Options—
465B Service
Option
07
General Description
The
dc source is applied to
the Turn-off Level circuit,
the Start circuit,
and the primary of
T14500 (see Figure
6-8) If the
dc source is above the level
set by Turn-off
Level
Adjustment
R1613, the Turn-off circuit
does not
operate.
The Start circuit
provides a large current surgethrough
T1631 secondary to the bases of
Q1652, Q1662, Q1654
and Q1664. This starts the inverter.
The
Turn-off
circuit is activated in
two ways. In 24 V
operation, Q1622
is
turned
on by the
source
voltage
dropping
below 22 volts. In 12 Vmodeof
operation, Q1626
is turned on
by
the accidental
application of 24 volts
dc.
Turn-Off Level
Circuit
For
the following
description, refer to Figure 6-9.
The voltage reference
for the base of Q1606
is set by
R1604, VR1604,
and VR1605
to approximately 9.1 volts.
This establishes
the junction of R1607 and
the
emitters
of
01606
and Q1608 at
about 9.7 volts. Power transients
that
occur
whenever the inverter
is turned on are prevented
from
activating the Turn-off
circuit by the filtering action
of
C1605 This filtering action
helps hold the 9.1 volt
supply base of Q1606
constant and prevents
Q1608 from
turning
on, thereby allowing
sufficient time for the power
source to recover after providing
the initial-start
surge.
Source
voltages higher than
22 volts dc cause in-
creased current through
R1607,
Q1606,
and
R1609. Q1608
is kepi
cut off by
the
increased
voltage across R1609 and
the resulting
change across the
divider comprised of
R1611, R1613, and
R1614.
This
permits no current
through
R1617.
Since
Rl617furnishes
biasto
01622,
thetransistor
is cut off This permits
the collector
of Q1622 and the rest
of the turn-off circuit
to rise to a voltage determined
by
the
inverter circuit and
the dc source voltage. The
collector of
Q1622 may be about 24 volts (with
respect to
-dc)
with
a
12-volt dc source and about
36 volts with a 24-volt
dc
source.
If the dc source voltage
drops to less than 22 volts, the
current through divider
R1609, R1611, Rl613and R1614is
decreased. Q1608 conducts, taking
current from Q1606,
and causing less drop across
R1609. This makes Q1608
conduct more,
and Q1606iscut off. Current flowthrough
R1617 turns 01622 on. Q1622 saturates,
dropping its
collector voltage
to about 0.2 volt. The maximum base
current of Q1622
is
limited
by R1618.
During 12-volt
dc operation,
there
is no current flow
through
VR1604
and
VR1605, since their series rating
(about 18 volts) exceeds the
applied voltage. The
base
current of
Q1606, through R1605, turns Q1606 on enough
to take all thecurrentthrough
R1607,
which
causesQl608
to be cut off.
SI
765
(1753-29)2757-57
Figure
6-8.
Option
07 simplified block diagram.
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