Cirrus SR20 Maintenance Manual page 15

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 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.
EFFECTIVITY:
All
M O D E L S R 2 0
24-30
Page 15
15 Dec 2014