What is a Transformer Rectifier Unit (TRU)?

A Transformer Rectifier Unit (TRU) is a power conversion device that combines a transformer and a rectifier in one compact component. It converts three-phase AC power into regulated DC power at the required voltage and current levels. In aircraft, a TRU typically converts 115/200 V, 400 Hz three-phase AC into approximately 28 V DC for avionics, lighting, control electronics and battery charging.

What does a TRU unit do on an aircraft?

On an aircraft, a TRU unit converts 115/200 V, 400 Hz three-phase AC power from an engine generator, APU generator, or external ground power into 28 V DC. This DC power is then distributed to DC buses that supply avionics, flight control computers, communication and navigation radios, interior lighting, and other critical electronic systems. In many designs, the TRU output also supports aircraft battery charging.

How does a Transformer Rectifier Unit work?

A Transformer Rectifier Unit works in two main stages. First, the transformer stage steps the three-phase AC input to the appropriate level and provides galvanic isolation. Second, a diode or thyristor rectifier bridge converts the AC voltage into DC. Filters and, in some cases, inter-phase transformers then smooth the DC output, control ripple, and improve power quality. Multi-pulse transformer and rectifier arrangements are often used to reduce harmonics and improve power factor.

How are TRUs used in the Airbus A320 electrical system?

In the Airbus A320 family, the electrical system uses a three-phase 115/200 V, 400 Hz AC network and a 28 V DC network. Engine and APU generators feed the AC buses, and several transformer rectifier units convert this AC power into DC. TR 1 supplies DC BUS 1, TR 2 supplies DC BUS 2, and the ESS TR (essential transformer rectifier) converts power from the AC ESS BUS to the DC ESS BUS. This ensures essential avionics and systems receive 28 V DC even in abnormal or emergency electrical configurations.

What is the difference between a TRU and a regulated TRU (RTRU)?

A standard TRU is largely passive: it uses a transformer and a diode bridge to produce an unregulated but filtered DC output whose exact voltage varies with load and input conditions. A regulated Transformer Rectifier Unit (RTRU or R-TRU) adds active control electronics and high-frequency conversion stages. This allows it to stabilise the DC output voltage, improve power factor, reduce harmonics at the AC source, and deliver better overall power quality, which is especially important in more-electric aircraft architectures.

What is an Auto-Transformer Rectifier Unit (ATRU)?

An Auto-Transformer Rectifier Unit (ATRU) is a variant of the transformer rectifier unit that uses an autotransformer instead of a fully isolated transformer. It still provides multi-pulse rectification but can reduce weight and size while maintaining good power quality. ATRUs are often used in high-power systems and more-electric aircraft architectures, including those that use ±270 V DC buses.

Why is 28 V DC commonly used in aircraft?

28 V DC is commonly used in aircraft because it offers a good balance between safety, wiring practicality, and equipment design. It is high enough to keep currents at manageable levels for typical loads, which helps limit cable size and weight, yet low enough to avoid the insulation and safety challenges associated with higher DC voltages. For these reasons, 28 V DC has become a standard for many aircraft DC systems.

What are the key design considerations for a Transformer Rectifier Unit?

Key design considerations for a Transformer Rectifier Unit include input voltage and frequency, required DC output voltage and current, overload capability, efficiency, power factor and harmonic distortion, DC ripple level, electromagnetic interference (EMI) performance, and environmental ratings such as temperature, vibration, and humidity. Weight and size are also critical in aircraft applications, so designs aim for high power density while meeting reliability and certification requirements.

Where are TRUs used besides onboard aircraft?

In addition to onboard use, Transformer Rectifier Units are widely used in ground power equipment. External 28 V DC ground power units supply DC power to aircraft on the ramp for APU start, engine start, maintenance, or powering avionics without running the engines or APU. Similar transformer rectifier concepts are also used in industrial power supplies and cathodic protection systems where reliable AC-to-DC conversion is required.

Transformer Rectifier Unit (TRU): How Aircraft Convert AC Power to Stable 28 V DC

Q: Is a TRU the same as a battery charger?

A TRU is not exactly the same as a battery charger. Its primary role is to convert AC power into DC power for aircraft DC buses. These DC buses may then feed a dedicated battery charger or battery management system. Although both may use transformer-rectifier front ends, a battery charger typically adds specialised control, monitoring, and charging profiles tailored to the specific battery type.

A Transformer Rectifier Unit (TRU) converts three-phase AC power into regulated DC power. It combines a transformer and a rectifier in one compact component. Most aircraft use 115/200 V, 400 Hz AC. The TRU converts this input into stable 28 V DC for avionics, lighting and essential electronics. As a result, the TRU forms a key link in modern aircraft applications.

Airbus A320 ELEC system page showing normal AC and DC power distribution — A320 ELEC page showing normal TR1, TR2 and ESS TR power distribution.

The Airbus A320 family uses several TRUs. TR 1 feeds DC BUS 1, TR 2 feeds DC BUS 2 and the ESS TR supports the essential DC network. This layout ensures stable voltage on all main and essential buses during both normal and abnormal electrical conditions.

1. What a Transformer Rectifier Unit Does

A TRU performs two primary functions. First, it reduces AC voltage through the transformer stage. Then the rectifier stage converts the AC output into DC. Most units also include filters to smooth the waveform and limit harmonic content. This design improves overall power quality.

Aircraft depend on a reliable 28 V DC power supply. The TRU feeds systems such as flight guidance computers, radios, cabin lighting, control electronics and monitoring equipment. It also stabilises current flow to DC buses and supports aircraft battery charging.

Typical TRUs deliver 200 A at 28 V DC. Many units can support higher load peaks during pump starts or rapid changes in electrical demand.

2. How a Transformer Rectifier Unit Works

Most aircraft TRUs use a multi-pulse rectifier design. This architecture reduces harmonic distortion and improves power factor. It also improves reliability and temperature stability.

Transformer section: It adapts the incoming three-phase AC input power and provides isolation.
Rectifier section: It uses diodes or thyristors to produce direct current.
Filters: They smooth the output voltage and reduce ripple and EMI.

Higher-pulse TRUs, such as 12-pulse or 18-pulse units, create better waveform quality. Consequently, they place fewer disturbances on the aircraft AC network.

A320 ELEC system overview display with TR1, TR2, ESS TR and AC ESS page — The A320 ELEC system page highlights AC and DC flow through TR1, TR2 and the ESS TR.

3. TRUs in the Airbus A320

The A320 electrical system uses two main 115/200 V AC buses. The system also uses a 28 V DC network for avionics and control electronics. Each TRU converts AC from its associated AC bus into DC for a specific DC bus.

TR 1 → DC BUS 1
TR 2 → DC BUS 2
ESS TR → DC ESS BUS (fed from AC ESS BUS)

A320 ELEC page displaying APU GEN supplying AC ESS and TRU network — The APU generator supplying the AC ESS bus, allowing the ESS TR to energize the DC ESS bus.

If both AC BUS 1 and AC BUS 2 fail, the Ram Air Turbine restores the AC ESS BUS. This power source feeds the ESS TR, which then restores the 28 V DC essential network.

4. Types of Transformer Rectifier Units

4.1 Standard TRU

A standard TRU uses a passive transformer and a diode bridge. It delivers well-filtered 28 V DC, and system load influences its exact output voltage. Operators value this design because it offers predictable behaviour and high reliability.

4.2 Regulated Transformer Rectifier Unit (RTRU)

A regulated transformer rectifier adds active control electronics. As a result, it maintains steady DC voltage even when load changes. It also improves power factor and reduces harmonic content.

4.3 Auto-Transformer Rectifier Unit (ATRU)

An Auto-Transformer Rectifier Unit replaces the isolated transformer with an autotransformer. This choice reduces weight and size. ATRUs remain common in high-power systems, especially those using ±270 V DC.

4.4 Next-Generation Smart TRUs

Smart TRUs use high-frequency conversion stages. They also improve power distribution across multiple DC buses.

5. Key Design Considerations

Engineers review several factors when they select a TRU. These factors influence stability, weight, efficiency and power quality.

Input voltage: Usually 115/200 V, 400 Hz AC.
DC output: 28 V or 270 V DC.
Overload capability: Peaks of 150–300% rated current.
Efficiency: Often above 85%.
EMI performance: Must meet certification limits.
Weight and size: Critical for installation.

6. TRUs in Ground and On-Board Use

Aircraft use TRUs both on board and on the ground. On-board units convert AC into 28 V DC for flight systems. Ground power TRUs provide 28 V DC for APU starts, maintenance work or avionics power during long turnarounds.

A320 ELEC page showing TR1 fault affecting DC BUS 1 power supply — TR1 fault displayed on the ELEC page, showing loss of DC BUS 1 power contribution.

A320 ECAM warning for TR1 failure displayed on E/WD — The ECAM E/WD page displays a TR1 fault message during an electrical malfunction.

7. Frequently Asked Questions

What is a TRU unit on an aircraft?

A TRU unit converts three-phase AC into 28 V DC. It supports avionics, lighting, communication systems and essential electronics.

What is the difference between a TRU and an RTRU?

A TRU uses a passive design. An RTRU includes active electronics that regulate output voltage and reduce distortion.

Is a TRU the same as a battery charger?

A TRU feeds the DC buses. A battery charger uses those buses and applies specific charging logic.

Why do aircraft use 28 V DC?

28 V DC keeps electrical current levels manageable, reduces cable weight and remains safe for technicians and equipment.

The transformer rectifier unit remains a key AC-to-DC converter in aviation. It supports clean power, stable voltage and reliable power distribution. As aircraft evolve, TRU designs continue to offer lighter, more efficient and more controlled solutions for future aircraft applications.

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Airbus A320, Electrics

Transformer Rectifier Unit (TRU): Working Principle, Types, and Aircraft Applications

1. What a Transformer Rectifier Unit Does

2. How a Transformer Rectifier Unit Works

3. TRUs in the Airbus A320