LED Driver
The electronic power supply that converts the constant current from the AGL series circuit into the precise regulated current required by LED modules. Features dimming control, surge protection, thermal management, and diagnostic feedback for reliable LED fixture operation.
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What Is It?
An LED Driver (also called an LED power supply or LED control gear) is an electronic device that provides the correct electrical conditions for operating LED modules in AGL fixtures. Unlike halogen lamps that run directly on the series circuit current, LEDs require a constant current source with precise voltage and current regulation. The LED driver takes the input from the series circuit (typically 6.6A constant current, stepped down via an isolating transformer) and converts it to a regulated constant current (e.g., 350mA, 700mA, or 1A) suitable for the LED module. It includes dimming control (responding to CCR current steps), surge protection, over-temperature protection, and often provides diagnostic feedback (e.g., status indication, current monitoring) for the ALCMS.
Purpose & Function
The LED driver's purpose is to make an LED module behave the way a pilot needs it to: producing exactly the intensity commanded by the CCR's current step, and continuing to do so reliably despite the electrical stresses of an outdoor airfield circuit. Because LEDs are current-driven devices rather than voltage-driven like halogen lamps, they cannot be connected to the series circuit directly — without a driver to regulate current and translate CCR steps into the correct LED brightness, the fixture could not deliver the graduated low/medium/high intensity settings that pilots rely on to match visibility conditions. The driver's surge and thermal protection functions serve a second purpose: shielding the comparatively delicate LED module from lightning-induced transients and overheating, which is what allows LED fixtures to achieve the long, low-maintenance service life that is their main advantage over halogen.
Where Is It Used?
LED drivers are used in every LED-based AGL fixture, including:
- Runway Edge & Centreline Lights: Drivers for white and yellow LED modules.
- Threshold & End Lights: Drivers for green and red LED modules.
- Taxiway Lights: Drivers for blue and green LED modules.
- Approach Lighting: Drivers for high-flux white LED barrettes and flashing lights.
- PAPI & VASI: Drivers for LED units in visual glide slope systems.
- Obstruction Lights: Drivers for red and white LED obstruction beacons.
- Beacons & Wind Cones: Drivers for rotating beacons and illuminated windsocks.
Installation & Location
The driver board is mounted on standoffs or a dedicated bracket inside the fixture housing, positioned clear of the optical path and away from direct exposure to condensation that can collect on the inside of the lens. It's wired between the incoming series-circuit or mains supply and the LED module, with input and output leads kept separated and routed away from sharp housing edges that could chafe the insulation over time. Where the driver generates significant heat, it's mounted against (or coupled to) a heat sink or the housing wall itself so the assembly can conduct heat away, and it's fitted with enough slack in its leads that the housing can be opened for servicing without disconnecting or straining the wiring.
How Does It Work?
The LED driver performs several critical functions:
- Current Regulation: Converts the varying input (from the series circuit) into a constant current output that matches the LED module's requirements (e.g., 350mA, 700mA). This ensures stable light output regardless of input voltage fluctuations.
- Dimming Control: Detects the CCR step (via input current level or voltage) and adjusts the output current accordingly to achieve the required brightness level (typically 5 steps: 2.8A, 3.4A, 4.1A, 5.2A, 6.6A input).
- Surge Protection: Incorporates surge suppression (MOVs, TVS diodes) to protect the LED module from lightning-induced transients and power surges.
- Thermal Protection: Monitors the driver temperature and reduces current or shuts down if overheating occurs, preventing damage.
- Diagnostic Feedback: Provides status signals (e.g., current output, temperature, fault conditions) to the ALCMS for remote monitoring and troubleshooting.
The driver is typically potted in a thermally conductive epoxy or housed in a sealed metal enclosure to withstand the harsh airfield environment (moisture, temperature extremes, vibration).
Regulation vs. Protection & Control Logic
Series Circuit Input
CCR steps the loop current from 2.8A to 6.6A across 5 dimming levels
Driver Regulates Constant Current Output
Converts input to a stable 350mA, 700mA, or 1A for the LED module
Output Tracks the CCR Step
Detected input current level sets the output current
Commanded Brightness Delivered
Status and current data reported to the ALCMS
Transient or Excess Temperature Detected
MOV/TVS clamps a surge; thermal sensor flags overheating
Output Reduced or Shut Down
LED module protected; fault reported to the ALCMS
The Driver Both Sets the Light Level and Protects the LED From Conditions a Halogen Lamp Would Simply Tolerate
Physical Construction
The driver electronics are built on a compact PCB populated with the input rectification, current-regulation, and protection circuitry, usually potted in an epoxy compound or enclosed in a small metal or polymer housing to protect against the fixture's internal condensation and vibration. Potting also improves heat spreading away from the switching components, which matters inside a sealed enclosure with no active airflow. The driver mounts inside the fixture's bottom housing on standoffs or a bracket, positioned clear of the hottest zone near the light source, and connects to the series-circuit side via the isolating transformer's secondary and to the LED module via a short, often keyed or polarised, wiring harness to prevent a reversed connection during maintenance. Input and output leads pass through the same sealed cable glands or grommets used elsewhere in the housing, keeping the electronics compartment at the fixture's rated IP level.
Technical Parameters
| Parameter | Typical Value |
|---|---|
| Input Type | Constant current from AGL series circuit (6.6A nominal via isolating transformer) |
| Input Voltage Range | 12V – 48V AC/DC (depending on transformer) |
| Output Current | 350mA, 700mA, 1A (programmable or fixed) |
| Output Voltage Range | 3V – 60V (depending on LED configuration) |
| Dimming Control | 5-step (detects CCR current levels) |
| Efficiency | >85% (typical) |
| Surge Protection | 4kV / 2kA (combination wave) |
| Protection Features | Over-voltage, over-current, over-temperature, short-circuit |
| Diagnostic Interface | Analogue voltage or PWM signal, or digital (I²C, DALI) for smart drivers |
| Operating Temperature | -40°C to +85°C |
| Protection Class | IP67 / IP68 (potting or sealed enclosure) |
Electrical Requirements
| Requirement | Typical Value |
|---|---|
| Circuit Position | Between isolating transformer secondary and LED module |
| Input | Constant current from AGL series circuit (6.6A nominal via transformer), 12V–48V AC/DC input |
| Output | Regulated current to LED module (350mA–1A typical) |
| Isolation | Provides electrical isolation between the primary series loop and the LED module |
| Surge Protection | 4kV / 2kA (combination wave) |
| Protection Features | Over-voltage, over-current, over-temperature, short-circuit shutdown |
| Dimming Response | Detects CCR current step and adjusts LED output current accordingly |
Applicable Standards
ICAO Annex 14
Photometric performance for LED AGL fixtures (driver impact on light output)
FAA AC 150/5345-46
Specification for LED AGL luminaires (includes driver requirements)
IEC 61827
Characteristics of inset and elevated AGL luminaires (electrical/photometric reference)
IES LM-79 / LM-80
Photometric measurement and long-term lumen maintenance testing feeding into driver output/current specification
Installation Requirements
Driver Mounting
MechanicalMount the driver on standoffs or a bracket inside the fixture's bottom housing, positioned clear of the hottest zone near the light source.
Isolating Transformer Connection
ElectricalConnect the driver's input to the isolating transformer's secondary on the series-circuit side, confirming voltage and current match the driver's rating.
Keyed Harness Connection
ElectricalConnect the driver to the LED module via the short, keyed or polarised wiring harness, preventing a reversed connection during installation or future maintenance.
Cable Gland Sealing
MechanicalRoute input and output leads through the sealed cable glands or grommets used elsewhere in the housing, keeping the electronics compartment at the fixture's rated IP level.
Pre-Energisation Sign-off
DocumentationRecord driver mounting, harness connection, and gland sealing as-built before the circuit is energised for output verification.
Testing & Commissioning
Insulation Resistance Test
Pre-EnergisationMegger-test the driver enclosure and wiring before the circuit is energised, confirming insulation resistance clears the acceptable minimum.
Output Current/Voltage Verification
EnergisationWith the CCR energised, confirm the driver's regulated output current and voltage match the LED module's specification at the commissioned step.
Dimming Step Response
EnergisationCycle the CCR through every intensity step and confirm the driver's output steps correctly and matches the corresponding LED module brightness.
Protection Function Check
EnergisationVerify surge and over-temperature protection functions operate as specified, using a functional test or manufacturer test certificate.
Enclosure Seal Verification
Final AcceptanceConfirm the driver enclosure maintains its rated IP protection once installed and all glands are sealed.
Documentation & As-Built Sign-off
Final AcceptanceRecord insulation, output, and protection test results in the commissioning dossier.
Inspection & Maintenance
Surge Protection Check
AnnualTest surge protection components and replace any that have degraded from repeated transient events.
Enclosure Seal Inspection
AnnualCheck driver enclosure seals and cable entries for degradation that could let moisture reach internal electronics.
Thermal Performance Check
Semi-annualVerify the driver is operating within its rated temperature range, since sustained overheating shortens component life.
Output & Dimming Verification
AnnualConfirm output current and dimming behaviour match specification, flagging drift before it affects the LED load.
Common Field Issues
Surge Damage
Lightning strikes and power surges are a primary cause of driver failure. Even with built-in surge protection, repeated surges can degrade components. Ensure proper earthing and consider external surge protectors in high-risk areas.
Moisture Ingress
Despite IP67/IP68 ratings, moisture can enter through cable glands or seal failures, causing corrosion and short circuits. Regular inspection of seals and cable entries is essential.
Overheating
If the driver is not properly heat-sinked or is covered by debris, it can overheat and reduce output or fail. Ensure adequate airflow and keep the driver housing clean.
Dimming Mismatch
If the driver does not correctly detect the CCR steps, the light intensity will be incorrect. Verify the driver's dimming calibration and compatibility with the specific CCR model.
Troubleshooting Guide
Driver Dead / No Output
Corrosion or Moisture Found Inside Driver Housing
Output Reduced or Driver Cycling On/Off
Light Intensity Wrong for the Selected CCR Step
Failure Modes & Root Causes
Surge Damage
Seal Failure / Moisture Ingress
Thermal Foldback / Overheating
Dimming Calibration Mismatch
Safety & Operational Notes
Series Circuit Isolation
Electrical SafetyLock out the CCR feeding the series circuit and verify zero energy before disconnecting or replacing the driver; the driver sits between the isolating transformer and the LED module and both sides must be confirmed de-energised.
ESD Precautions
Component HandlingUse anti-static precautions when handling the driver board; its regulation and diagnostic circuitry is sensitive to electrostatic discharge in a way a simple halogen lamp circuit is not.
Surge Exposure
Electrical SafetyConfirm surge protection is intact before re-energising, since the driver is a common failure point on circuits exposed to lightning-induced transients on the series loop.
Correct Output Current Setting
ComplianceVerify the replacement driver is configured for the correct output current (e.g., 350mA, 700mA, or 1A) matching the LED module it feeds; an incorrect setting can under-drive the light below intensity requirements or over-drive it into early failure.
Insulation Test Before Re-Energisation
Electrical SafetyAfter any driver replacement, megger test the affected series loop before restoring power to avoid tripping the CCR or damaging other fixtures on the circuit.
PPE
Personnel SafetyUse insulated tools when working on series-circuit connections.
Restore to Compliant State
ComplianceConfirm correct dimming response across all CCR intensity steps and any ALCMS diagnostic feedback before closing out the work.
Engineering & Expert Notes
Output Current Setting Errors Are Silent at Commissioning
ComplianceA driver configured to the wrong output current can still light the fixture and pass a casual "it turns on" check; the under-driven or over-driven condition only becomes apparent later as an intensity-tolerance failure or a shortened LED module lifespan, so verify the actual configured current against the module's rating at commissioning, not just visual operation.
Drivers Concentrate Surge-Related Failures on a Circuit
ReliabilityOn loops with a lightning-exposure history, the driver is often the first component to fail from induced transients, ahead of the LED module or the transformer, since its regulation circuitry is more sensitive to voltage spikes than a simple resistive load; if driver failures cluster on a specific loop, check upstream surge protection on that loop specifically rather than treating each failure as unrelated.
Dimming Response Curve Should Be Checked Across All Steps
VerificationA driver can perform correctly at full brightness while showing a non-linear or inconsistent response at intermediate CCR steps; verify dimming performance across the full range of intensity steps used operationally, not just the extremes, since a mismatched low-step response is easy to miss if only high and off states are checked.
Diagnostic Feedback Can Mask an Underlying LED Fault
DiagnosticsSome drivers report a healthy status to ALCMS even when the LED module they're feeding has partially failed, since the driver's self-diagnostics typically monitor its own output rather than confirming actual light emission; where a "no fault reported" fixture is visually dim or off-colour, treat the module as a suspect independently of what the driver's status feedback claims.
Ambient Temperature Affects Driver Life More Than Load Alone
ReliabilityA driver operating correctly within its rated current can still see shortened service life if mounted in a poorly ventilated housing that runs consistently hot; where nuisance driver failures cluster at a specific fixture design or location, check enclosure ventilation and ambient temperature before assuming the driver units themselves are defective.
Cross-Manufacturer Replacement Needs a Full Spec Check
ProcurementDrivers rated for the same nominal output current from different manufacturers don't always share identical dimming protocols, surge withstand ratings, or diagnostic reporting formats; verify full compatibility with the fixture's ALCMS interface before substituting a driver from a different supplier, not just the headline current rating.
Related Components
Frequently Asked Questions
Short answers about AGL LED drivers: regulation, dimming, protection, diagnostics, standards and maintenance.
Q1What is an LED driver in an AGL fixture?
An LED driver, also called an LED power supply or control gear, is the electronic device that provides the correct electrical conditions for the LED module. It takes the input from the AGL series circuit, typically 6.6 A constant current stepped down through an isolating transformer, and converts it to a regulated constant current such as 350 mA, 700 mA or 1 A.
Q2Why can't an LED module connect directly to the AGL series circuit?
LEDs are current-driven devices and need precise current regulation, unlike halogen lamps that run directly on the series circuit current. Without a driver to regulate current and translate CCR steps into the right LED brightness, the fixture could not deliver the graduated low, medium and high intensity settings pilots rely on.
Q3How does an LED driver handle dimming?
The driver detects the CCR step from the input current level and adjusts its output current to match. The CCR typically provides five steps at 2.8 A, 3.4 A, 4.1 A, 5.2 A and 6.6 A, and the driver output follows the selected step.
Q4What are typical LED driver specifications?
Typical output current is 350 mA, 700 mA or 1 A (programmable or fixed), with an output voltage range of 3 V to 60 V depending on the LED configuration. Efficiency is typically above 85%, operating temperature is -40°C to +85°C, and protection class is IP67 or IP68 through potting or a sealed enclosure.
Q5Where does the LED driver sit in the circuit?
It sits between the isolating transformer secondary and the LED module. It accepts a 12 V to 48 V AC/DC input depending on the transformer, and it provides electrical isolation between the primary series loop and the LED module.
Q6What protection functions does an LED driver provide?
It includes surge suppression using MOVs and TVS diodes (typically 4 kV / 2 kA combination wave), plus over-voltage, over-current, over-temperature and short-circuit protection. On overheating it reduces current or shuts down to protect the LED module, and it reports the fault to the ALCMS.
Q7Does the LED driver report diagnostics to the ALCMS?
Yes. It can provide status, output current, temperature and fault signals through an analogue voltage or PWM signal, or through digital interfaces such as I²C or DALI on smart drivers. Some drivers report a healthy status even when the LED module has partially failed, because they monitor their own output rather than confirming actual light emission, so a dim fixture should be checked visually regardless of the status feedback.
Q8Which standards apply to LED drivers in AGL fixtures?
The main references are ICAO Annex 14 (photometric performance), FAA AC 150/5345-46 (LED AGL luminaire specification, including driver requirements), IEC 61827 and IES LM-79/LM-80 (photometric measurement and lumen maintenance testing). Always verify the current edition and your aerodrome's approved requirements.
Q9How often should an LED driver be inspected and tested?
Typical intervals are annual for surge protection checks, enclosure seal inspection, and output and dimming verification, and semi-annual for thermal performance checks. Your airport's approved maintenance programme takes precedence.
Q10Why do LED drivers fail, and what are the common faults?
The most common causes are surge damage from lightning or repeated transients, moisture ingress through cable glands or seal failures, overheating from poor heat-sinking or debris, and dimming calibration mismatch with the CCR. On loops with a lightning history the driver is often the first component to fail, so clustered failures point to upstream surge protection on that loop.
Q11What should I check when replacing an LED driver?
Lock out the CCR and verify zero energy, confirm the replacement is set to the correct output current for the LED module, and megger test the series loop before re-energising. Then confirm the dimming response at every CCR step. If the replacement is from a different manufacturer, verify its dimming protocol, surge rating and ALCMS interface compatibility, not just the headline current rating.
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