TM 1-1520-238-T-6
9–84
Change 5
9–7.
SYSTEM DESCRIPTION (cont)
9–7
(b) DC essential bus 3 is powered by dc essential bus 1, dc essential bus 2, or both dc essential buses
1 and 2 through isolation diodes CR3 and CR4. The isolation diodes allow current to flow from dc essential bus 1
and 2 to dc essential bus 3 but prevents current from flowing from dc essential bus 3 to dc essential buses 1 or 2.
(c) The battery system utilizes the 28 VDC from bus 1 to charge the battery and control the battery
relay. If a complete dc system failure occurs, the battery charger uses the battery power to energize the battery
relay. The battery charger controls the HOT BATT and CHARGER indicators on the pilot’s caution/warning panel.
(d) The battery relay, when energized, connects the battery to the dc emergency bus. The relay can be
energized only when both T/Rs are inoperative, the pilot’s ELEC PWR panel’s BATT/EXT PWR switch is in BATT,
and the CPG’s power quadrant’s BAT OVRD switch is in NRML.
(e) The battery provides 24 VDC to the dc emergency bus if a complete 28 VDC failure occurs. The
battery can power the emergency loads until a safe landing can be made.
(f) The T/R (fig. 9–77) receives 115 VAC across the primary winding of transformer T1. Voltage induced
in the secondary winding of T1 is rectified by diodes CR1 through CR12. The rectified voltage is filtered by a
network consisting of capacitors C4, C5, C6, and C7, inductor L1 and resistor R1. The output voltage of 28 VDC
is then routed to the dc bus tie contactor.
(g) Fan motor B1 is driven by the applied ac power. Capacitors C1, C2, and C3 provide filtration for the
fan motor.
(h) In event of an overtemperature condition, the thermal sensor is activated which completes a circuit
to turn on the HOT RECT indicator on the pilot caution/warning panel.
(i) The dc bus tie contactor (fig. 9–78) connects the T/R outputs to the appropriate dc essential buses.
The dc bus tie contactor contains two contactors (K8 and K9), two switching relays (K3 and K4), an overcurrent
sensor and limiter (OC), and a bus tie relay (K5). The 28 VDC inputs from T/R 1 and T/R 2 energize the coils of
contactors K8 and K9 respectively. When K8 is energized, the T/R 1 is connected to dc essential bus 1 and the
path to the RECT 1 indicator on the pilot caution/warning panel is opened extinguishing the indicator. When K9 is
energized, T/R 2 is connected to dc essential bus 2 and the path to the RECT 2 indicator on the pilot
caution/warning panel is opened extinguishing the indicator. Both dc essential buses 1 and 2 are connected to dc
essential bus 3 through isolation diodes. Either one or both dc essential buses power dc essential bus 3.
(j) Switching relays K3 and K4 are energized by K8 and K9. K3 and K4 connect the emergency dc bus
power and open the path for K5 to energize. Keeping K5 deenergized with both T/Rs operating prevents
paralleling of the buses.
(k) When T/R 1 fails, voltage to the coil of K8 is removed, deenergizing K8 and lighting the
RECT 1 indicator on the pilot’s caution/warning panel. When K8 deenergizes, emergency dc bus voltage is
removed from the coil of K3. When K3 is deenergized and K4 is energized, K3 connects emergency dc bus
voltage to the coil of K5. With K5 energized, dc essential bus 1 is connected to and powered by dc essential bus
2. DC essential buses 1, 2, 3, and the emergency dc bus are now powered by T/R 2.
(l) When T/R 2 fails, voltage to the coil of K9 is removed, deenergizing K9 and lighting the RECT 2
indicator on the pilot’s caution/warning panel. When K9 deenergizes, emergency dc bus voltage is removed from
the coil of K4. When K4 is deenergized and K3 is energized, K4 connects emergency dc bus voltage to the coil of
K5. With K5 energized, dc essential bus 2 is connected to and powered by dc essential bus 1. DC essential buses
1, 2, 3, and the emergency dc bus are now powered by T/R 1.
