DC (Direct Current) system overview
TR (Tie Relay)
- The 737 uses 3 transformer rectifiers to supply 28 volt DC power
- The TRs transfer change 155 volts AC from the transfer busses into 28 volt DC for use by the DC busses
- The TRs change 115 volt AC from the transfer busses into 28 volt DC for use by the DC busses
TR (Tie Relay)
-The TRs change 115 volt AC from the transfer busses into 28 volt DC for use by the DC busses
TR (Tie Relay) overview
- TR 1 is supplied by transfer bus 1 and is the normal power source for DC bus 1
- TR 2 is supplied by transfer bus 2 and is the normal source for DC bus 2
- Transfer Bus 2 is the normal power source for TR 3 (Tie Relay) but transfer 1 can also be used
TR 3 (Tie relay)
- TR 3 is an auxiliary sourced of DC power if TR 1 and 2 fails
- TR 3 is also the normal source of power for the battery bus
TR failure (Tie relay)
-If a TR fails, the remaining TRs can supply power for the total DC load
DC busses distribution
-DC busses are connected by a cross bus tie relay
Relays on an ILS approach
- The relay opens automatically when the glide-slope is captured during the ILS approach
- The relay opens to isolate the navigation receivers and flight computers. This prevents the removal of all approach information because of only 1 electrical failure
Relays + bus transfer switches
-The relay also opens when the BUS TRANSFER switch is moved to off
Normal sources of power to the DC busses fail
-The emergency source of DC power is the batteries
Battery Power
-Battery power:
- 2 standby busses
- Battery Bus
- Hot battery bus
- Switched hit battery bus
The Hot Battery
- Hot battery bus is always connected to battery power
- It’s components operate as long as battery voltage is above minimum value
Hot Battery Source
- Power to the switched hot battery bus controlled by the battery switch
- The battery switch and battery operations are discussed as part of the standby power system
Automatic battery
-The automatic battery charger retorted and maintains full battery voltage
Battery charger
-The battery charger is supplied power from AC ground service bus 2 when a source of AC power is available
Second battery charger (left side)
-A different battery charger for the auxiliary battery is supplied power AC ground service bus 1
AC and DC metering panel
Normal indications:
-Normal voltage 26 volts
- Amps vary with elect
- The maintenance switch is used by maintenance personnel to do electrical system tests
