
Q 1
through
CR 13 and
voltage
amplifier
Q
3,
thus
preventing
any
further
increase
in
the
load
current.
Voltage
divider
R
5,
R 6
adjust
the
base-emitter
voltage
of
transistor
Q 2
if
the
power
supply
is
driven
in
the 36 V / 300 mA range. The
ammeter
(M
1)
indicates
the
output
current
by
measuring the
voltage
across R
22
and
the
voltmeter
M 2
indicates
directly
the
output
voltage
of
the
power
supply.
The
voltage
error
amplifier
consists
of
the
two
transistors Q 7
and
Q 8
besides
Q
3.
In
the
constant
voltage
mode
changes in
the
output
voltage
due
to
changes in
load
or
line
voltage
are
amplified
and
applied
to
the
base
of
Q 1
in
such a way
as
to
keep
the
output
voltage
constant. R
23
is used
to
adjust
the
positive
feedback
from
the
collector
of
Q 8
to
the base
of
Q
7.
A
proper
adjustment
of
this
feedback
will
result
in no
output
voltage
change when the load
current
is changed from no load
to
full
load.
It
can
even
be
adjusted
to
yield
a
negative
output
resistance,
i.
e., when a load is
applied,
the
output
voltage
increases
rather
than decreases. Normally, this is
adjusted
to
give
zero
output
resistance and
not
a
negative
resistance.
CR 14 and CR
13
are
disconnecting
diodes.
In
the
constant
voltage
mode
CR
13 is
closed, CR 14 open, so
that
the
current
error
amplifier
is
out
of
circuit.
In
the
constant
current
operation
CR 13 will be
automatically
opened
and CR 14
will
be
closed
to
isolate
the
voltage
error
amplifier.
The
reference
supply
consists
of
a
complete
feedback
amplifier,
Q 4 and Q
6.
The
reference
voltage
itself
is
controlled
by zener
diodes
CR 20 and
CR
21, which also
stabilizes
the
negative
auxiliary voltages. R
41
helps
to
supply
the
thermal
stability
of
the
base
potential
of
transistor
Q
5.
Manufacturing
tolerances
of
CR
21
are
compensated
by
factory
adjustment
of
R 27 by means
of
shunt
resistor
R 38. The value
of
R 17
allows
current
changes
proportional
to
line
voltage
changes.
These
current
changes
compensate
possible
variations
of
the
current
through CR
21
with line
voltage
variation
and minimize
changes in
output
voltage.
4-2
"'''ii'''''''''''''''=,",
_
..

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