Cirrus SR20 Maintenance Manual page 17

C I R R U S
A I R P L A N E M A I N T E N A N C E M A N U A L
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.
EFFECTIVITY:
All
M O D E L S R 2 0
24-30
Page 17
15 Dec 2014