HP 6177B Operating And Service Manual
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limi~.
As
soon
as
the
voltage
limit
threshold
is
ex-
ceeciec.,
the
voltage
limit
overshoot
control
clamps
the
output
voltage
to
a
few
Volts
above
the
thres-
hole
setting,
until
07
has
a
chance
to
tum
on
to
l1rni:
the
output.
4-Sl
\Vhen
the
voltage
limit
threshold
is
exceeded,
Q3
COD-ciucts
heavily
causing
CR12
to
become
for-
ward
biased
and
VR4
and
VR7
to
reach
their
break-
down
voltages.
This
draws
current
from
the
+5.6-
\/olt
reierence.
Since
the
+5.6
Volt
reference
is
tied
to
comrr:on
through
VR12,
the
above
action
actually
craws
C..1rrent
irom
common
away
from
the
load,
thus
lImiting
the
output
to
the
preset
voltage
limit
set-
tlTIg
plus
about
7
Volts.
4-52
METER
CIRCUIT
4-53
The
meter
circuit
provides
a
continuous
indi-
cation
0:
the
output
current,
in
anyone
of
three
ranges,
or
the
output
voltage.
The
meter
ranges
are
controlled
by
the
same
switch
tha:
controls
the
output
current
ranges.
For
this
reason,
when
the
range
switch
is
moved
to
the
next
higher
range,
the
output current
changes
to
ten
times
the
original
value,
but
the
needle
on
the
meter
will
not
move.
However.
it
should
be
understood
tna:
the
meter
is
now
reading
ten
times
the
indicated
value.
The
re-
verse
is
true
when
switching
to
the
next
lower
range.
4-5
4-54
Notice
that
the
ammete:
is
connected
from
common
~
to
guard
and
the
voltmeter
from (-)
out
to
guard.
]:"5
explaIned
previously,
the
(+)
output
terminal
is
kept
at
the
same
potential
as
the
guard.
Th'...ls,
the
ammeter
is
actually
reading
the
voltage
across
the
sampling
resistance
which
is
proportion-
al
to
the
outpu'!:
current,
and
the
voltmete:
reeds
the
actual
output
voltage.
Connecting
the
mete
to
guard
eliminates
any
shunt
current
paths
which
might
be
created
by
the
ITf€ter
CIfcuit.
1:
also
allows
the
operator
to
program
the
output
before
connecting
the
load,
since
the
meter
reads
the
programmeci
out-
put
current.
4-55
ADDITIONAL FEATURES
4-56
Two
diodes
(CR3S
and
CR36)
placed
in
series
with
the
negative
output
terminal,
prevent
reverse
current
from
flowing
into
the
supply
and
possibly
damaging
its
components.
They
also
allow
the
sup-
ply
voltage
limit
to
be
programmed
through
zero
by
offsetting
minor
base
to
emitter
voltage
drops
in
the,
voltage
feedback
loop.
4-57
R80,
connected
across
the
supply,
keeps
the
supply
from
oscillating
with
an
iI1ductive
load
con-
nected.
CR34
prevents
reverse
voltages
from
dam-
aging
the
supply.
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