7-68
P/N 11934-003
Section 7
Cirrus Design
Airplane Description
SR20
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 overcurrent annunciations. Power is distributed to the airplane
circuit panel buses through Main and Essential Buses in the MCU.
During normal operation, the alternators 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 ALT
2 / 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.
Power Distribution
The power distribution system for the SR20 consists of the Main
Distribution Bus and the Essential Distribution Bus in the MCU and
associated buses in the Circuit Breaker panel. The circuit breaker
panel is located on the left side of the console next to the pilots right
knee.
For normal operation, the Essential Buses in the circuit breaker panel
are powered from the Essential Distribution Bus in the MCU through
25-amp circuit breakers. BAT 2 is connected directly to the Essential
Bus in the circuit breaker panel and will power the bus should the
voltage coming from the MCU distribution buses drop below the
battery voltage. Additionally, in the event of an ALT 2 failure, the circuit
breaker panel Essential Bus will be powered from ALT 1 through the
Main distribution and Essential distribution buses in the MCU. Main
Bus 1, Main Bus 2, and the equipment Non-Essential Bus in the circuit
breaker panel are powered from ALT 1 through the Main Distribution
Bus in the MCU. The Avionics Non-Essential Bus in the circuit breaker
panel is powered from Main Bus 1.
Revision A5
Summary of Contents for SR20
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