the
base current
of
Ql,
effectively holding
it
in
saturation;
this
regen-
erative
switching
action can
only
be cancelled by
momentarily
disrupting
the supply
to pin
A.
(2)
A
second
holding
circuit
will
be
in effect when the typical
connections
are made
as
shown.
(Figure
3-6)
When
relay
Kl
is
energized,
one set
of
contacts will disconnect the
aircraft buss from
the regulator
section
(Pins F
and
G),
This
action
also places the external overvoltage
warning light in circuit. A second
set
of
contacts
will connect
the
buss voltage
to
the
top
of
RB;
the
resulting
current will add to the base
current of
Q2,
holding it in
saturation.
Note that both
holding
circuits
are
capable
of
independantly
holding
in the relay Kl,
thus
increasing
the
reliability
of
the overvoltage protection
system.
:
It would be undesirable for the
overvoltage
protection circuit
to
respond
to
short-duration
voltage
transients,
These
transients would
not be considered
a
fault condition
so,therefore, they
must not be allowed to
trip the
overvoltage
protection
circuit.
A
tripout
delay
of
approximately 50 to
200
milliseconds is
provided
by
capacitor
C2
and the combination of R4, Rl,
R2
and R3.
The action is
as
follows:
If
a transient appears
at
pin
A,
capacitor
C2
will appear as
a
low
impedance
and
will
couple 100% of the voltage change to
the
base
of Ql. As
Zener
diode
CR2
also couples 100%
of
the
voltage
change
to the emitter
of
Ql,
there will be no
overall change in
Ql
bias. However,
if
the voltage change
persists,
C2
will
charge
to
the level determined
by
Rl, R2,
and
R3,
and
Ql
will
conduct.
Cl serves to
reduce
incoming
transients and also to prevent
any
internally generated interference
from
entering
the aircraft supply
buss.
Rl
is
not
accessable from outside the unit.
It is preset
at the factory
to give
the
"overvaltage
adjust" control
R2
an adjustment
range
of 30.5
3-14
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