When homeowners and apprentices ask about LED how it works, they usually expect a lesson in semiconductor physics and P-N junction electron-hole recombination. But on the bench or the jobsite, the diode itself is only 10% of the story. In a 120V or 230V AC environment, an LED fixture is entirely defined by its driver circuitry. An LED is a constant-current device; feed it constant voltage, and its forward voltage drop ($V_f$) will plummet as it heats up, causing thermal runaway and instant failure. Therefore, understanding how an LED works in practice means understanding switched-mode power supplies (SMPS), power factor correction (PFC), and non-linear load behavior.
The Core Circuit: Efficacy, Watts, and Lumens
The raw light output of an LED chip is measured in lumens, but the electrical cost to produce those lumens is dictated by the driver's efficacy. Modern high-efficacy LEDs (as of 2026) easily surpass 180 lumens per watt (lm/W) at the chip level, though system-level efficacy (including driver losses) typically lands between 120 and 150 lm/W. When replacing legacy lighting, you cannot simply match wattages; you must match lumen output while accounting for the massive drop in heat dissipation.
| Lighting Technology | Nominal Wattage | Output (Lumens) | System Efficacy (lm/W) | Heat Dissipation (% of Input) |
|---|---|---|---|---|
| Incandescent (A19) | 60W | 800 lm | 13.3 lm/W | ~90% (Infrared/Radiant) |
| Halogen (PAR38) | 90W | 1,300 lm | 14.4 lm/W | ~85% (Infrared/Radiant) |
| Compact Fluorescent (CFL) | 15W | 800 lm | 53.3 lm/W | ~60% (Convection/Conduction) |
| Standard LED (2020-era) | 9W | 800 lm | 88.8 lm/W | ~40% (Conduction to Heatsink) |
| High-Efficacy LED (2026) | 5.5W | 850 lm | 154.5 lm/W | ~25% (Conduction to Heatsink) |
Circuit Impact Math: Power Factor and Inrush Current
Because an LED driver rectifies AC to DC and stores energy in a bulk electrolytic capacitor, it presents a highly non-linear load to the grid. This creates two major circuit design challenges: poor Power Factor (PF) and massive inrush current.
Power Factor (PF) and Apparent Power
Cheap, non-PFC LED drivers draw current in sharp spikes at the peak of the AC voltage waveform, resulting in a Power Factor as low as 0.5. This means a 10W LED fixture might draw 20VA (Volt-Amps) of apparent power. While residential utility meters only bill for real power (Watts), the wiring and breakers must be sized for the apparent current. If you wire 20 fixtures with a 0.5 PF on a single 15A branch circuit, your neutral conductor can overheat due to triplen harmonic currents stacking up, even if the real power draw seems safely within limits.
Inrush Current and Breaker Tripping
When you flip a switch, the driver's bulk capacitor looks like a dead short until it charges. The inrush current ($I_{inrush}$) is limited only by the Equivalent Series Resistance (ESR) of the capacitor and the trace resistance of the PCB.
Consider a 120V AC circuit. The peak voltage is $120 \times \sqrt{2} \approx 169V$. If the driver's internal resistance is $0.5\Omega$, the inrush current per fixture is:
$I_{inrush} = \frac{169V}{0.5\Omega} = 338A$
If you switch on 10 of these fixtures simultaneously via a single smart relay or contactor, the instantaneous inrush is 3,380 Amps. A standard 15A Type B circuit breaker has a magnetic trip threshold of 3x to 5x its rating (45A to 75A). The 3,380A spike will instantly trip the magnetic latch, even though the steady-state draw is only a few amps.
Dimmer Compatibility and Fixture Count Limits
Dimming an LED is not as simple as lowering the voltage. Dimming requires either Pulse Width Modulation (PWM) at the driver level or phase-cutting the AC waveform at the switch. The most common point of failure in DIY and retrofit projects is mismatching the dimmer topology to the driver type.
Which Dimmer for Which Fixture Count?
Legacy dimmers use Leading-Edge (TRIAC) topology, designed for resistive incandescent loads. Modern LED drivers require Trailing-Edge (ELV/MOSFET) dimmers. Furthermore, you must calculate the fixture count based on the dimmer's minimum load requirement, not just its maximum.
| Criteria | Leading-Edge (TRIAC / Incandescent) | Trailing-Edge (ELV / Electronic Low Voltage) |
|---|---|---|
| Switching Mechanism | Silicon Controlled Rectifier (SCR) / TRIAC | MOSFET or IGBT transistors |
| Minimum Load | High (Typically 25W - 40W real power) | Low (Typically 5W - 10W real power) |
| Max LED Fixture Count | Divide max wattage by 5 (e.g., 600W dimmer = 120W LED max) | Divide max wattage by 1.2 (e.g., 300W dimmer = 250W LED max) |
| Best Application | Legacy retrofits with mixed incandescent/LED loads | Pure LED circuits, low-wattage smart lighting |
Why Flicker Happens and the Fix
If you install three 5W LED bulbs (15W total) on a legacy 600W TRIAC dimmer, the lights will likely strobe or flicker violently at low dim levels. This happens because a TRIAC requires a minimum holding current (usually 10mA to 50mA) to stay latched in the "on" state. Highly efficient LED drivers draw current in such short, sharp pulses that the current drops below the TRIAC's holding threshold before the AC waveform reaches the zero-crossing point. The TRIAC turns off prematurely, the driver's capacitor recharges, and the cycle repeats at 120Hz, causing visible flicker.
The Fixes:
- Upgrade the Dimmer: Swap the TRIAC dimmer for a Trailing-Edge (ELV) dimmer, which uses MOSFETs that do not rely on load current to maintain their state.
- Add a Dummy Load: If you must keep the TRIAC dimmer, wire a dummy load resistor (e.g., a 50kΩ 2W resistor or a commercial "LED load compensator" module) in parallel across the Line and Load terminals to draw enough bleed current to keep the TRIAC latched.
- Check for PWM Interference: If the flicker looks like a rolling wave on camera, the driver's internal PWM frequency (often 200Hz - 500Hz on cheap drivers) is beating against your camera's shutter speed. The fix is to upgrade to a high-frequency PWM driver (>20kHz) or a constant-current reduction (CCR) analog dimming driver.
Thermal Constraints and Enclosure Derating
A common misconception is that because LEDs don't emit infrared heat in their light beam like halogens do, they don't produce heat. In reality, while the light beam is cool, the semiconductor junction ($T_j$) and the driver components generate significant conductive heat. The efficacy and lifespan of an LED are inversely proportional to its junction temperature. For every 10°C rise in $T_j$ above the rated baseline, the expected lifespan of the phosphor and the silicon is roughly halved.
Enclosed Fixture Derating
When installing LED retrofit kits or integrated fixtures into enclosed recessed cans (like older 6-inch IC-rated housings), the ambient temperature inside the enclosure can easily exceed 50°C (122°F). Most standard LED drivers are rated for a maximum ambient temperature ($T_a$) of 40°C to 45°C.
If you place a 40°C-rated driver in a 55°C enclosure, the driver's internal thermal protection will trigger, causing the light to "breathe" (dim down to 50% output to cool off, then brighten, then repeat). To prevent this:
- Verify the Rating: Only use LED fixtures explicitly marked "Suitable for Enclosed Fixtures." These utilize drivers rated for $T_a$ = 60°C or higher, and often feature potted (epoxy-encased) driver boards for superior thermal mass and heat transfer to the metal housing.
- Derate the Circuit: If you are designing a custom fixture or using a remote driver, derate the driver's maximum output current by 15-20% when it will be housed in an insulated, enclosed space. A driver rated for 1000mA should be jumpered or programmed to output 800mA to ensure the thermal headroom survives the enclosure's ambient heat.
- Separate the Heat Sources: In high-end architectural lighting, use remote drivers. Mount the LED engine in the ceiling aperture for optical performance, but run low-voltage DC wires (e.g., 24V or 48V) back to a central, ventilated driver enclosure in the attic or mechanical room.
Mastering how an LED works in a real-world circuit means respecting the math. By sizing your breakers for inrush, matching your dimmer topology to the driver's minimum load, and respecting thermal derating in enclosed spaces, you eliminate the flickering, tripping, and premature failures that plague poorly designed solid-state lighting installations.
For further reading on solid-state lighting performance standards, refer to the Department of Energy's Solid-State Lighting program. For electrical code compliance regarding branch circuit sizing and breaker selection, always consult the latest edition of the National Electrical Code (NFPA 70) and your local Authority Having Jurisdiction (AHJ).
