341
A
343A
circuit is
activated,
which occurs in
less
than
100 milli-
seconds. The sink supply
is
connected across the series
regulator, through diodes CR32 and CR33.
When,
the vol-
tage
across
the series regulator approaches 730 volts,
CR32
and
CR33
conduct, and the sink supply clamps the
voltage
across
the regulator to 730 volts.
As
soon as the crowbar
circuit
is
activated,
the crowbar discharges the high voltage
filter capacitors and the clamping function of the sink
supply ceases.
3-32.
The crowbar circuit consists of Schmitt trigger
Q28,
Q29,
relay driver
Q30,
series regulator
Q31,
crowbar
relays K2A
and
K3A and associated components. Under
normal
conditions,
Q28
is cutoff
and Q29 is
conducting.
Thus,
Q30
is also off and the crowbar relays are de-ener-
gized. Assume the instrument is downranged. The output
capacitor €49 begins
to discharge and
the voltage
across
the series regulator begins to rise. As this
voltage
rises,
current flow increases through
CR51,
CR52, R88, and
R89,
thereby
increasing the
positive voltage
at
the base
of
Q28.
When
the voltage across the series regulator rises to
within 40 volts of the sink supply voltage,
Q28
is biased on
and
the Schmitt
trigger
changes state. This turns
on the
relay driver
Q30,
which
activates
the crowbar
relays.
The
crowbar relays
place R81 and R84 directly across
filter
capacitors €55
through C58, and the capacitors quickly
begin to discharge.
When
the voltage
across
the filter
capacitors has decreased by approximately 80 volts, the
decreased
voltage at the base of Q28
resets the Schmitt
trigger, and the crowbar
relays deenergize. The
continued
discharge of
€49,
however, causes
the
voltage across the
series regulator to
rise again. When the
voltage rises to
within 40
volts
of
the sink supply voltage,
the crowbar
circuit is again
activated,
and
the filter capacitors are
dis-
charged another 80
volt step. This action
continues
until
C49
and
C55
through C58
have stabilized at the selected
downrange voltage. The
step discharge of the
filter capaci-
tors is a function of
the hysterisis exhibited by
the Schmitt
trigger, that is,
the Schmitt trigger is activated by a
much
higher voltage than
is
required to
deactivate it. Thus, the
crowbar
relays are energized for
a
finite time
period,which
contributes greatly to minimum arcing at
the contacts and,
consequently, long contact
life. Transistor
Q31 is
a series
regulator for the
Schmitt trigger and relay driver stages.
3-33.
A second function of
the crowbar circuit
is
to pro-
vide a trip signal to the control circuit
whenever crowbar
relay closure time
becomes excessive.
This
will occur
if
the instrument output is
short-circuited at voltage
settings
above
300 on the 1000
volt
range.
It will
also
occur if the
instrument is downranged
excessively. Closure
of
the crow-
bar relays applies a
voltage
to
the
RC
time constant
circuit
consisting of R10I, R102, and
C5,
which determines the
crowbar relay closure limits. When
crowbar
operation
exceeds this limit, a
positive
trip
voltage is coupled through
CR67
to the
control circuit.
3-34.
CONTROL
CIRCUIT
3-35.
The
control circuit consists of AND gate
Q26,
Q27,
high
voltage relay K1C, silicon controlled rectifier
CR29,
and associated components.
When
the
instrument
FUNCTION
switch
is
set from OFF
to
STANDBY/RESET,
voltage is applied to
the control circuit,
to
the +15 volt
reference supply, and to
the auxiliary power supplies. At
this time, the high voltage is disabled by contacts KIA of
the high voltage relay; and the instrument output
terminals
are shorted
by
contacts K1B of the
high
voltage relay,
which prevents any uncontrolled voltage from appearing
on the output terminals while the amplifiers are
stabilizing.
Rectified voltage for the control circuit is
provided
by
CR26, CR27, and CR24.
This voltage
is
applied to the
base of Q51 through a time constant circuit
consisting
of R86 and
C25,
thus
delaying turn-on
of Q51
for 3
to
5
seconds. This delay ensures that the
auxiliary power
supplies will be on and stabilized before high voltage is
applied. The delayed voltage then turns on
Q51
and Q27.
The other input to the AND gate is
controlled
by
the
FUNCTION
switch and is not present until the FUNCTION
switch
is set to ON. A
positive
voltage is then applied
through R87 to the base of
Q26,
turning it on. The high
voltage relay then
operates,
high voltage is applied to the
main
rectifier
and filter, and normal operation
ensues.
The purpose of diode CR68 is to discharge
C25
quickly
in
the event
of
power failure
or other interruption of
main
power, thus assuring a full 3 to 5 second delay cycle.
3-36.
In the
STANDBY/RESET
condition,
the series
regulator is cut off to ensure that the main control ampli-
fier is within its linear operating range when the high
voltage is applied. This is accomplished by
Q13,
which
activates
the
controlled current
limiter as
described
in
paragraph
3-25.
When the high
voltage
is off, there is no
zener
current for CR48. Consequently, transistor
Q13
will
be on and the controlled current limiter will be activated.
The main feedback loop at this time
is
provided by the
negative limiter, which clamps the output of the differential
amplifier
to
-2.5 volts.
3-37.
The trip
function
of the control circuit is accom-
plished
by silicon
controlled rectifier CR29. CR29 receives
an
enabling trigger under two
conditions.
When the crow-
bar
relay
closure is excessively
long, a
trigger voltage
is
developed
as
described
in paragraph
3-33.
A
second means
of triggering CR29 is
provided
by the circuit consisting of
3-5
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