so that each preregulator control
transistor feels one-fourth
of
the high voltage.
The base
of
the
final
preregulator
control transistor
Q42
is held at 12.4 volts
by
zener diode
CR4S.
Consequently,
the emitter
of Q42 will be constant
at approximately 11.6 volts, and the
main control transis-
tor
Q40 will see no more than 1 1
.6
volts
for any value of
high voltage output. The
control transistor
Q40
together
with
Q4i
form
a Darlington pair. The purpose
of diode
CR65 is twofold:
It
prevents breakdown
of Q42 when
Q40
is suddenly cut
off, and it clamps the voltage
at the
collector
of
Q40
to a safe value.
Diodes CR41 through
CR45 prevent excessive voltage
drops across
current sens-
ing
resistor
R70,
which
could
occur under certain output
load conditions,
such as
short circuited
output.
3-22.
Fixed
current limiting is provided
by Q50 and
associated components
and is designed to protect
the
series
regulator against
catastrophic failure.
Q50 is con-
nected as
a shunt regulator across
the base of
Q41 and
limits control transistor
current to
a maximum of 60 ma.
When
the current through
R70 exceeds
60 ma, transistor
Q50 conducts, drawing current
away from the base of
Q41,
thereby limiting
output current to 60 ma.
The
purpose
of
diode
CR50 is to permit the preamplifier
output voltage to
swing below zero volts.
3-23.
CONTROLLED CURRENT
LIMITER
3-24.
The
controlled current limiter
utilizes a differ-
ential
amplifier
Q! I,
Q12,
a voltage
amplifier
Q10,
and
a
diode-connected
transistor
Q8
to accomplish the current
limit function. The voltage
developed
across current
sensing resistor
R70 is applied
to the base of
Q12.
The
base of
Qll,
which is the other
input to the
differential
amplifier, is connected
to the current
control potentio-
meter
R41. When the
voltage
developed
across
R70
exceeds the voltage
set by R41,
transistor 012 ceases
conduction. When
Q12
stops
conducting, a shunt current
path is offered to the reference
current flowing into
the
summing junction. This
path is through
Q8
and
010 and
into
the
-15
volt supply.
The main amplifier input current
is
reduced accordingly,
and the instrument output current
is thus limited
to
the
preset value. Resistor
R44 and
transistor
Q8
form
a
coupling network,
which prevents the
leakage
current of
Q10
from
affecting the summing
junction current.
Transistor
Q8
is used as a diode because
of its
inherently low
leakage.
3-25.
Transistor Q13 is used to activate the controlled
current
limiter
during instrument warmup and
STANDBY
conditions. The base
of Q13 is
connected
to CR48, and
cutoff
bias is
applied
to
Q13
as long as the
voltage
across
CR48
exceeds
8
volts. When
the
voltage
across
CR48 drops
below 8 volts, Q13 conducts, cutoff bias is applied
to
Q12,
and the current limiter is activated. The
purpose of
Q13
is
covered in the control circuit
discussion, paragraph
3-36.
3-26.
The current limit lamp
driver turns
on the current
limit light whenever the controlled current
limiter
is
activa-
ted. When the current through
Q!2 is reduced at the
be-
ginning of the current limit operation, the
current in Qll
increases. The
current change in
Q1
1 is amplified
by
Q9,
which
turns on switch
Q7,
lighting the current
limit lamp
DSL
3-27.
NEGATIVE VOLTAGE
LIMITER
3-28.
The negative voltage limiter
consists
of a two-
stage inverting
amplifier
Q5, Q6,
and a coupling diode
Q4,
The limiter is connected between
the output
and
input
of
the chopper-stabilized
amplifier. When the
output of the
chopper-stabilized
amplifier attempts
to go more negative
than approximately
-2.5
volts, transistor
Q6
conducts
hard enough
to
turn
on
Q5;
and current
flows from the
+15
volt supply through
Q5,
through the
coupling diode
Q4,
and
into the summing junction at
the input
to
the
amplifier, litis limiting current
balances the
input
refer-
ence and beta
string currents, thereby limiting
the output
of"
the
amplifier to approximately
-2.5
volts. Coupling
diode
Q4
and resistor R52 are
used to limit
the
effects
of leakage current in
Q5. Resistor R51 and
capacitor Cl
4
control the frequency response of this
servo loop. Again,
transistor Q4 is used as
a
diode, because
of its inherently
low
leakage.
3-29.
SINK SUPPLY AND
CROWBAR
3-30.
The 730 volt
sink supply is
composed
of two
power supplies
connected
in
series.
The
570
volt supply
is
a
full-wave
voltage doubler
consisting
of diodes
CR1,
CR2, CR4, CR5 and capacitors
Cl and
C2. The 160 volt
supply
is a
half-wave
rectifier
consisting of
diode CR3
and
capacitor
C3.
The load
resistors for the sink
supply
are
R96 and
R97. The
sink
supply
is connected
through
R96
and R97 across
output capacitor
C49.
When
the
instru-
ment
is downranged,
current flows through
the sink
supply
and its
load resistors
to
quickly
discharge
C49
to the
selected
downrange voltage.
3-3
1 . When
the
instrument is downranged
or the output
is short circuited, the voltage
across
the
series
regulator will
attempt
to
rise,
because
of
the
charge on
the
high
voltage
filter capacitors.
The charge
on these
capacitors
can be
as high as 1500 volts.
The sink supply
clamps the voltage
across
the series regulator
to a safe value until
the
crowbar
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