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can 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 also be turned on. Positioning the ALT
switch to the off position isolates the alternator from the electrical
system and the entire electrical load is placed on the battery.
• Note •
Continued operation with the alternator switched off will
reduce battery power low enough to open the battery relay,
remove power from the alternator field, and prevent alternator
restart.
March 2010
7-52 Information Manual
Section 7 Cirrus Design
Airplane Description SR20
Avionics Power Switch
A rocker switch, labeled AVIONICS POWER, controls electrical power
from the airplane primary bus to the avionics bus. The switch is
located next to the ALT and BAT Master switches and is ON in the up
position and off in the down position. Typically, the switch is used to
energize or de-energize all avionics on the Avionics Non-Essential and
Avionics Essential buses simultaneously. 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. For normal operations, 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.
Volt / Amp Meter
A 2¼" combination Volts and Ampere meter is mounted on the right
instrument panel immediately outboard of the oil temperature and
pressure gage. The indicator is internally lighted. 28 VDC for
instrument lighting is supplied through the 2-amp Instrument Lights
circuit breaker on Main Bus 1.
The VOLT pointer sweeps a scale from 16 to 32 volts. Refer to Section
2 (Limitations) for instrument limit markings. The voltage indication is
measured off the essential bus.
The AMP pointer sweeps a scale from -60 to +60 amps with zero at
the 9 o'clock position. The amps indication is derived from a current
shunt located in the electrical system master control unit (MCU). 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.
March 2010
Information Manual 7-53
Cirrus Design Section 7
SR20 Airplane Description
Low Volts Warning Light
The airplane is equipped with a red LOW VOLTS warning light in the
annunciator panel located on the left side of the instrument panel. The
alternator control unit (ACU) located within the master control unit
(MCU), which is mounted on the engine side of the firewall, operates
the warning light.
In the event an over voltage condition occurs, the alternator control
unit (ACU) automatically removes alternator field current to shut down
the alternator. With the alternator off line, the battery supplies system
current and a discharge rate is indicated on the ammeter. Under these
conditions, depending on electrical system load, the LOW VOLTS
warning light will illuminate when system voltage drops below 25.5 ±
0.35 volts. Turning the ALT MASTER switch off and back on again may
reset the alternator control unit. If the warning light does not illuminate,
normal alternator charging has resumed. If the light illuminates again,
a malfunction has occurred.
• Note •
Illumination of the LOW VOLTS warning light and ammeter
discharge indications may occur during low RPM conditions
with an electrical load on the system, such as during a low
RPM taxi. Under these conditions, the light will go out at
higher RPM. The master switch need not be recycled since an
over voltage condition has not occurred to de activate the
alternator system.
Warning light operation can be tested by turning the landing light on
and momentarily turning off the master switch ALT portion while
leaving the BAT portion ON.
March 2010
7-54 Information Manual
Section 7 Cirrus Design
Airplane Description SR20
Circuit Breakers and Fuses
Individual electrical circuits connected to the Main, Essential, and
Non-Essential buses in the airplane are protected by re-settable circuit
breakers mounted on the left side of the center console. The airplane
Essential bus is supplied from the Main Buses through the 20-amp
ESSENTIAL 1 and ESSENTIAL 2 circuit breakers. Avionics loads on
the Non-Essential Avionics Bus and Essential Avionics Bus are
　
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