PAPI Power Supply

The PAPI Power Supply encompasses the electrical systems that provide energy to the PAPI light units. The specific configuration depends on the type of light source used:

• Halogen PAPI Systems: Powered by a Constant Current Regulator (CCR) that supplies a regulated 6.6A current to the series circuit. Each PAPI unit is connected via an isolating transformer that steps down the voltage while maintaining the constant current. The CCR receives mains power (typically 115/230 VAC) and converts it to the required series current, with selectable intensity steps.
• LED PAPI Systems: May use a CCR (if designed for series circuit compatibility) with LED drivers in each unit, or a dedicated low‑voltage DC power supply that feeds the LED modules directly. In some modern systems, a central power supply unit converts mains AC to a regulated DC voltage (e.g., 24V or 48V) and distributes it to the LED units via a parallel or distributed power architecture.
• Portable PAPI: Uses battery packs (often rechargeable Li‑ion or lead‑acid) with integrated charging circuits, sometimes supplemented by solar panels for extended operation.

The power supply is critical to the performance and reliability of the PAPI system. It must deliver clean, stable power with precise current regulation to maintain the required luminous intensity and ensure that all units in the array produce consistent light output.

The power supply's purpose goes beyond simply keeping the lamps lit — PAPI works by a pilot comparing the relative brightness and colour of four units side by side, so what matters most is that every unit receives exactly the same, stable current as the others. If one unit runs even slightly dimmer because of an underpowered or poorly regulated supply, the red/white balance a pilot relies on to judge glide slope becomes distorted in a way that has nothing to do with the aircraft's actual position. This is why constant-current regulation, rather than simple constant-voltage power, is central to the design: it holds every unit in the array to the same output regardless of cable length or connection resistance differences between units. The same regulation also has to support clean switching between day, twilight, and night intensity steps without one unit lagging or overshooting relative to the others during the transition.

PAPI power supplies come in several configurations depending on the system type and installation environment:

The PAPI power supply is an essential component of every PAPI installation, with variations for different operational requirements:

• Precision runways (CAT I, II, III) – CCR-based systems with UPS backup, providing reliable 6.6A series power and multiple intensity steps.
• Non‑precision runways – simpler CCR or DC power supplies, often with manual intensity control.
• Heliports – compact power supplies, sometimes with lower power ratings for helicopter PAPI units.
• Portable or temporary PAPI – battery or solar power supplies for rapid deployment in remote locations.
• Military airfields – ruggedized power supplies with high reliability and EMI filtering.
• Retrofit upgrades – LED drivers and power supplies that allow existing halogen PAPI systems to be upgraded to LED without replacing the entire infrastructure.

The power supply for a permanent PAPI installation is housed in the CCR room or equipment shelter serving that runway, wired into the series circuit (or DC distribution, depending on topology) that feeds the wing bar, with UPS backup added on precision runways where continuous availability is mandated. Cabling runs from the shelter out to each unit on the bar via underground ducting and the same isolating transformers used elsewhere in the AGL network. On portable and temporary installations, the power supply is instead integrated into or mounted alongside the unit itself — a battery pack, solar panel, or small generator — sited close to the wing bar rather than in a fixed building, so the whole system can be set up and struck without permanent infrastructure. In every case, the supply's location is chosen to keep cable runs to the wing bar within the system's voltage-drop limits.

The PAPI power supply performs several critical functions to ensure reliable and consistent illumination:

• AC to DC/Current Conversion: For CCR‑based systems, the input mains AC is converted to a regulated 6.6A constant current, regardless of load variations. This is achieved using a saturable reactor, thyristor, or IGBT-based regulator. For DC‑distributed systems, the AC is rectified and filtered to produce a stable DC voltage.
• Intensity Control: The power supply receives brightness commands from the control unit (or ALCMS) and adjusts the output current or PWM duty cycle accordingly. In CCRs, this is done by switching transformer taps or electronically controlling the current setpoint. In LED drivers, it's typically achieved via PWM dimming.
• Protection: The power supply includes overcurrent, overvoltage, short‑circuit, and thermal protection to prevent damage to the PAPI units and the power supply itself. Faults are reported to the control system.
• Monitoring: The power supply monitors its own status (input voltage, output current, temperature) and, in advanced systems, the status of each connected PAPI unit. This data is made available to the control unit for diagnostics and maintenance planning.
• Isolation: In CCR‑based systems, isolating transformers at each PAPI unit provide galvanic isolation between the primary series circuit and the secondary lamp circuit, enhancing safety.
• Filtering: EMI filters and surge protection devices are integrated to protect the power supply from transients and to prevent electromagnetic interference with other airport systems.

The power supply is where the intensity command is actually converted into current — it receives its brightness setting from the control unit or ALCMS and adjusts either the CCR's output current (legacy series-circuit topology) or a PWM duty cycle (modern DC bus / LED-driver topology) to match. Sitting alongside that command path is a set of protection functions — overcurrent, overvoltage, short-circuit and thermal cutoffs — that run independently of the intensity logic and take precedence over it whenever a limit is exceeded.

Because a PAPI's glide-slope function depends on continuous, correctly-graduated illumination, the supply's monitoring — input voltage, output current, internal temperature, and, on advanced units, per-fixture status — feeds back into the same fault-reporting chain as the rest of the AGL system, so a developing problem is flagged to the control unit and ALCMS well before it becomes a hard outage.

A CCR-fed halogen installation places the constant current regulator in an equipment room enclosure, converting mains input into a regulated series current fed out through cable glands to the primary loop, with each PAPI unit's isolating transformer built into its own base can or junction box along that loop. An LED PAPI system built around a central DC power supply instead houses a switch-mode power supply in a cabinet or equipment room, rectifying and regulating mains input down to a low DC voltage that is distributed by parallel wiring — a heavier-gauge cable than a series loop, since parallel distribution carries the full current to every branch point rather than a single shared loop current. Portable PAPI units carry their own sealed rechargeable battery pack and charge controller inside the fixture's own housing or a companion equipment box, with a solar panel, where fitted, mounted above or beside the unit and wired through a charge regulator to trickle-charge the battery during daylight. In every configuration, the power conversion and regulation electronics are housed separately from the light-emitting optics, connected to them only by the cable carrying the regulated current or voltage.