Cessna 172N - Page 131

CESSNA
MODEL
172N
SECTION 7
AIRPLANE
&
SYSTEMS
DESCRIPTIONS
except when
either an external
power
source
is
connected
or
the
starter
switch
is turned
on;
then
a
power
contactor
is
automatically
actirated to
open
the
circuit
to the electronic bus.
Isolating
the electronic
circuits
in
this
marmer
prevents
harmful
transient
voltages from damaging
the
tran-
sistors
in the electronic
equipment.
IU
A
STER SW
IICH
The master
switch
is a
split-rocker
type
switch
labeled
IVIASTER, and
is
ON in
the
up
position
and
OFF in the
down
position.
The right
haU of
the
switch,
labeled
BAT, controls
all
electrical
power
to the
airplane.
The
left half,
Iabeled
ALT,
controls
the
alternator.
Normally,
both
sides
of the master
switch should
be used
simulta-
neously;
however,
the BAT side
of the
switch
cotrld be turned
ON separate-
ly to
check equipment
while
on the
ground.
The
ALT
side of the
switch,
when
placed
in
the OFF
position,
removes
the
alternator
from
the electri-
cal system.
With this switch
in the
OFF
position,
the entire
electrical
load is
placed
on the battery. Continued operation
with
the alternator
switch in the
OFF
position
will
reduce
battery
power
low enough to
open
the
battery contactor, remove
power
from the
alternator
field, and
pre-
vent
alternator
restart.
A
'N
'N
ETER
The
ammeter
indicates
the
flow
of
current, in
amperes,
from the al-
ternator
to
the
battery or from the
battery
to the airplane
electrical
sys-
tem.
When the
engine is operating and
the
master switch
is turned on,
the ammeter
indicates
the charging
rate applied
to the battery.
In the
event the alternator
is not
functioning
or the
electrical
load
exceeds the
output of
the alternator,
the ammeter
indicates the battery
discharge rate.
OVER.VOTTAGE
SENSOR
AND
WARNING LIGHT
The airplane
is
equipped
with
an
automatic
over-voltage
protection
system consisting
of an over-voltage
sensor
behind the
instrument
panel
and a red
warning light, labeled HIGH
VOLTAGE,
adjacent
to the ammeter.
In the event an over-voltage
condition
occurs, the over-voltage
sen-
sor
automatically removes
alternator
field
current and
shuts down the al-
ternator. The red
warning
light
will then turn on,
indicating to the
pilot
that the alternator
is
not
operating
and
the battery is
supplying
all
electri-
cal
power.
The over-voltage
sensor
may
be reset
by
turning the
master
switch
off
and
back on
again.
If the warning
light does
not illuminate,
normal
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