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3330B
The
unregulated
negative
,
output
is
also
applied
through
R8
and
R26
to terminal 6
for
use
in
the
A3
assembly
indi-
cator
lamps.
3-78.
CURRENT
LIMIT CIRCUITRY.
The
current
limit
circuitry
receives
a
dc
control voltage
at
terminal
10
which
is
proportional to the output
current
of
the
instrument.
Diode
CR12
applies
this signal
to the
base of
Q5.
The
emitter
of
Q5
is
connected through
R21
to terminal 11,
which
receives a control voltage
from
the
wiper of
the
CURRENT
LIMIT
VERNIER
control or
Ladder. Variable
resistors
R23
and
R24
are
also
part
of
this
circuit
and
allow
calibration
of
the
CURRENT
LIMIT
LO
and
HI
settings.
When
the
output
current
of
the
instrument
attempts to
exceed
the value preset
with
the
CURRENT
LIMIT
control,
Q5
is
driven into
conduction
by
the
in-
crease
in
voltage
at
terminal
10,
which
drives
Q4
and
Q3
into
conduction.
This action bypasses
a
portion of the
Ladder
current through
the
A4A3
Differential
Amplifier
range
resistor
connected
to
terminal
16,
and
thereby
limits
the
output
current of
the
instrument.
The
regenera-
tive
pair
of
Q6
and
Q7
are also
driven into
conduction
at this
time
and
supply
a drive
current
to
the
A3
1
Limit/V
Trip
lamp which
is
also
illuminated
on
the
STDBY/RESET
or
VOLTAGE
TRIP mode
by
a zero volt control
signal
at
terminal
5.
This input
also
brings
Q3, Q4,
Q6
and
Q7
into
conduction.
3-79.
L25
and
—15
Volt
Power Supply (A4A8)
3-80.
The +25 and —15
Volt
Power
Supply
consists
of
two
separate low-ripple
power
supplies
used
to
produce
operating voltages
for
the instrument.
The +25
volt
power
supply
is
composed
of
a
bridge
rectifier
CRl
throu^
CR4
and
the regulator
Q2
through
Q6.
The —15
volt
power
supply
consists
of
a bridge
rectifier
CR7
through
CRIO
and
the regulator
Q7
through Q9.
Transistor
Q1
and
associated
components
are
not used
in
this
instrument.
3-81.
+25
VOLT POWER
SUPPLY.
AC
voltage
at
terminals
13
and
14
is
rectified
and
filtered
by
CRl
through
CR4, C2
,
R4, and C3
.
Transistor
Q2
is
a
constant
current source
for the regulator consisting
of
Q3
through
Q6.
The
differential
comparator
amplifier
of
Q5
and
Q6
receives a
reference voltage
at
the base
of
Q6
derived
from
zener
diode
CR6.
Variations
at
the
+25
volt
output
(terminal
10)
are
developed
across the voltage
divider
of
R8
through
RIO
and
are
applied to the base
of Q5.
Any
difference
between
the base voltages
of
Q5
and
Q6
is
amplified
and
used
to control the
conduction of
the
series-pass
driver
Q3.
Conduction
of
Q3
controls the
conduction of
the
series-pass
regulator
transistor
Q4, which
establishes
a low-ripple
+25
volt
dc output
at
terminal
10.
Variable
resistor
R9
allows
adjustment of
the regulator
output.
3-82.
-15
VOLT POWER
SUPPLY.
AC
voltage
at
terminals
1
and
6
is
rectified
and
filtered
by
CR7
through
CRIO, R14,
and
C8
to
produce
a
negative operating
vol-
tage for the regulator
of
Q7
through Q9.
A
voltage
divider
comprised of
R18
and
R19
references the base of
Q9
to a
potential
dependent on
the
+25 and
-15
volt
output.
Variations
in
the
output
are
amplified
by
Q9
and
used
to
control the
conduction of
Q8. The
conduction of
Q8
con-
trols
the
series-pass
regulator
Q7, which
establishes a
low-
ripple
-15
volt
dc output
at
terminals 3
and
9.
3-83.
Relay
Driver
(A4A9)
3-84.
The
Relay
Driver
activates
the
various relays
within the instrument
on
commands
from
the
A3
assembly.
The
relay drivers
Ql,
Q3, Q5, and
Q7
are identical
PNP
switches
used
to activate relays.
Transistors
Q4, QIO,
Ql
1
and
Q12
are
used
to
invoke
the
STDBY/RESET
mode
whenever
a
mode
or polarity
change
occurs.
A
Logic
"1"
(0
to
+0.5
volts) will
turn the relay
drivers
on,
and
a
Logic
"0"
(+4.5
to
+20
volts) will
turn
them
off.
3-84.
RELAY
DRIVERS.
Relay
drivers
Ql, Q3, Q5,
and
Q7
are identical
PNP
switches.
Each
transistor
re-
ceives
-25
volts
at
its
collector derived
from
the asso-
ciated
relay.
Base
voltage
is
dependent on
the input
logic signal
from
the
A3
assembly.
Zener
diodes located
in
the
collector
circuit
protect the
transistors
from
the
induced
voltages
developed
by
the
relay solenoids.
The
drivers
consisting
of
Q2,
Q8
and Q6,
Q9
provide current
for relay
operation.
Circuitry
for these drivers
is
essen-
tially
the
same
as the
other
drivers,
with
the exception
of
the additional
base
circuitry
used
to
invoke
the
STDBY/
RESET
mode.
3-86.
STDBY/RESET MODE.
When
changing
a
pro-
gram between
the voltage or
current
mode,
it
is
necessary
to
temporarily
place the
instrument
in
standby
to
prevent
uncontrolled
voltages
from
appearing
on
the
output
ter-
minals.
This
is
accomplished
by
circuitry
located
on
the
relay driver
assembly.
When
the negative or
low
input
is
applied
to the
base
of
Q9
(current
mode
command),
it
is
also
applied
to a differentiator consisting
of
C6
and
R30.
The
negative differentiator
output
is
coupled through
diode
CR12
to the base
of
Ql
1,
turning
it
on.
Q12
turns
on
at
the
time
that
Qll
does,
and
stays
on
until
C7
dis-
charges
through
R26
to a
value
below
0.6
volts,
a period
of approximately
100
milliseconds.
In the interval that
Q12
is
on, the base of
Q3
is
effectively
connected
to
ground,
which
de-energizes relay
A4K5
and
places
the
instrument
in
standby.
When
the positive or
high input
is
applied to
the
base
of
Q9
(voltage
mode
command),
it
is
also
applied
to the
base
of
Q4.
The
signal
is
inverted
in
Q4,
differentiated
by C5 and R28,
and
applied
to
Qll.
3-10
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