Power
Supp/y/RF
Board
4-26
the
ground
referenced
low
voltage circuitry. Each
handswitching
output
of
the
generator
is
associated
with
its
own
IsoBloc
power
source
and
isolated signal
paths.
Oscillator
The oscillator circuit consists
of
a
chain
of
74HC14 inverters. The
output
of
the
oscillator yields a 67.5
kHz
square
wave
that
is
applied
to
the
input
of
three 4081 buffers (U29).
Power
Supply
The three 4081 buffers (U29)
drive
three VN0808L FETs connected
to
transformers
which
are
operated
in
a quasi-resonant flyback
mode
with
their associated 6800
pF
capacitors.
The
voltages
at
the
secondaries
of
the
three transformers
are
half-wave rectified
and
referenced to three
separate
isolated
grounds
to
provide
-8
V for
operating
the
isolated activation
circuitry.
Optoisolators
The
isolated
power
supply
voltages
produced
by
the
lsoBloc
power
supplies
are connected to
the
active
output
terminals
of
the
generator.
Handswitch
activation is accomplished
by
sensing
active to CUT
or
active
to
COAG
switch
closure
in
a
handheld
accessory. Switch closure
is
detected
by
comparing
the
voltage across
the
switch
to
a
divider
reference
.vith
comparators
U32, U33,
and
U34.
Current
limiting resistors,
in
series
with
the
LEDs
in
the optoisolators, cause
the
LEDs
to
light.
The
photo
transistor
in
the
optoisolator detects this light.
The
photo
transistor,
which
is connected
to
an
input
on
the
82C55
expansion
port
in
the
main
microcontroller circuit,
turns
on,
pulling
the
associated
input
low. This is
interpreted
by
the
software
as
an
activation request,
and
the
generator
is
activated accordingly.
Temperature
Sense
Circuits
The
Force FX-C
generator
features
two
temperature
sense circuits.
The first
temperature
sense
circuit
measures
the air
temperature
adjacent
to
the
RF
output
FET
heat
sink.
When
the
temperature
at
this location
reaches approximately
40° C, a cooling fan switches
on
to
minimize
heat
buildup.
The
fan
switches off
when
the
temperature
drops
below
approximately 35° C.
The
second
temperature
sense circuit
measures
the
air
temperature
near
the
control
board,
which
is
located
in
the
forward
third
of
the
generator
enclosure.
When
this
temperature
reaches
approximately
50° C,
the
main
microcontroller flashes
number
451 alternately
with
the
power
settings to
indicate
that
the
generator
is
too
hot.
Generator
operation
reverts to
normal
when
the
temperature
decreases to
below
40° C.
Force
FXTM-C
Service
Manual
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