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Valleylab Force FX-C - Page 77

Valleylab Force FX-C
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Force
FXTIILG
Service
Manual
Power
SupplylRF
Board
Thermal
Sensing
(Fan
Control)
A reference
voltage
is
applied
to
the
noninverting
input
of
comparator
U2A. Resistors R15
and
R16
determine
the
reference
voltage.
\Vhen
the
temperature
of
NTC
thermistor
R19 is 40° C
the
reference
voltage
is
set
to
be
equal
to
the
voltage
at
the
inverting
input
of
U2A. Resistor RIO
provides
positive
feedback
causing
approximately
5° C
of
hysteresis.
At
temperatures
below
40° C,
the
output
of
U2A
is
low.
When
the
thermistor
exceeds
the
threshold,
the
voltage
comparator
changes
state
causing
the
output
at
U2A to
go
high,
turning
on
FET Q2.
This
applies
12
V
to
the
cooling fan.
When
the
temperature
drops
to 35° C
the
circuit
reverts
to
its
low
temperature
state,
and
the
cooling
fan
switches
off.
Thermal
Sensing
(High
Temperature
Limit)
A reference
voltage
is
applied
to
the
non-inverting
input
of
comparator
U26A. Resistors
RIll
and
RllO
determine
the
reference voltage.
When
the
temperature
of
mc
thermistor
RI09 is 50° C,
the
reference
voltage
is
set
to
be
equal
to
the
voltage
at
the
inverting
input
of
U26A. Resistor R94
provides
positive
feedback
causing
approximately
10° C
of
hysteresis.
\-Vhen
the
thermistor
exceeds
the
threshold,
the
voltage
comparator
changes
state,
causing
the
output
at
U26A
to
go
high.
The
main
rnicrocontroller
reads
this
signal
(TEMP_HI)
and
flashes
number
451
alternately
with
the
power
settings.
When
the
temperahtre
drops
to 40° C,
the
circuit
reverts
to
its
low
temperature
state,
and
Ule
generator
allows
activation
for
an
unlimited
duration.
4-27

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