RN-120U

Aircraft voltage regulator RN-120U

RN-120U is an aircraft carbon voltage regulator of the rheostat type. It is used in DC power generation systems to stabilize the output voltage of onboard starter-generators and DC generators. The regulator changes the excitation current of the generator and keeps the bus voltage within the required range when engine speed or electrical load changes. This is older electromechanical technology, not a modern electronic regulator, so condition of the carbon column, electromagnetic system, contacts and mounting base matters during inspection.

RN-120U voltage control function

In operation, RN-120U controls the generator field circuit. When the generator output tends to rise or fall, the regulator changes resistance in the excitation circuit and brings the voltage back to the set value. The unit is also used in systems with parallel generator operation, where it helps distribute load current between generators. This function is important on aircraft with several DC power sources, because unequal load sharing can overload one generator while the other remains underused.

RN-120U with STG-3 and GS-24B generators

RN-120U is associated with onboard starter-generators and DC generators such as STG-3 and GS-24B. In these systems it works together with generator control equipment, contactors, protection relays, cockpit switches, voltage indication circuits and the aircraft DC bus. The regulator does not generate electrical power by itself. It controls the excitation circuit of the generator that supplies the onboard network. During maintenance, a voltage fault should be checked as a full circuit issue, not only as a regulator failure.

Aircraft application of RN-120U

The RN-120U regulator is used on Soviet and Ukrainian aircraft and helicopters with DC generator systems. Typical applications include Mi-2, Mi-8 and Mi-24 helicopters, as well as Yak-40, An-24, An-26 and An-32 aircraft. The exact installation depends on the aircraft modification, generator type and power system configuration. On some upgraded Mi-8AMT / Mi-171 aircraft, older RN-120U regulators may be replaced by modern electronic voltage regulation units according to applicable service bulletins and modification documentation.

Identification and ordering of RN-120U

Before ordering RN-120U, the unit marking should be compared with the aircraft parts catalog, wiring diagram and generator equipment list. This is especially important for aircraft that have passed through overhaul or electrical system modernization. The installed regulator may differ from the original configuration. For correct selection, provide the aircraft type, generator type, installed regulator marking and, if possible, a photo of the data plate. The product is also known under alternative designations RN-120U and РН-120У.

Product Type: Aircraft carbon voltage regulator, rheostat type
Function: Automatic stabilization of DC generator and starter-generator voltage; current sharing during parallel generator operation
Application / Where Used: Aircraft DC power generation systems
System / Installation Area: Generator excitation and voltage regulation circuit
Compatible Aircraft / Equipment: Mi-2, Mi-8, Mi-24, Yak-40, An-24, An-26, An-32
Compatible Equipment: STG-3 starter-generator, GS-24B DC generator
Regulated Voltage: 28.5 V DC
Maximum Dissipated Power: 120 W on carbon column
Construction: Electromagnetic system, variable-resistance carbon column, mounting plate with shock absorbers, plug connector, diode and additional resistors
Dimensions: 200.5 × 110 × 105 mm
Weight: Not more than 2.1 kg
Production Status: Discontinued; may be replaced by electronic voltage regulation units on modernized aircraft according to applicable documentation
Technical Documentation: Aircraft electrical system documentation, parts catalog, repair and adjustment documentation

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