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3330B
limiter
continuously
monitors output
current
and com-
pares
it
to
the
setting
of
the
CURRENT
LIMIT
control.
Wlien
the voltage across
the current
sensing
resistor
exceeds
the reference voltage
set
by
the
CURRENT
LIMIT
control,
the
limiter
is
activated.
A
portion of
the
current flowing
from
the reference
supply
into the
summing
junction
is
then
bypassed,
and
the
output
current
is
limited or
clamped
to
the
preset
value.
3-29.
The
series
pass
driver
performs
two
functions:
It
amplifies control
signals sufficiently
to drive the
series
pass,
and
it
disables
the
output
in
the
event
of an
over-
voltage or catastrophic over-current condition.
When
the
voltage across the
current sensing
resistor
exceeds
a pre-
determined
value,
the
trip
circuitry acts to
de-energize
relays
A5A2K1
and
A5A2K2
which removes
ac
power
from
the high voltage
transformer primary.
The
voltage
trip
circuit
is
activated
whenever
the
output
voltage ex-
ceeds
the
selected
trip
voltage
set
by
the
VOLTAGE
TRIP
VERNIER.
Again
A5A2K1
and
A5A2K2
are
de-energized
and
the
instrument
is
tripped
to
standby.
The
instrument
is
reset
by
first
pressing the
STDBY/RESET
switch
to
de-energize
A4K5
and
then the
OPR
switch.
3-30.
The
series
pass regulates
instrument
output
current
in
response
to
control
signals
from
the
series
pass
driver.
The
series
pass
is
actually eight
transistors
connected
in
series,
an arrangement
which
provides the regulated high
voltage capability.
The
series
pass
assembly
also
contains
a voltage controlled
oscillator
(VCO)
which
provides
a
pulsed timing
signal
to the pre-regulator.
The
pre-regulator
increases the
efficiency
of
the instrument
by
regulating
the input to the
high
voltage
power
supply.
It
utilizes
voltage
level
information
from
the
VCO
to
provide
full-
wave
phase control of the input
line
voltage
of
the high
voltage transformer.
Thus,
the
power
supplied
by
the high
voltage
transformer
is
controlled
to
provide only
that
amount
necessary
for
load
requirements.
3-31.
The
high voltage
power
supply
is
connected
as
a fuli-wave bridge
rectifier,
obtaining
its
input
from
the
pre-regulator
and
supplying dc
voltages to the
series
pass
regulators.
Contained
on
the high voltage
assembly
is
a
rate
limiter
for
the
automatic crowbar
which
prevents
excessive
crowbar
activation,
thereby providing protection
for
the
series
pass
transistors.
See paragraphs
3-67,
3-70
and
3-74.
3-32.
The
function
of
the
automatic crowbar
circuitry
is
to
quickly discharge the high voltage
filter
capacitors
when
down-ranging
the instrument. This
circuit also
provides
protection
for
the
instrument
during
abnormal
load con-
ditions,
such
as short-circuited
output.
A
second crowbar
circuit,
which
is
manually
operated,
is
connected
across
the
output
terminals.
It
is
used
to limit
compliance
volt-
age while changing
loads.
3-33.
Operating
voltages
for
the
instrument
are
provided
by
the
low
voltage
power
supplies.
The +25
volt
and -15
volt supplies
provide operating voltages
for
the
+10
volt
reference,
chopper
amplifier,
and
differential
amplifier.
A
separate
-25
volt
dc supply furnishes operating
voltages
for
all
decade
control
relays.
Operating
voltages
for
the
series
pass
driver are
provided
by
a
±35
volt
supply
located
on
the current
limiter
assembly.
A
—5
volt
supply provides
the
program
supply
voltage.
3-34.
The 50 program
input/output
(I/O) lines
provide
remote
control
of
all
front panel
control functions
and
remote
indication
of instrument
power
condition
and
vol-
tage trip/current
limit
operation.
Mode,
standby/operate,
polarity,
manual
crowbar,
and
voltage/current range input
lines
terminate
in
a relay driver,
which
supplies
the
driving
current
for
the
selected
relay.
Output
voltage
level
is
con-
trolled
by
a
28-line 8-4-2-1
BCD
input.
The
front panel
controls
are
connected
in
parallel
with
the
program
I/O
lines,
and
the
same
circuits are
activated
in
either
remote
or
local
operation.
The
selection
of remote
or
local
opera-
tion
is
made
only
at
the fiont panel.
When
the
OPR
switch
is
pressed, front panel controls
are
enabled;
when
the
REMOTE
switch
is
pressed,
instrument
control
is
switched
to the
programmed
input
at
J8.
3-35.
Mode
input
commands
are
applied through the
relay
driver to
relays
A4K3
and
A4K6.
These
relays
change
the
instrument
circuitry
to
provide
either voltage
or
constant
current operation.
Together
with
the
range
relays
A4K1
and
A4K2,
they
select
the appropriate
vol-
tage range
resistor,
current shunt,
and chopper
amplifier
zero
adjust control.
3-36.
The
standby/operate input
is
applied
through
the
relay driver to relay
A4K5.
This
relay
controls the opera-
tion
of
relay
A5A2K2,
which
in
turn, controls relay
A5A2K1. When
operated,
A5A2K1
connects
main power
to the pre-regulator.
The
remote
polarity
input
is
applied
through
the relay driver to relay
A4K4.
Upon
command.
A4K4
reverses
the
output
voltage polarity
at
the front
and
rear terminals.
The manual
crowbar
input operates
relay
A6K2.
When
activated,
this
relay
shorts the
output
ter-
minals through
a 5
ohm
resistor.
3-37.
The
28-line
output
level
input
is
applied
to the
seven-deck
resistive
ladder
network and
a
set
of
relay
switches, wliich
are
used
to
connect
the reference voltage
11/72
3-5
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