Flasher Power Supply

A Flasher Power Supply (also referred to as an SFL Power Supply or Flash Power Supply) is an electrical unit that provides the necessary power to Sequenced Flashing Light (SFL) units within an approach lighting system. Unlike standard constant‑current regulators (CCRs) that supply steady power to continuous‑burning lights, the Flasher Power Supply is specifically designed to deliver short, high‑energy pulses of power that produce the brilliant flashes required for SFL systems. These units often incorporate high‑voltage capacitors that discharge rapidly to generate the intense light output needed for visibility in fog, rain, and snow. The Flasher Power Supply is a critical component of precision (ALSF‑1/2) and medium‑intensity (MALSF/MALSR) approach lighting systems, ensuring the SFL units flash with consistent intensity and timing.

Modern Flasher Power Supplies use advanced switched‑mode technology, offering high efficiency, compact size, and precise output control. They are designed to interface with the Flasher Control Unit, receiving trigger signals to synchronise the power pulses with the desired flash sequence. Many units include built‑in diagnostics, remote monitoring, and redundant power stages for critical applications.

The flasher power supply's purpose is to deliver the brief, intense burst of energy an SFL unit needs to produce a flash bright enough to cut through fog, rain, or snow — a fundamentally different job from the steady, continuous current a CCR supplies to conventional runway and approach lights. Because that flash has to be both intense and precisely timed to align with the rest of the sequence, the unit's capacitor-based energy storage exists specifically to build up power between flashes and then release it almost instantaneously, achieving a peak brightness that a continuously-drawn current could never produce from the same average power budget. Its role is therefore purpose-built around a single job: convert steady input power into a burst that is bright enough, at exactly the right moment, to remain visible in the low-visibility conditions where SFL guidance matters most.

Flasher Power Supplies are deployed as part of approach lighting systems that require sequenced flashing guidance. They are used in:

• ALSF‑1 and ALSF‑2 systems – providing power for SFL units in CAT I, II, and III precision approaches.
• MALSF and MALSR systems – powering SFL for medium‑intensity approach lighting.
• Calvert II systems – supplying power for SFL in British approach lighting configurations.
• Runways with low‑visibility operations (RVR below 800 m) – where reliable high‑intensity flashing is essential for pilot guidance.
• Major international airports – with CAT II/III capability requiring robust SFL power systems.
• Military airbases – requiring all‑weather approach capability with SFL guidance.
• Airfields with remote monitoring – where the Flasher Power Supply integrates with ALCMS for centralised control and diagnostics.

The flasher power supply is installed in the same equipment shelter or CCR room as the flasher control unit, mounted on a rack or panel with adequate ventilation for the heat generated during its charge cycle. It is fed from the shelter's distribution board through its own protective device, and its high-energy discharge output is wired out to the sequenced flashing lights on the approach lighting structures using cabling rated for the peak discharge current. Because it stores a significant charge between flashes, its mounting position keeps the unit's discharge terminals and capacitor bank away from routine foot traffic in the equipment room, and access is arranged so a technician can safely isolate and discharge it before any maintenance.

The Flasher Power Supply operates through a combination of energy storage, pulse generation, and precise control:

• AC to DC Conversion: The unit converts incoming AC mains power (typically 120V or 230V) to a regulated DC voltage using a high‑efficiency switched‑mode power supply (SMPS) or linear regulator.
• Energy Storage: The DC power charges a bank of high‑voltage capacitors (or a pulse‑forming network) that store energy for the flash. These capacitors are designed to discharge rapidly, delivering a high‑current pulse to the SFL units.
• Pulse Generation: When the Flasher Control Unit sends a trigger signal, the power supply discharges the stored energy through a switching circuit (IGBT or thyristor) into the SFL unit, generating a high‑intensity flash. The pulse duration and peak current are precisely controlled to achieve the required luminous intensity.
• Intensity Control: The Flasher Power Supply interfaces with the CCR or intensity control system to adjust the flash intensity (brightness) based on ATC commands or automatic RVR‑based dimming. This is achieved by varying the capacitor charging voltage or pulse width.
• Multiple Outputs: Some Flasher Power Supplies have multiple independent output channels, each feeding a separate SFL unit or group of units. This allows for individual control and redundancy.
• Monitoring and Diagnostics: The unit continuously monitors its own operation, including capacitor voltage, output current, and temperature. It can detect faults such as capacitor failure, overcurrent, or overheating, and report them to the ALCMS or Flasher Control Unit.
• Redundancy: Critical installations often use dual Flasher Power Supplies with automatic failover, ensuring continuous SFL operation in the event of a primary unit failure.

The Flasher Power Supply is the energy stage of the SFL control chain — it holds no sequencing logic of its own and acts only on commands issued above it. The ALCMS or ATC sets the intensity step, which the Flasher Control Unit converts into a timed trigger for each SFL position in the sequence. On receipt of its trigger, the power supply discharges its capacitor bank (or pulse-forming network) through the switching circuit into the SFL, at a charging voltage or pulse width set by the current intensity step — this is where the commanded brightness step actually becomes flash energy. Where a site uses multiple independent output channels or dual power supplies for redundancy, each channel discharges only on its own trigger, so a fault or overload on one channel does not interrupt the others. Capacitor voltage, output current, and temperature are monitored continuously and reported to the Flasher Control Unit or ALCMS; on a fault such as capacitor failure or overcurrent, a standby power supply takes over automatically in redundant installations.

The Flasher Power Supply is housed in a floor-standing or wall-mounted steel cabinet, similar in outward form to a CCR cabinet, sized to accommodate its high-voltage capacitor bank alongside the switched-mode charging and discharge circuitry. A bank of high-voltage capacitors, mounted in a dedicated compartment with adequate clearance and venting, stores the energy for each flash pulse, while a separate power section houses the charging circuit (rectifier and switched-mode converter) that replenishes the capacitors between flashes. A discharge/trigger circuit, typically a thyristor or IGBT switch, connects the capacitor bank to the output terminals and is fired by the trigger signal from the Flasher Control Unit. The control and monitoring electronics occupy a separate, isolated section of the cabinet to protect them from the high-voltage switching transients in the power section, and heavy-duty output terminals, an input isolator/breaker, and surge protection are provided consistent with other AGL power equipment.