Cessna 172N Operating Handbook page 131

C ESSNA
MODEL 172N
S EC TION 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 IIC H
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.VO TTA G E S E N SOR AN D W A RNING L IGH T
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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