Cessna 172 P Pilot Operating Handbook page 45

SECTION: | 1 | 2 | 3 | 4 | 5
source, the avionics power switch,
labeled AVIONICS POWER, should
be turned off to prevent any
harmful transient voltage from
d a m a g i n g t h e a v i o n i c s
equipment.
MASTER SWITCH
The master switch is a split
rocker-type switch labeled
MASTER, and is ON in the up
position. The right half of the
switch labeled BAT controls all
the electrical power to the
airplane. The left half, labeled
ALT controls the alternator.
Normally, both sides of the master
s w i t c h s h o u l d b e
simultaneously; however, the BAT
side of the switch could be turned
on seperately to check equipment
while on the ground. To check or
use avionics equipment or radios
while on the ground, the avionics
power switch must also be turned
on. The ALT side of the switch,
when placed in the OFF position,
removes the alternator from the
electrical system. While 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
prevent alternator restart.
AVIONICS POWER SWITCH
Electrical power from the airplane
primary bus to the avionics bus
i s c o n t r o l l e d b y a t o g g l e
switch/circuit braker labeled
AVIONICS POWER. The switch is
located on the left side of the
switch and control panel and is
ON in the up position. With the
switch in the OFF position, no
electrical power will be applied
to the avionics equipment,
regardless of the position of the
master switch or the individual
equipment switches. The avionics
power switch also functions as a
circuit breaker. If an electrical
malfunction should occur and
cause the circuit braker to open,
electrical power to the avionics
equipment will be interrupted
and the switch will automatically
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move to OFF position. If this
occurs, allow the circuit breaker
to cool for about two minutes
before placing the switch in the
ON position again. If the circuit
breaker opens again, do not reset
it. The avionics power switch
should be placed in the OFF
position prior to turning the
master switch ON or OFF, starting
the engine, or applying an
external power sourve, and may
be utilized in place of the
individual avionics equipment
switches.
AMMETER
u s e d The ammeter, located on the
lower left side of the instrument
panel, indicates the amount of
current in amperes, from the
alternator to the battery or from
the battery to the airplane
electrical system. 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 elctrical load
exceeds the output of the
alternator, the ammeter indicates
the battery discharge rate.
ALTERNATOR CONTROL UNIT
A N D L O W - V O L T A G E
WARNING LIGHT
The airplane is equipped with a
combination alternator regular
high-low voltage control unit
mounted on the engine side of
the firewall and a red warning
light , labeled LOW VOLTAGE, on
the left side of the instrument
panel below the ammeter.
In the event an over-voltage
condition occurs, the alternator
control unit automatically
removes alternator field current
which shuts down the alternator.
The battery will then supply
system current as shown by a
discharge rate on the ammeter.
U n d e r t h e s e
depending on electrical system
load, the low-voltage warning
light will illuminate when system
voltage drops below normal. The
alternator control unit may be
reset by turning the master switch
AIRPLANE & SYSTEMS DESCRIPTION
off and back on again. If the
warning light does not illuminate
again, a malfunction has occured,
a n d t h e f l i g h t s h o u l d b e
terminated as soon as practical.
CIRCUIT BREAKERS AND
FUSES
Most of the elecrical circuits in
the airplane are protected by
„push-to-reset" type circuit
breakers mounted on the left side
of the switch and control panel.
H o w e v e r , c i r c u i t b r e a k e r s
protecting the alternator output
and the strobe light/avionics
cooling fan circuits are the „pull-
off" type. In addition to the
individual circuit breakers, a
toggle switch/circuit breaker,
labeled AVIONICS POWER, on the
left side of the switch and control
panel, also protects the avionics
system. The control wheel map
light (if installed) is protected by
the NAV LT circuit breaker and
a fuse behind the instrument
panel. Electrical circuits which
are not protected by circuit
breakers are the battery contactor
closing (external power) circuit,
clock circuit, and flight hour
recorder circuit. These circuits
are protected by fuses mounted
adjacent to the battery.
c o n d i t i o n s ,
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