15W Isolating Transformer

The 15W Isolating Transformer is a single-phase, hermetically sealed transformer used in constant‑current AGL series circuits to power low‑wattage light fixtures, guidance signs, or auxiliary loads. Per FAA AC 150/5345‑47, it is officially a dual-rated 10/15W unit (type L‑830‑16 for 60 Hz, L‑831‑16 for 50 Hz), permitting either a 10W or 15W lamp on the same 6.6A secondary — it is the smallest standard isolating transformer in the AGL family, ideal for applications where only a minimal light output is needed—such as taxiway edge lights (low intensity), obstruction lights on small structures, or illuminated airfield signs.

Like all isolating transformers, it provides galvanic isolation between the high‑voltage series loop (primary) and the low‑voltage fixture (secondary), ensuring safety and fault tolerance. Its compact size and lower material usage make it cost‑effective for large installations with many low‑power fixtures, while still maintaining the reliability and environmental sealing required for buried or pit‑mounted operation.

The 15W isolating transformer exists for the lowest-demand fixtures on the airfield — low-intensity taxiway edge lights, small obstruction lights, and illuminated signs — where using a larger transformer than necessary would add cost and material across an installation that might include hundreds of these units. Its purpose remains the same isolation and fault-tolerance role as every transformer in the family, simply scaled down to match a genuinely low-power fixture.

Because large installations often deploy many 15W units at once, its compact size and lower material use directly affect overall project cost, while its hermetic sealing and buried- or pit-mounted reliability ensure that using the smallest standard rating doesn't mean accepting a less dependable component.

The 15W isolating transformer is typically deployed in scenarios where the fixture power demand is 15W or less. Common applications include:

• Low‑Intensity Edge Lights: On non‑precision runways or taxiways where only a basic visual cue is required, using miniature low-voltage lamps or LED modules matched to the transformer's fixed 6.6A secondary current.
• Illuminated Airfield Signs: Mandatory instruction signs, information signs, and direction signs that incorporate internal lighting (often LED or small halogen).
• Obstruction Lights: Low‑intensity obstruction lights on small structures (e.g., perimeter fences, short masts) where 15W is sufficient.
• Apron and Gate Guidance: Low‑power guidance fixtures on aprons or at gate positions.
• Retrofit Replacements: Upgrading older 15W halogen fixtures to LED equivalents while keeping the same transformer.

The transformer is typically installed in a handhole, pit, or buried directly adjacent to the fixture, using standard L‑823 primary connectors and secondary cables for termination.

Given its low secondary load, the 15W transformer is usually the smallest unit in the enclosure and is installed in a small handhole or shallow pit set close to the fixture or sign it feeds, keeping the low-power secondary run as short as possible. The primary side is spliced into the series loop with a standard L-823 connector, while the light-gauge secondary cable terminates directly at the fixture connector. Where it feeds an illuminated sign or low-intensity light on a fence line or short mast, the transformer is often housed in the same base or foundation as the fixture rather than a separate pit, simplifying cabling on low-power circuits.

Operation is identical to other isolating transformers: the primary winding is connected in series with the 6.6A constant‑current loop. The primary current creates a magnetic flux that induces a secondary voltage — since secondary current is also fixed at 6.6A, this works out to roughly 1.5V on the 10W tap and 2.3V on the 15W tap (V = P/I). The secondary is connected directly to the lamp or LED module, which draws the rated power at the rated voltage.

Key operational aspects specific to the 15W variant:

• Low Core Loss: The smaller core and lower flux density result in very low no‑load losses, making it highly efficient even at the minimum loop current (2.8A).
• Thermal Performance: With only 15W of power transfer, heat generation is minimal, allowing for more compact encapsulation and less reliance on large heat sinks.
• Voltage Regulation: Designed to maintain secondary voltage within ±5% across the normal current range (2.8A to 6.6A), ensuring consistent light output even at reduced brightness settings.
• Short‑Circuit and Overload Protection: The transformer’s impedance naturally limits secondary fault currents; however, it is not intended for sustained overload—proper fuse or circuit protection on the secondary side is recommended when used with sensitive electronics.

The 15W transformer occupies the same position in the control chain as any other isolating transformer: an Airfield Lighting Control and Monitoring System (ALCMS) selects the required brightness step based on prevailing RVR, ATC instruction, or pilot-controlled lighting input, and the CCR regulates the whole loop to the corresponding fixed current (one of 5 steps between 2.8A and 6.6A). The transformer has no addressable identity of its own — it converts whatever current the loop is carrying at that moment into roughly 1.5V–2.3V on the secondary (10W/15W taps), and cannot be commanded independently of the rest of the loop. A fault here is detected the same way as any other transformer on the circuit: the CCR monitors loop voltage against its commanded current, and a voltage rise flags increased impedance without distinguishing an open-circuit lamp from a failed transformer, leaving sectionalised field testing to pinpoint which of the many low-power 15W units on a taxiway or sign loop is at fault.

At this low wattage, the transformer is built as a compact single-piece encapsulated unit: a small laminated core and windings sized for a 15W secondary sit inside a moulded shell that is vacuum-impregnated and back-filled with epoxy or polyurethane resin, leaving no internal air gap for moisture to collect in. The full encapsulation is what earns the fully-sealed IP68 rating rather than a gasketed enclosure, and the compact size — typically 1.5–2.5 kg complete — suits the sign, low-intensity edge light, and low-intensity obstruction light applications this rating serves. Depending on the lamp it feeds, the winding insulation is specified to either Class F or Class H, and primary and secondary leads terminate in sealed L-823 splice connectors rated for continuous burial. The small size means these units are typically grouped several to a handhole, so lead length and connector colour-coding matter more here than on larger single-fixture transformers.