CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
24-30
Page 1
All
EFFECTIVITY:
DC GENERATION
1. DESCRIPTION
This section covers the systems to generate, regulate, control, and indicate DC electrical power. The DC
generation portion of the system includes the batteries, alternators (with internal rectifiers), and regulator.
A. DC Generation - Serials 1005 thru 1267 w/ Single Alternator (See Figure 24-301)
Primary power for the airplane is supplied by a 28-VDC negative-ground electrical system. The electri-
cal power generation system consists of a 24-volt, 10-amp-hour battery, a 75-ampere alternator, a volt-
age regulator and an over-voltage protection system. The battery is an aviation grade, 12-cell lead-
acid type with non-spill vent caps. The battery is used for engine starting and as an emergency power
source in the event of alternator failure. The 75-ampere alternator provides constant charging current
for the battery and primary power to the aircraft electrical system during normal system operation. The
voltage regulator provides transient suppression and constant voltage regulation of the unfiltered alter-
nator power. To protect sensitive instruments, the over-voltage protection system monitors the primary
power bus and automatically limits the peak voltage to 28.5 volts.
B. DC Generation - Serials 1005 thru 1267 w/ Dual Alternators (See Figure 24-302)
Primary power for the airplane is supplied by a 28-VDC negative-ground electrical system. The electri-
cal power generation system consists of a 24-volt, 10-amp-hour battery, two alternators, and a master
Control Unit (MCU). The MCU contains an Alternator Control Unit (ACU) for each alternator, contac-
tors for starter, battery, and ground power, a landing light relay, circuit protection for the circuit breaker
panel buses, and modules for other protection and annunciation functions. The battery is an aviation
grade, 12-cell lead-acid type with non-spill vent caps. The battery is used for engine starting and as an
emergency power source in the event of alternator failure.
Two rectified alternators provide constant charging current for the battery and primary power to the air-
craft electrical system during normal system operation. The forward, belt-driven alternator is desig-
nated ALT 1. The aft, engine-driven alternator is designated ALT 2. Although each alternator produces
the same amount of power at a given rotational speed, ALT 1 rotates faster and is rated at 75 amperes
while ALT 2 is rated at 40 amperes. Paralleling circuits in the function modules balance alternator out-
put so that, under normal operating conditions, ALT 1 provides 60% of the electrical power and ALT 2
provides the remaining 40%.
Each alternator's ACU provides transient suppression and constant voltage regulation of the alternator
output. To protect sensitive instruments, over-voltage circuits monitor each alternator's output and
automatically limit peak voltage to 28.5 volts. In the event an over-voltage or an overload condition
occurs, the associated ACU automatically opens the affected alternator's circuit breaker. With the alter-
nator off line, the associated ALT FAIL light illuminates, and the other alternator will provide 100% of
the electrical power requirements.
Should both alternators fail, the battery will supply system current and a discharge rate will be indi-
cated on the ammeter. Under these conditions, depending on electrical system load, the LOW VOLTS
warning light will illuminate when system voltage drops below approximately 24.5 volts.
C. DC Generation - Serials 1268 thru 2015, 2016 & subs w/o Perspective Avionics (See Figure 24-
303)
Primary power for the airplane is supplied by a 28-VDC, negative-ground electrical system. The electri-
cal power generation system consists of two alternators controlled by a Master Control Unit (MCU)
mounted on the left side of the firewall and two batteries for starting and electrical power storage.
Alternator 1 (ALT 1) is a belt-driven, internally rectified, 75-amp alternator mounted on the RH front of
the engine. Alternator 2 (ALT 2) is a gear-driven, internally rectified, 20-amp alternator mounted on the
accessory drive at the rear of the engine. ALT 1 is regulated to 28 volts and ALT 2 is regulated to 28.75
volts. The output from ALT 1 is connected to the main distribution bus in the MCU through an 80-amp
fuse. The output from ALT 2 is connected to the essential distribution bus in the MCU through a 40-
15 Jun 2010
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EFFECTIVITY:
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
15 Dec 2014
amp fuse. Both alternators are self-exciting (not self-starting) and require battery voltage for field exci-
tation in order to start up - for this reason, the batteries should not be turned off in flight.
Battery 1 (BAT 1) is an aviation grade 12-cell, lead-acid, 24-volt, 10-amp-hour battery mounted on the
right firewall. BAT 1 is charged from the main distribution bus in the MCU. Battery 2 (BAT 2) is com-
posed of two 12-volt, 7-amp-hour, sealed, lead-acid batteries connected in series to provide 24 volts.
Both BAT 2 units are located in a vented, acid-resistant container mounted behind the aft cabin bulk-
head (FS 222) below the parachute canister. BAT 2 is charged from the circuit breaker panel Essential
bus.
The Master Control Unit (MCU) is located on the left firewall. The MCU controls ALT 1, ALT 2, starter,
landing light, external power, and power generation functions. In addition to ALT 1 and ALT 2 voltage
regulation, the MCU also provides external power reverse polarity protection, alternator overvoltage
protection, as well as alternator fail and over-current annunciations. Power is distributed to the airplane
circuit panel buses through main and essential buses in the MCU. During normal operation, the alter-
nators feed their respective distribution bus independently (ALT 1 feeds main distribution bus and ALT
2 feeds the essential distribution bus). The distribution buses are interconnected by two 50-amp fuses
and a diode. The diode prevents ALT 2 from feeding the main distribution bus. Additionally, since ALT2
/ essential distribution bus voltage is slightly higher than ALT 1 / main distribution bus voltage, ALT 1
will not feed the essential distribution bus unless ALT 2 fails.
D. DC Generation - Serials 2016 & subs w/ Perspective Avionics (See Figure 24-306)
During normal operation, the alternators feed their respective distribution bus independently (ALT 1
feeds Main Distribution Bus 1 and ALT 2 feeds Main Distribution Bus 2). The main distribution buses
are interconnected by an 80-amp (Serials 2016 thru 2240) or 60-amp (Serials 2241 & subs) fuse and
diode. The diode prevents ALT 2 from feeding Main Distribution Bus 1. Additionally, since ALT 2 voltage
is slightly higher than ALT 1 voltage, bus voltage is further assured.
The alternators are three-phase AC generator type; each alternator has an internal rectifier. Each
alternator uses two diodes for each phase to rectify the output. In addition, these diodes will block
reverse current. If a single output diode were to fail, the winding junction will be held to the output
potential, reducing the quantity of current the alternator can produce.
The alternators are regulated by the MCU and power the MCU via 6 AWG tin plated copper wires.
Alternator 1 is protected by an 100-amp fuse within the MCU while alternator 2 is protected by a 80-
amp fuse within the MCU. ALT 1 is connected to the Main Distribution Bus 1 and ALT 2 is connected to
the Main Distribution Bus 2. Each alternator system has its own 5-amp alternator circuit breaker
located in the circuit breaker panel. Each alternator is individually protected against overvoltage gener-
ation by the field control module, located within the MCU. ALT 1 is regulated to 28 volts and ALT 2 is
regulated to 28.75 volts.
Voltage output of each alternator is a function of engine RPM, alternator design, and load on the alter-
nator. During low RPM operation, the alternator will require higher engine RPM to provide the same
voltage to increased electrical loads.
The drop off RPM for each alternator will change whenever the electrical system loads are altered from
nominal. Any time an alternator drops off line, a Crew Alerting System (CAS) message will appear on
the MFD. If an alternator drops off line due to low engine RPM, the alternator can be restarted by sim-
ply increasing engine RPM. When alternator 2 drops off line, alternator 1 will power the Essential Bus
through the Main Distribution Bus 1 in the MCU. When alternator 1 drops off line, alternator 2 will
power the Essential Bus through the Main Distribution Bus 2 in the MCU. For additional information
concerning power distribution, see Electrical Load Distribution. (Refer to 24-50)
E. DC Generation - Serials 1337 & subs w/ Single Alternator SRV (See Figure 24-304)
Primary power for the airplane is supplied by a 28-VDC, negative-ground electrical system. The electri-
cal power generation system consists of an alternator controlled by a Master Control Unit (MCU)
mounted on the left side of the firewall and two batteries for starting and electrical power storage.
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
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Page 3
Serials 1337 & subs w/ Dual Alternator SRV
EFFECTIVITY:
Alternator 1 (ALT 1) is a belt-driven, internally rectified, 75-amp alternator regulated to 28 volts, and
mounted on the RH front of the engine. Alternator power is routed into the MCU, where it is distributed
to the Main Distribution Bus and the Essential Distribution Bus. The alternator is regulated by a voltage
regulator within the MCU, and can be disconnected from the distribution bus system by the ALT 1
switch located on the pilot's bolster switch panel. The alternator is self-exciting (not self-starting) and
requires battery voltage for field excitation in order to start up - for this reason, the batteries should not
be turned off in flight.
Battery 1 (BAT 1) is an aviation grade 12-cell, lead-acid, 24-volt, 10-amp-hour battery mounted on the
right firewall. BAT 1 is charged from the main distribution bus in the MCU. Battery 2 (BAT 2) is com-
posed of two 12-volt, 7-amp-hour, sealed, lead-acid batteries connected in series to provide 24 volts.
Both BAT 2 units are located in a vented, acid-resistant container mounted behind the aft cabin bulk-
head (FS 222) below the parachute canister. BAT 2 is charged from the circuit breaker panel Essential
bus.
The Master Control Unit (MCU) is located on the left firewall. The MCU controls ALT 1, starter, landing
light, external power, and power generation functions. In addition to ALT 1 voltage regulation, the MCU
also provides external power reverse polarity protection, alternator overvoltage protection, as well as
alternator fail and over-current annunciations. Alternator power is distributed to the airplane circuit
panel buses through main and essential buses in the MCU. The distribution buses are interconnected
by two 50-amp fuses and a diode. The diode prevents BAT 2 from feeding the main distribution bus.
F. DC Generation - Serials 1337 & subs w/ Dual Alternator SRV (See Figure 24-305)
Primary power for the airplane is supplied by a 28-VDC, negative-ground electrical system. The electri-
cal power generation system consists of two alternators controlled by a Master Control Unit (MCU)
mounted on the left side of the firewall and two batteries for starting and electrical power storage.
Alternator 1 (ALT 1) is a belt-driven, internally rectified, 75-amp alternator mounted on the RH front of
the engine. Alternator 2 (ALT 2) is a gear-driven, internally rectified, 20-amp alternator mounted on the
accessory drive at the rear of the engine. ALT 1 is regulated to 28 volts and ALT 2 is regulated to 28.75
volts. The output from ALT 1 is connected to the main distribution bus in the MCU through an 80-amp
fuse. The output from ALT 2 is connected to the essential distribution bus in the MCU through a 40-
amp fuse. Both alternators are self-exciting (not self-starting) and require battery voltage for field exci-
tation in order to start up - for this reason, the batteries should not be turned off in flight.
Battery 1 (BAT 1) is an aviation grade 12-cell, lead-acid, 24-volt, 10-amp-hour battery mounted on the
right firewall. BAT 1 is charged from the main distribution bus in the MCU. Battery 2 (BAT 2) is com-
posed of two 12-volt, 7-amp-hour, sealed, lead-acid batteries connected in series to provide 24 volts.
Both BAT 2 units are located in a vented, acid-resistant container mounted behind the aft cabin bulk-
head (FS 222) below the parachute canister. BAT 2 is charged from the circuit breaker panel Essential
bus.
The Master Control Unit (MCU) is located on the left firewall. The MCU controls ALT 1, ALT 2, starter,
landing light, external power, and power generation functions. In addition to ALT 1 and ALT 2 voltage
regulation, the MCU also provides external power reverse polarity protection, alternator overvoltage
protection, as well as alternator fail and over-current annunciations. Power is distributed to the airplane
circuit panel buses through main and essential buses in the MCU. During normal operation, the alter-
nators feed their respective distribution bus independently (ALT 1 feeds main distribution bus and ALT
2 feeds the essential distribution bus). The distribution buses are interconnected by two 50-amp fuses
and a diode. The diode prevents ALT 2 from feeding the main distribution bus. Additionally, since ALT2
/ essential distribution bus voltage is slightly higher than ALT 1 / main distribution bus voltage, ALT 1
will not feed the essential distribution bus unless ALT 2 fails.
15 Jun 2010
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Page 4
Serials 1005 thru 1267
EFFECTIVITY:
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
15 Jun 2010
Figure 24-301
Single Alternator Electrical System Schematic - Serials 1005 thru 1267 (Sheet 1 of 2)
SR20_MM24_1316B
Earlier serials have fuses in place of circuit breakers.
ALTERNATOR
ANNUN. POWER
NOTE
AVIONICS
25A
STANDBY VACUUM
15A
MASTER CONTROL UNIT
(MCU)
INST. LIGHTS
PITCH TRIM
ROLL TRIM
ESSENTIAL I
AVIONICS
CIRCUIT BREAKER PANEL
COM I
GPS 1
ENCODER/TRANS
AUDIO PANEL
HSI
TURN COORD.
ENGINE INST.
ESSENTIAL
AVIONICS
ESSENTIAL
MAIN BUS 1
NON-ESSENTIAL
AVIONICS
NON-ESSENTIAL
MAIN BUS 2
25A25A
LANDING LIGHT
STANDBY VACUUM
SEE VACUUM SYSTEM
ESSENTIAL 2
AVIONICS
COM 2
GPS 2
MFD
ADF
STORMSCOPE
DME
AUTO PILOT
FUEL PUMP
NAV LIGHTS
CABIN FLOOD LIGHTS
STROBE LIGHTS
AUXILIARY PWR. 28VDC
CONVENIENCE
POWER 12VDC
PITOT HT./
COOLING FAN
FLAPS
15A
ALT SWITCH
LOW VOLTS
STARTER RELAY
ALT RELAY
(SEE SHEET 2)
MAIN BUS 2
NON-ESSENTIAL
MAIN BUS 1
Serials 1005 thru 1267.
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
24-30
Page 5
Serials 1005 thru 1267
EFFECTIVITY:
Figure 24-301
Single Alternator Electrical System Schematic - Serials 1005 thru 1267 (Sheet 2 of 2)
SR20_MM24_1313B
FLD
BAT
ALT 1
ACU
POWER
BUS
BAR
+ 28V
BATTERY SWITCH
MCU
STARTER
RELAY
IGNITION
SWITCH
STARTER
MOTOR
BATTERY
RELAY
GND
POWER
RELAY
CURRENT
SENSOR
FUNCTION
MODULE
ANNUNC.
EXTERNAL
POWER
RECEPTACLE
CIRCUIT BREAKER
PANEL
+
M
BATTERY
-
LOW VOLTS
VOLTS
AMPS
+
-
V
O
L
T
A
M
P
+
-
ALT SWITCH
+ 28V
25
25
25
15
15
ALT
RELAY
+ 28V
LANDING
LIGHT
LANDING
LIGHT
RELAY
LANDING
LIGHT
SWITCH
STANDBY
VACUUM
STUD (MCU)
(SEE SHEET 1)
Serials 1005 thru 1267.
15 Jun 2010
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Serials 1005 thru 1267
EFFECTIVITY:
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
15 Jun 2010
Figure 24-302
Dual Alternator Electrical System Schematic - Serials 1005 thru 1267 (Sheet 1 of 2)
Serials 1005 thru 1267.
SR20_MM24_1317B
ALTERNATOR 1
ANNUN. POWER
AVIONICS
25A
STANDBY VACUUM
15A
MASTER CONTROL UNIT
(MCU)
INST. LIGHTS
PITCH TRIM
ROLL TRIM
ESSENTIAL I
AVIONICS
CIRCUIT BREAKER PANEL
COM I
GPS 1
ENCODER/TRANS
AUDIO PANEL
HSI
TURN COORD.
ENGINE INST.
ESSENTIAL
AVIONICS
ESSENTIAL
MAIN BUS 1
NON-ESSENTIAL
AVIONICS
NON-ESSENTIAL
MAIN BUS 2
25A25A
LANDING LIGHT
STANDBY VACUUM
SEE VACUUM SYSTEM
ESSENTIAL 2
AVIONICS
COM 2
GPS 2
MFD
ADF
STORMSCOPE
DME
AUTO PILOT
FUEL PUMP
NAV LIGHTS
CABIN FLOOD LIGHTS
STROBE LIGHTS
AUXILIARY PWR. 28VDC
CONVENIENCE
POWER 12VDC
PITOT HT./
COOLING FAN
FLAPS
15A
ALT 2 SWITCH
STARTER RELAY
ALT 1 FAIL LED
ALT 2 FAIL LED
ALTERNATOR 2
ALT 2 RELAY
ALT 1 SWITCH
ALT 1 RELAY
(SEE SHEET 2)
LOW VOLTS
MAIN BUS 2
NON-ESSENTIAL
MAIN BUS 1
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
24-30
Page 7
Serials 1005 thru 1267
EFFECTIVITY:
Figure 24-302
Dual Alternator Electrical System Schematic - Serials 1005 thru 1267 (Sheet 2 of 2)
SR20_MM24_1314B
AMMETER
SELECTOR
SWITCH
BATTERY
-
LOW VOLTS
ACU
1
ALT 1 SWITCH
BATTERY SWITCH
MCU
STARTER
RELAY
IGNITION
SWITCH
STARTER
MOTOR
BATTERY
RELAY
GND
POWER
RELAY
ANNUNC.
EXTERNAL
POWER
RECEPTACLE
CIRCUIT BREAKER
PANEL
+
M
LANDING
LIGHT
2
ANNUNC
ALT 1 FAIL LED
ALT 2 FAIL LED
80A
LANDING
LIGHT
RELAY
LANDING
LIGHT
SWITCH
POWER
BUS
BAR
25
25
25
15
15
80A
FUNCTION
MODULE/
CURRENT
SENSOR
FUNCTION
MODULE/
CURRENT
SENSOR
STANDBY
VACUUM
STUD (MCU)
+
-
ALT 1 +
ALT 2 +
ALT -
ALT 1
RELAY
ALT 2
RELAY
ALT 2 SWITCH
+28 V
5
5
ALT 1
ALT 2
(SEE SHEET 1)
ACU
2
ALT 2
BAT
FLD
ALT 1
BAT
FLD
V
O
L
T
AMPS
VOLTS
-
+
A
M
P
+ 28V
Serials 1005 thru 1267.
15 Jun 2010
24-30
Page 8
Serials 1268 & subs w/o SRV & w/o Perspective Avion-
ics
EFFECTIVITY:
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
15 Jun 2010
Figure 24-303
Electrical System Schematic - Serials 1268 & subs w/o SRV & w/o Perspective Avionics
ATT
GYRO
2A
2A
STARTER
50A
50A
125A
EXTERNAL
POWER
A/C 2
FAN
COMPR/CTRL
SR20_MM24_1599E
BAT 1
SWITCH
LANDING
LIGHT
LANDING LIGHT
SWITCH
15A
30A
ALT 2
SWITCH
BAT 2 SWITCH
AVIONICS
ESSENTIAL RELAY
AVIONICS
NON-ESSENTIAL
RELAY
AVIONICS
SWITCH
BAT 1
ALT 1
RELAY
FUEL
OIL
ALT 1 ALT 2
PITOT
HEAT
ESSENTIAL
DIST
MAIN DIST
25A
25A
25A
25A
25A
VOLT REG
ALT 2
F
B
ALT 1
F
B
100A
40A
VOLT REG
GPS 2
ANN/ENG IND
MASTER CONTROL UNIT
CIRCUIT BREAKER PANEL
TRN CRD #1
AVIONICS
ESSENTIAL
BAT 2
ALT 1
SWITCH
LOW
VOLTS
ATTITUDE #1
PFD #1
STALL WARN
BATTERY 2
ALT 2
ESS POWER
ESSENTIAL
AUTOPILOT
AVIONICS
COM 1
GPS 1
FUEL PUMP
TRN CRD #2
MAIN BUS 2
ATTITUDE #2
PFD #2
CABIN LTS
ALT 1
START RELAY
NON-ESSENTIAL
AVIONICS NON-ESSENTIAL
MAIN BUS 1
PITOT/FAN
12V/ANR
STROBE LTS
NAV LTS
FLAPS
TRAFF/TAWS
COM 2
XPNDER
XM/WX
AUDIO PANEL
MFD
AVIONICS
FUEL QUANTITY
REC/INST LTS
PITCH TRIM
ROLL TRIM
ICE PROTECT
NOTE
Serials 1268 thru 1581: 80A fuse.
Serials 1268 thru 1581: 100A fuse.
Serials 1633, 1639 & subs w/ optional Blower Fan installed.
25A
2A
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
24-30
Page 9
Serials 1337 & subs w/ Single Alternator SRV
EFFECTIVITY:
Figure 24-304
Electrical System Schematic - Serials 1337 & subs w/ Single Alternator SRV
ANN/ENG IND
STARTER
50A
50A
125A
EXTERNAL
POWER
A/C 2
FAN
COMPR/CTRL
SR20_MM24_1733D
BAT 1
SWITCH
LANDING
LIGHT
LANDING LIGHT
SWITCH
15A
30A
BAT 2 SWITCH
AVIONICS
ESSENTIAL RELAY
AVIONICS
NON-ESSENTIAL
RELAY
AVIONICS
SWITCH
BAT 1
ALT 1
RELAY
FUEL
OIL
ALT 1
PITOT
HEAT
ESSENTIAL
DIST
MAIN DIST
25A
25A
25A
25A
25A
ALT 1
F
B
100A
VOLT REG
GPS 2
ATTITUDE #1
MASTER CONTROL UNIT
CIRCUIT BREAKER PANEL
TRN CRD #1
AVIONICS
ESSENTIAL
BAT 2
ALT 1
SWITCH
LOW
VOLTS
PFD #1
STALL WARN
BATTERY 2
ESS POWER
ESSENTIAL
AUTOPILOT
AVIONICS
COM 1
GPS 1
FUEL PUMP
TRN CRD #2
MAIN BUS 2
PFD #2
CABIN LTS
ALT 1
START RELAY
NON-ESSENTIAL
AVIONICS NON-ESSENTIAL
MAIN BUS 1
PITOT/FAN
12V/ANR
STROBE LTS
NAV LTS
FLAPS
TRAFF/TAWS
COM 2
XPNDER
XM/WX
AUDIO PANEL
MFD
AVIONICS
FUEL QUANTITY
REC/INST LTS
PITCH TRIM
ROLL TRIM
NOTE
Serials 1337 thru 1581: 80A fuse.
Serials 1337 thru 1581: 100A fuse.
Serials 1633, 1639 & subs w/ optional Blower Fan installed.
25A
ATTITUDE #2
ALT 2
2A
2A
15 Jun 2010
24-30
Page 10
Serials 1337 & subs w/ Dual Alternator SRV
EFFECTIVITY:
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
15 Jun 2010
Figure 24-305
Electrical System Schematic - Serials 1337 & subs w/ Dual Alternator SRV
2A
2A
STARTER
50A
50A
125A
EXTERNAL
POWER
A/C 2
FAN
COMPR/CTRL
SR20_MM24_1734D
BAT 1
SWITCH
LANDING
LIGHT
LANDING LIGHT
SWITCH
15A
30A
ALT 2
SWITCH
BAT 2 SWITCH
AVIONICS
ESSENTIAL RELAY
AVIONICS
NON-ESSENTIAL
RELAY
AVIONICS
SWITCH
BAT 1
ALT 1
RELAY
FUEL
OIL
ALT 1 ALT 2
PITOT
HEAT
ESSENTIAL
DIST
MAIN DIST
25A
25A
25A
25A
25A
VOLT REG
ALT 2
F
B
ALT 1
F
B
100A
40A
VOLT REG
GPS 2
ANN/ENG IND
MASTER CONTROL UNIT
CIRCUIT BREAKER PANEL
TRN CRD #1
AVIONICS
ESSENTIAL
BAT 2
ALT 1
SWITCH
LOW
VOLTS
ATTITUDE #1
PFD #1
STALL WARN
BATTERY 2
ALT 2
ESS POWER
ESSENTIAL
AUTOPILOT
AVIONICS
COM 1
GPS 1
FUEL PUMP
TRN CRD #2
MAIN BUS 2
ATTITUDE #2
PFD #2
CABIN LTS
ALT 1
START RELAY
NON-ESSENTIAL
AVIONICS NON-ESSENTIAL
MAIN BUS 1
PITOT/FAN
12V/ANR
STROBE LTS
NAV LTS
FLAPS
TRAFF/TAWS
COM 2
XPNDER
XM/WX
AUDIO PANEL
MFD
AVIONICS
FUEL QUANTITY
REC/INST LTS
PITCH TRIM
ROLL TRIM
NOTE
Serials 1337 thru 1581: 80A fuse.
Serials 1337 thru 1581: 100A fuse.
Serials 1633, 1639 & subs w/ optional Blower Fan installed.
25A
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
24-30
Page 11
Serials 2016 thru 2064 w/ Perspective Avionics
EFFECTIVITY:
Figure 24-306
Electrical System Schematic - Serials 2016 thru 2064 w/ Perspective Avionics (Sheet 1 of 3)
EVS
CAMERA
30A
BAT 2
SWITCH
BAT 2
AVIONICS
SWITCH
20A
8A
8A
20A
VOLT REG
BAT 1
SWITCH
EXTERNAL
POWER
MAIN DIST BUS 2
50A
50A
80A
BAT 1
RELAY
30A
30A
30A
STARTER
SWITCH
AVIONICS
NON-ESSENTIAL
RELAY
30A
30A
STARTER
125A
EXTERNAL
POWER RELAY
STARTER
RELAY
LANDING
LIGHT
LANDING LIGHT
SWITCH
7.5A
BAT 1
ALT 1
RELAY
MAIN DIST BUS 1
30A
ALT 1
F
B
100A
VOLT REG
MASTER CONTROL UNIT
ALT 1
SWITCH
DME / ADF
AVIONICS BUS
AUDIO
PANEL
WEATHER
TRAFFIC
XPONDER
STDBY
ATTD #2
MFD #2
MAIN BUS 1
CABIN LIGHTS
FUEL QTY
KEYPADS /
AP CTRL
AP SERVOS
AVIONICS
ESSENTIAL BUS 2
ENGINE
INSTR
STALL
WARNING
PITCH
TRIM
ROLL
TRIM
GPS NAV
GIA 1
COM 1
ESSENTIAL BUS 1
ADC 1
AHRS 1
PFD #1
STDBY
ATTD #1
ESSENTIAL
POWER
BAT 2
A/C BUS 1
ALT 1
MFD #1
MAIN BUS 3
12V DC
OUTLET
GPS NAV
GIA 2
COM 2
MAIN BUS 2
AHRS 2/
ADC 2
PFD #2
FUEL
PUMP
AVIONICS
FAN 2
ALT 2
A/C BUS 2
CABIN
FAN
CABIN AIR
CONTROL
NAV
LIGHTS
STROBE
LIGHTS
NON-ESSENTIAL BUS
PITOT
HEAT
FLAPS
STARTER
AVIONICS
FAN 1
RECOG
LIGHTS
ALT 2
SWITCH
CIRCUIT BREAKER PANEL
ESSENTIAL DIST BUS
30A
30A
ALT 2
F
B
80A
50A
SR20_MM24_2900A
Serials 2016 thru 2064 w/ Perspective Avionics.
15 Dec 2014
24-30
Page 12
Serials 2065 thru 2240 w/ Perspective Avionics
EFFECTIVITY:
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
15 Dec 2014
Figure 24-306
Electrical System Schematic - Serials 2065 thru 2240 w/ Perspective Avionics (Sheet 2 of 3)
CABIN AIR
CONTROL
EVS
CAMERA
30A
BAT 2
SWITCH
BAT 2
AVIONICS
SWITCH
20A
8A
8A
20A
VOLT REG
BAT 1
SWITCH
EXTERNAL
POWER
MAIN DIST BUS 2
50A
50A
80A
BAT 1
RELAY
30A
30A
30A
STARTER
SWITCH
AVIONICS
NON-ESSENTIAL
RELAY
30A
30A
STARTER
125A
EXTERNAL
POWER RELAY
STARTER
RELAY
LANDING
LIGHT
LANDING LIGHT
SWITCH
7.5A
BAT 1
ALT 1
RELAY
MAIN DIST BUS 1
30A
ALT 1
F
B
100A
VOLT REG
MASTER CONTROL UNIT
ALT 1
SWITCH
DME / ADF
AVIONICS BUS
AUDIO
PANEL
WEATHER
/DATA LINK
TRAFFIC
XPONDER
STDBY
ATTD #2
MFD #2
MAIN BUS 1
CABIN LIGHTS
/ OXYGEN
FUEL QTY
KEYPADS /
AP CTRL
AP SERVOS
AVIONICS
ESSENTIAL BUS 2
ENGINE
INSTR
STALL
WARNING
PITCH
TRIM
ROLL
TRIM
GPS NAV
GIA 1
COM 1
ESSENTIAL BUS 1
ADC 1
AHRS 1
PFD #1
STDBY
ATTD #1
ESSENTIAL
POWER
BAT 2
A/C BUS 1
A/C COND
ALT 1
MFD #1
MAIN BUS 3
12V DC
OUTLET
GPS NAV
GIA 2
COM 2
MAIN BUS 2
AHRS 2/
ADC 2
PFD #2
FUEL
PUMP
AVIONICS
FAN 2
ALT 2
A/C BUS 2
CABIN
FAN
A/C COMPR
NAV
LIGHTS
STROBE
LIGHTS
NON-ESSENTIAL BUS
PITOT
HEAT
FLAPS
STARTER
AVIONICS
FAN 1
RECOG
LIGHTS
ALT 2
SWITCH
CIRCUIT BREAKER PANEL
ESSENTIAL DIST BUS
30A
30A
ALT 2
F
B
80A
50A
SR20_MM24_3359B
Serials 2065 thru 2240 w/ Perspective Avionics.
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EFFECTIVITY:
Figure 24-306
Electrical System Schematic - Serials 2241 & subs w/ Perspective Avionics (Sheet 3 of 3)
VOLT REG
BAT 1
RELAY
EXTERNAL
POWER RELAY
RECOG
LIGHTS
5A
CONS
CONV
LIGHTS
CABIN AIR
CONTROL
30A
BAT 2
SWITCH
BAT 2
AVIONICS
SWITCH
20A
8A
8A
20A
VOLT REG
BAT 1
SWITCH
EXTERNAL
POWER
MAIN DIST BUS 2
50A
50A
60A
30A
30A
30A
STARTER
SWITCH
AVIONICS
NON-ESSENTIAL RELAY
30A
30A
STARTER
125A
STARTER
RELAY
LANDING
LIGHT
LANDING LIGHT
SWITCH
7.5A
BAT 1
ALT 1
RELAY
MAIN DIST BUS 1
30A
ALT 1
F
B
100A
MASTER CONTROL UNIT
ALT 1
SWITCH
DME / ADF
AVIONICS BUS
AUDIO
PANEL
DATA LINK/
WEATHER
TRAFFIC
XPONDER
STDBY
ATTD #2
MFD #2
MAIN BUS 1
CABIN
LIGHTS
FUEL QTY
KEYPADS
/ AP CTRL
AP SERVOS
AVIONICS
ICE
LIGHTS
ESSENTIAL BUS 2
ENGINE
INSTR
STALL
WARNING
PITCH
TRIM
ROLL
TRIM
GPS NAV
GIA 1
COM 1
ESSENTIAL BUS 1
ADC 1
AHRS 1
PFD #1
STDBY
ATTD #1
ESSENTIAL
POWER
BAT 2
A/C BUS 1
A/C COND
ALT 1
MFD #1
MAIN BUS 3
12V DC
OUTLET
GPS NAV
GIA 2
COM 2
MAIN BUS 2
AHRS 2/
ADC 2
PFD #2
FUEL
PUMP
AVIONICS
FAN 2
ALT 2
A/C BUS 2
CABIN
FAN
A/C COMPR
NAV
LIGHTS
STROBE
LIGHTS
NON-ESSENTIAL BUS
PITOT
HEAT
FLAPS
STARTER
AVIONICS
FAN 1
ALT 2
SWITCH
CIRCUIT BREAKER PANEL
ESSENTIAL DIST BUS
30A
30A
ALT 2
F
B
60A
50A
EVS
CAMERA
UAT
1
1
NOTE
Serials 2303 & subs.
SR20_MM24_3636C
Serials 2241 & subs w/ Perspective Avionics.
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CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
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G. Alternator 1 (See Figure 24-307)
Alternator 1 is mounted directly to the front, LH side of the engine and is used in all electrical configu-
rations.
Alternator 1 is a 75-amp belt-driven alternator. Alternator 1 has sufficient capacity to power the entire
aircraft electrical load and charge the battery during normal system operation. The alternator and bat-
tery are designed to function in parallel or independently and can be alternately connected to the pri-
mary bus. Alternator 1 will continue to generate electrical power in the case of battery failure or
malfunction. Alternator 1 is self-exciting, not self-starting. The battery must energize Alternator 1
before it can produce electrical power.
H. Alternator 2 (See Figure 24-308)
Serials 1005 thru 1267: The optional dual alternator system adds a 40-amp engine-driven alternator 2
to the single alternator system. An alternator switch, ammeter switch, circuit breaker, function module,
alternator fail lights, and alternator relay is also added in this configuration. Alternator 2 is mounted
directly to the aft engine accessory pad.
On the optional dual alternator system, alternator 1 produces approximately 60% of the system voltage
and alternator 2 produces approximately 40%. If alternator 1 fails during heavy loading, it may be nec-
essary to shed electrical load to prevent over-burdening alternator 2. The alternator fail lights are
located to the immediate right of the annunciator panel. The lights provide warning of a failed or inop-
erative alternator. If an alternator is not providing adequate proportional power, the appropriate alter-
nator fail LED will illuminate. In addition, both lights will illuminate during a battery discharge situation
(approximately a 3-amp draw).
Serials 1268 & subs w/o single alternator SRV: Alternator 2 is a 20-amp engine-driven alternator
mounted directly to the aft engine accessory pad. Alternator 2 supplies electrical power to the essen-
tial bus through a 25-amp circuit breaker located in the MCU.
I. Master Control Unit (MCU) (See Figure 24-309)
The MCU is an integrated component which ensures proper electrical output over the entire tempera-
ture and RPM range of the alternator system by regulating and distributing alternator output. The MCU
is mounted to the firewall, directly behind the engine.
(1) Master Control Unit - Serials 1005 thru 1267
The MCU used in the single alternator system contains a landing light relay, ammeter trans-
ducer, alternator control unit (ACU), three 25-amp distribution feeder fuses, buses, and contac-
tors for the alternator, start, master battery, and ground power circuits.
The MCU used in the optional dual alternator system contains a landing light relay, 15-amp land-
ing light circuit breaker, 15-amp auxiliary power circuit breaker, two ACUs, two function modules,
three 25-amp distribution feeder circuit breakers, buses, and contactors for the start, master bat-
tery, and ground power circuits.
Note: The MCU on these airplanes is not field repairable and must be replaced if faulty.
The MCU will trip the alternator circuit breaker(s) whenever there is more than 6-10 amps
reverse current. The pilot has access to the alternator circuit breaker(s), which are located in the
circuit breaker panel, during flight. If the system is generating more than 31.75 ± 0.5 V, the corre-
sponding alternator circuit breaker will open, preventing further power generation by that alterna-
tor. The other alternator will regulate at a higher rate, 29.0 +0.25, -0.5 V during the overvoltage.
After the circuit breaker opens, the other alternator will provide 100% of the load at the nominal
voltage.
The MCU contains a current sensor in the battery bus, this current sensor sends a signal to the
ammeter (battery charge or discharge). The ammeter only indicates a discharge when the elec-
trical draw on the system exceeds the quantity of power generated by system. In the dual alter-
nator system, the ammeter will only indicate a discharge when the ammeter select switch is in
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the BATT position and the electrical draw on the system exceeds the quantity of power gener-
ated by system. Diodes in the alternator(s) block reverse current making negative current indica-
tion unnecessary. The current value displayed by the ammeter corresponds directly to the
quantity of power generated by the alternator system. When the ammeter select switch is in the
ALT 1 or ALT 2 position (dual alternator system only), the ammeter drives will only allow the
ammeter to display a positive current value because of the diodes within the alternator block.
The master switch arrangement, located in the pilot's bolster contains the battery, alternator 1,
and alternator 2 control switches. Each switch disconnects the associated device from the pri-
mary bus. The switches are located in a side-by-side arrangement and are labeled as to their
function. The BATT master switch, when closed, connects battery 1 to the primary bus. This
switch activates a contactor within the MCU, providing the ability to connect and disconnect bat-
tery power from the aircraft. The battery switch controls a contactor, which isolates battery 1,
while the alternator switch removes field excitation from the alternator thereby isolating the pri-
mary bus.
Six buses are powered from the MCU. These buses are designed to ensure essential flight and
avionics systems remain powered during a malfunction of any one of the buses. Three bus wires
run from the MCU, through the firewall, and to the circuit breaker panel. Three bus wires in the
MCU are overload protected by fuses and circuit breakers. Three buses are used to power the
majority of aircraft loads and supply power to the circuit breaker panel. The fourth bus is for the
landing light. The fifth is the clock bus which is fused at 5 amperes and is the only bus powered
directly from battery 1 and is not controlled by the master switch arrangement. The clock bus
fuse is externally removable for long term storage situations to prevent battery drain. The sixth
bus is for the standby vacuum pump.
Alternator power flows from the alternator into the MCU. Inside the MCU is where the ACU regu-
lates alternator output. The ACU (voltage regulator) provides transient suppression and constant
voltage regulation of unfiltered alternator power. To protect sensitive instruments, the over-volt-
age protection system monitors the primary power bus and automatically limits peak voltage to
approximately 31.75 volts. During sustained over-voltage and under-voltage periods, the over-
voltage system provides a warning to the pilot. The ACU will cause the alternator circuit breaker
to open in cases of reverse current, field output overloads, and overvoltage. In the event an over-
voltage condition occurs, the ACU automatically removes alternator field current to shut down
the alternator. The ACU has been integrated into the MCU case, for durability and reliability.
(2) Master Control Unit 100 and 120 - Serials 1268 thru 1581
The MCU controls alternator 1, alternator 2 (Serials w/ dual alternators), starter, landing light,
external power, and the power generation system annunciation. The MCU provides protection
for external power reverse polarity and alternator overvoltage situations. The MCU also provides
low voltage annunciation and an overload annunciation for ALT 1 and ALT 2 (Serials w/ dual
alternators) fail annunciators in the instrument panel.
The bus structure of the MCU and the circuit breaker panel allows all power devices to feed into
the Essential Bus during emergencies. This action is automatic and does not require pilot inter-
vention. The Main and Essential Buses are separated via diodes which allow the Main Bus to
feed into the Essential Bus during failures of Alternator 2 (Serials w/ dual alternators) or Battery
2. The MCU regulates Alternator 1 to 28 VDC, while Alternator 2 (Serials w/ dual alternators) is
regulated to 28.75 VDC which ensures the diode separates the buses during normal operation.
The MCU used on this airplane contains a landing light relay, battery relay, ground power relay,
starter relay, ammeter transducer, 15-amp landing light circuit breaker, two buses (one essential
and one non-essential), two field control modules, three current sensors, and five 25-amp distri-
bution feeder circuit breakers.
The pilot has access to the alternator circuit breaker(s), which are located in the circuit breaker
panel, during flight. If the system is generating more than 31.75 ± 0.25 V, the corresponding
alternator circuit breaker will open, preventing further power generation by that alternator.
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CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
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The MCU contains a current sensor in the battery bus, this current sensor sends a signal to the
ammeter (battery charge or discharge). The ammeter will only indicate a discharge when the
ammeter select switch is in the BATT position and the electrical draw on the system exceeds the
quantity of power generated by the system. Diodes in the alternators block reverse current mak-
ing negative current indication unnecessary. The current value displayed by the ammeter corre-
sponds directly to the quantity of power generated by the alternator system. When the ammeter
select switch is in the ALT 1 or ALT 2 (Serials w/ dual alternators) position, the ammeter drives
will only allow the ammeter to display a positive current value because of the diodes within the
alternator block.
The master switch arrangement, located in the pilot's bolster contains battery 1, battery 2, alter-
nator 1, and alternator 2 (Serials w/ dual alternators) control switches. Each switch disconnects
the associated device from the corresponding bus. The switches are located in a side-by-side
arrangement and are labeled as to their function. The BAT 2 switch, when closed, connects bat-
tery 2 to the Essential Bus. This switch activates a relay located next to BAT 2, providing the abil-
ity to connect and disconnect battery 2 from the aircraft.
Seven power distribution buses with associated protection devices are located in the MCU.
These buses are designed to ensure essential flight and avionics systems remain powered
during a malfunction of any one of the buses. Five bus wires run from the MCU, through the fire-
wall, and to the circuit breaker panel for overload protection by fuses or circuit breakers. The five
buses are used to power the majority of aircraft loads and supply power to the circuit breaker
panel. Three buses are used as distribution feeders from the Main Distribution Bus. Two buses
are used as distribution feeders from the Essential Distribution Bus. One bus is used to power
the landing light directly from the Main Distribution Bus through a 15 amp circuit breaker. One
bus is used to power the clock which is fused at 5 amperes and is the only bus powered directly
from battery 1 and is not controlled by master switch arrangement. The clock bus fuse is exter-
nally removable for long term storage situations to prevent battery drain. On PFD equipped air-
planes, the clock bus is not used.
Alternator power flows from alternator(s) into the MCU. Inside the MCU is where the field control
module regulates alternators output. Each field control module provides transient suppression
and constant voltage regulation of unfiltered alternator power. To protect sensitive instruments,
the over-voltage protection system monitors the primary power bus and automatically limits peak
voltage to approximately 31.75 volts. During sustained over-voltage periods, the over-voltage
system provides a warning to the pilot. Each field control module will cause the corresponding
alternator circuit breaker to open in cases of field output overloads, and overvoltage. In the event
an over-voltage condition occurs, the corresponding field control module automatically removes
alternator field current to shut down the corresponding alternator. Each field control module has
been integrated into the MCU case, for durability and reliability.
(3) Master Control Unit 130 and 140 - Serials 1582 & subs
The MCU 130 (Serials w/o Perspective) and 140 (Serials w/ Perspective) controls alternator 1,
alternator 2 (Serials w/ dual alternators), starter, landing light, and external power. The MCU
provides protection for external power reverse polarity and alternator overvoltage situations. The
bus structure of the MCU and the circuit breaker panel allows all power devices to feed into the
Essential Bus during emergencies. This action is automatic and does not require pilot interven-
tion.
MCU 130: The Main and Essential Buses are separated via diodes which allow the Main Bus to
feed into the Essential Bus during failures of Alternator 2 (Serials w/ dual alternators) or Battery
2. The MCU used on this airplane contains a landing light relay, battery relay, ground power
relay, starter relay, 15-amp landing light circuit breaker, two buses (one essential and one non-
essential), two field control modules, two (Serials w/ single alternator) or three (Serials w/ dual
alternators) current shunts, and seven 25-amp distribution feeder circuit breakers.
MCU 140: The Main Distribution Bus 1 and Main Distribution Bus 2 are separated via a diode
which allows the Main Bus 1 to feed into Main Bus 2 during failures of Alternator 2. The MCU
CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
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EFFECTIVITY:
used on this airplane contains a landing light relay, battery relay, ground power relay, starter
relay, 7.5-amp landing light fuse, three buses (one essential and two non-essential), two field
control modules, three current shunts, and eight 30-amp distribution feeder fuses.
The MCU regulates Alternator 1 to 28 VDC. while Alternator 2 (Serials w/ dual alternators) is
regulated to 28.75 VDC which ensures the diode separates the buses during normal operation.
The pilot has access to the alternator circuit breaker(s), which are located in the circuit breaker
panel, during flight. If the system is generating more than 31.75 ± 0.25 V, the corresponding
alternator circuit breaker will open, preventing further power generation by that alternator.
The MCU contains a current shunt in the battery bus. This current shunt sends a signal to the
data acquisition unit (Serials w/o Perspective) or the GEA 71 engine airframe unit (Serials w/
Perspective). Diodes in the alternators block reverse current making negative current indication
unnecessary. The current value displayed by the ammeter (Serials w/o Perspective) or MFD
(Serials w/ Perspective) corresponds directly to the quantity of power generated by the alternator
system.
The master switch arrangement, located in the pilot's bolster contains battery 1, battery 2, alter-
nator 1, and alternator 2 (Serials w/ dual alternators) control switches. Each switch disconnects
the associated device from the corresponding bus. The switches are located in a side-by-side
arrangement and are labeled as to their function. The BAT 2 switch, when closed, connects bat-
tery 2 to the Essential Bus. This switch activates a relay located next to BAT 2, providing the abil-
ity to connect and disconnect battery 2 from the aircraft.
Nine (Serials 2016 thru 2240) or ten (Serials 2241 & subs) power distribution buses with associ-
ated protection devices are located in the MCU. The buses are designed to ensure essential
flight and avionics systems remain powered during a malfunction of any one of the buses.
MCU 130: Seven bus wires run from the MCU, through the firewall, and to the circuit breaker
panel for overload protection by fuses or circuit breakers. The seven buses are used to power
the majority of aircraft loads and supply power to the circuit breaker panel. Five buses are used
as distribution feeders from the Main Distribution Bus. Two buses are used as distribution feed-
ers from the Essential Distribution Bus. One bus is used to power the landing light directly from
the Main Distribution Bus through a 15 amp circuit breaker. One bus was used to power the
clock which was fused at 5 amperes. On PFD equipped airplanes, the clock bus is not used.
MCU 140: Eight bus wires run from the MCU, through the firewall, and to the circuit breaker
panel for overload protection by fuses or circuit breakers. The eight buses are used to power the
majority of aircraft loads and supply power to the circuit breaker panel. Three buses are used as
distribution feeders from the Main Distribution Bus 1. Three buses are used as distribution feed-
ers from the Main Distribution Bus 2. Two buses are used as distribution feeders from the Essen-
tial Distribution Bus. One bus is used to power the landing light directly from the Main
Distribution Bus 1 through a 7.5 amp fuse.
Serials 22-2241 & subs: One bus is used to power the constant power devices including the
interface between the CAPS and the ELT system which is fused at 5 amperes and is not con-
trolled by master switch arrangement.
Alternator power flows from alternator(s) into the MCU. Inside the MCU is where the field control
module regulates alternator(s) output. Each field control module provides transient suppression
and constant voltage regulation of unfiltered alternator power. To protect sensitive instruments,
the over-voltage protection system monitors the primary power bus and automatically limits peak
voltage to approximately 31.75 volts. Each field control module will cause the corresponding
alternator circuit breaker to open in cases of field output overloads, and overvoltage. In the event
an over-voltage condition occurs, the corresponding field control module automatically removes
alternator field current to shut down the corresponding alternator. Each field control module has
been integrated into the MCU case, for durability and reliability.
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CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
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J. 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.
Serials 1005 thru 1581: If bus voltage drops to approximately 24.5 volts, the field control module in the
MCU will cause the LOW VOLTS warning light will illuminate red.
Serials 1582 & subs: If Essential Bus voltage drops to approximately 24.5 volts, the data acquisition
unit will cause the LOW VOLTS warning light to illuminate red.
In twin alternator configurations, resetting the ALT 1 and ALT 2 switches from off and back on again
may reset the field control module. If the warning light does not illuminate again, normal alternator
charging has resumed. If the light illuminates again, a malfunction has occurred. (Refer to 31-50)
Illumination of the LOW VOLTS warning light along with 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 will not need to be recycled
since an over-voltage condition has not occurred to de-activate the alternator system.
K. ALT 1 and ALT 2 Fail Lights
Serials 1005 thru 1267 w/ optional dual alternator system: In conjunction with the red LOW VOLTS
warning light, the MCU will illuminate the appropriate amber alternator fail light, notifying the pilot of the
system at fault. (Refer to 31-50)
Serials 1268 thru 1581 w/o single alternator SRV: Two amber colored alternator fail lights are located
in the annunciator panel. The lights provide warning of a overloaded or inoperative alternator. The ALT
1 and ALT 2 fail lights are operated by circuits in the MCU and current sensors on the ALT 1 and ALT 2
output lines. If either alternator generates less than 2 amps (approximately), the corresponding annun-
ciator light will illuminate steady.
Serials 1582 & subs w/o single alternator SRV: Two amber colored alternator fail lights are located in
the annunciator panel. The lights provide warning of a overloaded or inoperative alternator. The ALT 1
and ALT 2 fail lights are operated by the data acquisition unit. The DAU monitors a voltage drop across
the current shunts in the MCU on the ALT 1 and ALT 2 output lines. If either alternator generates less
than 2 amps (approximately), the corresponding annunciator light will illuminate steady.
Serials 1337 thru 1581 w/ single alternator SRV: In conjunction with the red LOW VOLTS warning light,
the MCU controls the illumination of the alternator fail light, notifying the pilot of the system at fault.
Serials 1582 & subs w/ single alternator SRV: In conjunction with the red LOW VOLTS warning light,
the data acquisition unit controls the illumination of the alternator fail light, notifying the pilot of the sys-
tem at fault.
L. Volt and Ampere Meter (See Figure 24-3011)
A combination Volt and Ampere meter is mounted on the right instrument panel immediately outboard
of the oil temperature and pressure gage.
Serials 1005 thru 1382: The AMP pointer sweeps a scale from -60 to +60 amps with zero at the 9
o'clock position.
Serials 1383 thru 1581: The AMP pointer sweeps a scale from -100 to +100 amps with zero at the 9
o'clock position.
Serials 1005 thru 1267: The ammeter will indicate the current generation provided by Alternator 1,
Alternator 2, and the charge or discharge status of battery 1.
The VOLT pointer sweeps a scale from 16 to 32 volts. The voltage indication is measured off the
essential bus. The voltage required to operate the internal meter lighting is supplied through the 2-amp
Instrument Lights circuit breaker on Main Bus 1.
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Serials 1268 thru 1581 w/o single alternator SRV: The ammeter will indicate the current generation
provided by Alternator 1, Alternator 2, and the charge or discharge status of battery 1.
The VOLT pointer sweeps a scale from 16 to 32 volts. The voltage indication for the Volt / Ampere
Meter is measured off the annunciator circuit breaker which is on the Essential Bus. The voltage
required to operate the internal meter lighting is supplied through the 2-amp instrument lights circuit
breaker which is on Main Bus 1. Main Bus voltage (measured at the engine instrument circuit breaker)
is displayed on the clock voltmeter for reference.
Serials 1337 thru 1581 w/ single alternator SRV: The ammeter will indicate the current generation pro-
vided by Alternator 1, and the charge or discharge status of battery 1.
The VOLT pointer sweeps a scale from 16 to 32 volts. The voltage indication for the Volt / Ampere
Meter is measured off the annunciator circuit breaker which is on the Essential Bus. The voltage
required to operate the internal meter lighting is supplied through the 2-amp instrument lights circuit
breaker which is on Main Bus 1. Main Bus voltage (measured at the engine instrument circuit breaker)
is displayed on the clock voltmeter for reference.
Serials 1582 & subs: Main and Essential Bus voltages are shown as text in the electrical data field
located in the mid-right section of the MFD and are also continuously displayed in the voltage parame-
ters field located in the upper left corner of the PFD. The MFD and PFD receive the voltage signals via
the DAU as measured directly off the Main and Essential Buses.
In the event Main Bus voltage is less than 24.5v or exceeds 32.0v the MFD will display “Check Main
Bus” in a yellow advisory box in the lower right corner of the MFD.
In the event Essential Bus voltage is less than 24.5v or exceeds 32.0v the MFD will display “Check
Essential Bus” in a red advisory box in the lower right corner of the MFD.
Alternator 1 and Alternator 2 ampere output are shown as text in the electrical data field located in the
mid-right section of the MFD. The MFD and PFD receive the amp signals via the DAU as derived from
current shunts located in the MCU.
In the event Alternator 1 or Alternator 2 ampere output is less than 2 amps for 20 seconds or more, the
MFD will display “Check ALT 1” or “Check ALT 2” respectively, in a yellow advisory box in the lower
right corner of the MFD.
28 VDC for the digital instrument operation is supplied through the 2-amp ANNUN / ENGINE INST cir-
cuit breaker on the Avionics Essential Bus.
M. Ammeter Select Switch (See Figure 24-3011)
Serials 1005 thru 1267 w/ optional dual alternator system: An ammeter select switch is located in the
pilot's primary view and is labeled distinctly. The ammeter select switch controls which output reading
the ammeter will display, Alternator 1, Alternator 2, or the state of charge or discharge battery 1 is in.
The amps indication is derived from a current transducer located in the electrical system MCU. When
the engine is operating and the master switch is turned on, the ammeter indicates the charging rate
applied to the battery. Due to the inability of the alternators to dissipate current, the alternator ammeter
indications are positive only.
Serials 1268 thru 1581 w/o single alternator SRV: An ammeter select switch is located on the right
instrument panel and is labeled distinctly. The ammeter select switch controls which output reading the
ammeter will display the voltage from, ALT 1, ALT 2 or BAT 1. The amps indication is derived from a
current transducer located in the electrical system MCU. When the engine is operating and the BAT 1
switch is turned on, the ammeter indicates the charging rate applied to battery 1. Due to the inability of
the alternators to dissipate current, the alternator ammeter indications are positive only.
Serials 1337 thru 1581 w/ single alternator SRV: An ammeter select switch is located on the right
instrument panel and is labeled distinctly. The ammeter select switch controls which output reading the
ammeter will display the voltage from, ALT 1 or BAT 1. The amps indication is derived from a current
transducer located in the electrical system MCU. When the engine is operating and the BAT 1 switch is
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CIRRUS AIRPLANE MAINTENANCE MANUAL MODEL SR20
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turned on, the ammeter indicates the charging rate applied to battery 1. Due to the inability of the alter-
nators to dissipate current, the alternator ammeter indications are positive only.
Serials 1582 & subs: Main and Essential Bus voltages are displayed on the MFD and PFD, therefore,
no ammeter select switch is necessary.
N. Turn Coordinator Batteries - Serials 1005 thru 1267 (See Figure 24-3012)
Turn coordinator batteries provide emergency power to the turn coordinator in the event of essential
power bus failure. Turn coordinator batteries consist of three date stamped 9-VDC Duracell MN1604
batteries connected in series to provide 27-VDC to the turn coordinator and the stall warning horn.
When the turn coordinator battery switch is manually selected the turn coordinator batteries will pro-
vide at least 30 minutes of power for the turn coordinator. When replacement is required, replace all
turn coordinator batteries at the same time, ensure expiration dates are the same, and expiration date
is valid through the next annual inspection date.
Turn coordinator batteries are independently controlled and wired to the turn coordinator and stall
warning horn power source allowing it to function properly when connected with battery 1 and the
alternator. Failure of the battery 1 will not affect the ability of the turn coordinator batteries to provide
necessary power. Because of diode protection, a failure or malfunction of the turn coordinator batteries
will not affect Battery 1 or the alternator.
O. Battery 1 (See Figure 24-3013)
Serials w/ TCM Battery: Battery 1 is a 24-volt, 12-cell, 10-ampere hour, lead-acid aviation-grade type
battery with non-spill vent caps. The battery is mounted in the engine compartment and has a top vent
with an acid-resistant tube. The tube discharges out the bottom of the engine cowling, preventing the
build up of dangerous or explosive gasses within the engine cowl.
Serials w/ Concorde Battery: Battery 1 is a 24-volt, valve regulated, lead-acid aviation-grade type bat-
tery. The battery is mounted in the engine compartment.
Serials 1268 & subs: Battery 1 is used for engine starting and can also be used as an emergency
power source in the event Battery 2 or either alternator fails. Battery 1 provides all the electrical power
for starting the aircraft. Battery 1 also supplies the electrical power to the landing light in the event
Alternator 1 fails. Battery 1 is independently controlled by the BAT 1 switch, located in the pilot’s bolster
panel. The BAT 1 switch energizes a relay in the MCU which will connect BAT 1 to the Main Distribution
Bus.
P. Battery 2 - Serials 1268 & subs (See Figure 24-3014)
Battery 2 is a maintenance free rechargeable sealed lead acid battery. Battery 2 consists of two 12-
volt, 6-cell, 7-amp-hour batteries connected in series to provide 24-VDC to the Essential Bus. There is
no need to check the specific gravity of the electrolyte or add water to these batteries during their ser-
vice life.
Battery 2 is independently controlled by using the BAT 2 switch, located in the pilot’s bolster panel. The
BAT 2 switch energizes a relay located just aft of bulkhead 222 in the MCU which will connect BAT 2 to
the Essential Bus. The electrical power from BAT 2 reaches the Essential Bus through the circuit
breaker panel. Battery 2 is mounted directly behind bulkhead 222 in an acid resistant battery container.
Battery 2 is primarily used to power the Essential Bus. The Essential Bus delivers electrical power to
the annunciator lights, turn coordinator, attitude indicator, horizontal situation indicator (HSI), stall
warning system, and alternator 2.
Failure of the BAT 1 will not affect the ability of BAT 2 to provide necessary power to the Essential Bus.
Because of diode protection, a failure or malfunction of the BAT 1 will not affect BAT 2 or either alterna-
tor output.
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