BFGoodrich Avionics Systems,
Inc.
Component Maintenance
Manual
P/N 501-1075-()
1.
Theory
Of
Operation
A.
General
This section provides
a
general
theory
of
operation
for
Model
PS-823
Emergency
Power
Supply.
A
block
diagram
(Figure
4)
is
included to support the text.
For
detailed circuit
descriptions refer
to
schematic
diagrams in the Testing
and Fault
Isolation
section.
B.
Power
Input
Aircraft 28.0Vde
is
connected
to the emergency
power
bus through diode A2CRl8. Diode
A2CR18 isolates the
internal
batteries in case of a short
circuit
on
the aircraft main DC bus. If
aircraft
power
is
lost,
the emergency
switch
is
placed
in
emergency
position.
C. Oscillator
Application of 28.0Vde to connector pin 13 activates a
voltage
divider
consisting of resistors
AlR12,
AlR13
and
zener diode AlCR8. Transistor
AlQ5
is a
voltage regulator that
holdsthe
DC voltage on
the oscillator
at 16
volts. Current flow
through capacitor
A lCl and
resistors
AlR1 and AlR2 is
gradually decreased as
capacitor AlCl
charges. The
junctionof capacitor
AlCl and resistor AlR1 reaches a
point where
transistor
AlQl will fire. Current flow
through
the
junctiondischarges capacitor A1Cl
and develops a
negative
spike on
transistor AlQ2base.
Transistor AlQ2conducts
for
the
duration of the spike and
develops a
positive pulse at
capacitor
AlC2 and diodes AlCR5
and AlCR7.
The
switching
portion ofthe
oscillator consists
of transistors
AlQ3
and AlQ4. The
unijunction transistor AlQl
fires
at 800 pulses
per second. Each
spike determines
the
startof a
half-cycle of
square-wave
on transformer Tl.
An
800pps
spike
frequency then
produces
a
400Hz
square-wave
on transformer Tl to
drive the output
transistors.
To understand
the
operations
of the free-running
oscillator
consider one cycle without the
application
of
pulses. When
power
is
initially
applied, the natural
unbalance of
the components
starts either transistors AlQ3
or
AlQ4
conducting.
(The
example
will
start
with
AlQ3
conducting). The initial current flow is
through
diode
AlCR3,
transistor
AlQ3
and halfof
transformer
Tl
to ground.
Transformer
Tl
saturates
immediately
and terminal
P3
becomes
+16Vdc.
As current continues to
increase,
transformer
action causes the
opposite
halfof
transformer
Tl
to become
-16.0Vde
at
terminal
Pl, which
turns AlQ3ON. When
current can
no
longer
continue to
increase,
transformer Tl no longer presents a
high
reactance
and terminal
P3 becomes more
negative.
The
magnetic field of transformer
T1 then collapses
and reverses
the
voltages
on its
two
halves.
Terminal
Pl
becomes
+16.0Vde
and terminal P3 becomes
-16.0
Vdc.
The
positive
voltage from terminal Pl
shuts OFF
AlQ3and the negative voltage
on P3
turns
transistor AlQ4ON.
As
current
through
transistor AlQ4
increases, the
same sequence
occurs as
described above. This is the second half-cycle and the polarities
on
transformer Tl
are opposite those of
the
first half-cycle. The time
duration of
each half-cycle
is
determined by
transformer Tl.
To operate at 400Hz,
the oscillator
functions as
described
above,
except that
the time duration
for each
half-cycle is determined
by
the
frequency ofthe spikes
from transistor AlQ2.
A
positive
spike applied to both
bases
of transistors AlQ3and
AlQ4
will turn
the conducting
transistor
OFF. This triggers
the same
events
described above
on each
half-cycle.
The
result is
a 400Hz
square wave
on
transformer
Tl.
msmijtp202DescriptOpsdiskl59
24-20-03
Page
5
Revision
A
June
30, 1999
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