SECTION 7
CESSNA
AIRPLANE & SYSTEMS DESCRIPTIONS
MODEL 172RG
Figure 7-8. Electrical System
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1 July 1979
CESSNA
SECTION 7
MODEL 172RG
AIRPLANE & SYSTEMS DESCRIPTIONS
switch, labeled BAT, controls all electrical power to the airplane. The left
half, labeled ALT, controls the alternator.
Normally, both sides of the master switch should be used simultane-
ously; however, the BAT side of the switch could be turned ON separately
to check equipment while on the ground. To check or use avionics
equipment or radios while on the ground, the avionics power switch must
be turned ON. The ALT side of the switch, when placed in the off position,
removes the alternator from the electrical 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 prevent alternator restart.
AVIONICS POWER SWITCH
Electrical power from the airplane primary bus to the avionics bus
(see figure 7-8) is controlled by a single-rocker switch/circuit breaker
labeled AVN PWR. The switch is located on the left sidewall avionics
circuit breaker panel and is ON in the up position and OFF in the down
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 breaker to open, electrical power to the avionics
equipment will be interrupted and the switch will automatically move to
the OFF position. If this occurs, allow the circuit breaker to cool approxi-
mately 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 source, and may
be utilized in place of the individual avionics equipment switches.
AMMETER
The ammeter, located beneath the fuel gages, 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
electrical load exceeds the output of the alternator, the ammeter indicates
the battery discharge rate.
ALTERNATOR CONTROL UNIT AND
LOW-VOLTAGE WARNING LIGHT
The airplane is equipped with a combination alternator regulator
1 July 1979
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