FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A technician is servicing an aircraft electrical system in which the engine-driven generators produce 115-volt, 400-Hz AC power. Which component converts this AC power into 28-volt DC power to charge the battery and supply DC buses?
- AAutotransformer
- BTransformer-rectifier unit (TRU)
- CStatic inverter
- DCarbon pile regulator
Show answer & explanationAnswer & explanation
Correct answer: B. Transformer-rectifier unit (TRU)
A transformer-rectifier unit (TRU) steps down 115-volt AC to a lower AC voltage with a transformer, then rectifies it to usable 28-volt DC power for battery charging and DC bus loads on AC-primary electrical systems.
Why the other options are wrong
- A. An autotransformer changes AC voltage but does not convert it to DC.
- C. An inverter does the opposite, converting DC to AC power.
- D. A carbon pile regulator controls DC generator field current, not AC-to-DC conversion.
Transformer-Rectifier Unit (TRU)
A device that steps down AC voltage using a transformer and converts it to DC using rectifier diodes, commonly used in AC-primary electrical systems to supply DC buses and charge the battery.
- Combines a step-down transformer with a rectifier
- Converts 115V AC to 28V DC typically
- Used on aircraft where AC generators are the primary power source
Memory trick: TRU = 'Turn to DC, Reduce voltage, Use it' — transform then rectify.