Cirrus SR20 Maintenance Manual page 16

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
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
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15 Dec 2014
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