25W Isolating Transformer

The 25W Isolating Transformer is a single‑phase, fully encapsulated transformer designed for AGL series circuits. Per FAA AC 150/5345‑47, it is officially a dual-rated 20/25W unit (type L‑830‑17 for 60 Hz, L‑831‑17 for 50 Hz), permitting either a 20W or 25W lamp on the same 6.6A secondary — one of the more commonly used transformer sizes, powering a range of airfield lighting fixtures—from medium‑intensity runway edge lights to illuminated signs and small obstruction lights.

Like all isolating transformers, it provides primary‑secondary galvanic isolation, ensuring that the high‑voltage series loop is electrically separated from the low‑voltage lamp circuit. This improves safety, reduces fault propagation, and allows individual fixture removal without affecting the rest of the loop. The 25W rating offers a good balance between size, cost, and power capability, making it a versatile choice for both runway and taxiway applications.

The 25W isolating transformer exists to serve fixtures that sit between the smallest low-intensity applications and the common 45W standard — medium-intensity runway edge lights, illuminated signs, and small obstruction lights — giving the airfield a size- and cost-appropriate option rather than over- or under-powering a fixture that doesn't need a larger or smaller unit. Its function is identical to every transformer in the family: keep the fixture isolated from the shared series current so it can be serviced independently.

Because it balances size, cost, and capability well, it ends up being one of the most commonly deployed ratings across both runway and taxiway lighting, making its reliability and consistent performance across large quantities especially important to overall system uptime.

The 25W isolating transformer is deployed in a variety of AGL applications requiring moderate power levels, including:

• Runway Edge Lights: Medium‑intensity elevated or inset edge lights using 25W halogen or LED equivalents.
• Taxiway Edge and Centreline Lights: Many taxiway fixtures are rated at 25W, especially for older halogen systems.
• Illuminated Airfield Signs: Direction signs, information signs, and mandatory instruction signs with internal lighting (small fluorescent or LED).
• Obstruction Lights: Medium‑intensity obstruction beacons on structures up to 45m tall.
• Low‑intensity Approach Lights: Some non‑precision approach lighting configurations use 25W fixtures.
• Retrofit of Older Systems: Replacing existing 25W halogen lamps with LED equivalents while keeping the same transformer.

Typical installation is in handholes, pits, or direct burial adjacent to the fixture, using standard L‑823 primary connectors and secondary cabling.

The 25W transformer is installed in a standard handhole, pit, or direct-buried enclosure sited next to the fixture it serves, following the same pattern as most medium-power AGL transformers. The primary winding is spliced into the series circuit via an L-823 connector, and the secondary cable is run a short distance to the fixture's connector. Because many 25W applications are retrofits of older halogen taxiway and sign circuits, the existing enclosure and cable route from the original installation can usually be reused when the transformer is swapped, provided the pit and connectors remain in serviceable condition.

The 25W transformer operates on the same electromagnetic induction principle as other isolating transformers. The primary winding is connected in series with the 6.6A constant‑current loop. The primary current generates a magnetic flux that induces a secondary voltage — since secondary current is also fixed at 6.6A, this works out to roughly 3.0V on the 20W tap and 3.8V on the 25W tap (V = P/I). The secondary winding is connected to the lamp or LED driver, which draws power at the rated current and voltage.

Specific design features for the 25W variant include:

• Efficient Core Design: Optimised for low core losses across the 2.8A to 6.6A dimming range, ensuring good regulation and consistent light output.
• Thermal Management: The 25W power level allows for a compact form factor while still maintaining adequate thermal dissipation through encapsulation.
• Fixed Secondary Voltage by Tap: Delivers roughly 3.0V (20W tap) or 3.8V (25W tap) at the fixed 6.6A secondary current, matched to the connected lamp's rating.
• High Insulation Integrity: Typically rated for 3kV dielectric strength, providing robust protection against voltage surges and insulation breakdown.
• Short‑Circuit Robustness: The transformer’s impedance limits secondary fault currents, protecting the loop and fixture.

The 25W 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 (2.8A–6.6A, 5-step). The transformer has no addressable identity of its own — it converts whatever current the loop is carrying at that moment into roughly 3.0V–3.8V on the secondary (20W/25W 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 25W units on a taxiway or sign loop is at fault.

The 25W transformer follows the same encapsulated, potted construction as the rest of the range: windings and a laminated core sized for a 25W secondary are enclosed in a moulded shell, vacuum-impregnated and back-filled with epoxy or polyurethane resin so the winding is fully sealed rather than gasketed, giving the unit its IP68 rating directly from the potting rather than from a separate enclosure. Complete units typically weigh 2.0–3.5 kg, and the winding insulation is specified to Class F or Class H depending on the fixture it serves. Leads terminate in sealed L-823 splice connectors suited to continuous burial, and the unit sits in the fixture's own base can rather than a dedicated housing. This wattage covers medium-intensity edge and centreline lights, illuminated signs, and medium obstruction beacons, so it is one of the more commonly stocked ratings in a mixed AGL inventory.