The Semiconductor Physics: How LED Lights Work
At the component level, understanding how LED lights work requires looking at electroluminescence within a solid-state semiconductor. Unlike incandescent bulbs that rely on heating a tungsten filament until it glows (blackbody radiation), a Light Emitting Diode (LED) generates photons through electron-hole recombination. When a forward voltage is applied across the P-N junction, electrons from the N-type material cross the depletion region and recombine with holes in the P-type material. This transition drops the electrons from a higher energy conduction band to a lower valence band, releasing the energy difference as a photon.
The color of the emitted light is strictly determined by the semiconductor's bandgap energy. For example, Indium Gallium Nitride (InGaN) is used to create the blue light in modern white LEDs, which then passes through a cerium-doped YAG (Yttrium Aluminum Garnet) phosphor layer to convert a portion of the blue spectrum into yellow, resulting in perceived white light. According to the U.S. Department of Energy's Solid-State Lighting program, modern phosphor-converted white LEDs have pushed the theoretical efficacy limits far beyond legacy discharge and filament technologies.
When selecting fixtures, you must look beyond raw wattage and evaluate luminous efficacy (lumens per watt). The table below contextualizes how LED lights work to deliver equivalent illumination with drastically reduced thermal and electrical loads.
| Technology | Nominal Wattage | Luminous Efficacy (lm/W) | Heat Dissipation Profile |
|---|---|---|---|
| Incandescent | 60W | 13.3 lm/W | 90% Radiant IR (Forward Heat) |
| Halogen | 43W | 18.6 lm/W | 85% Radiant IR (Forward Heat) |
| CFL | 13W | 61.5 lm/W | Omnidirectional Convection |
| Standard LED (2020 spec) | 9W | 88.8 lm/W | Conductive (Junction to Heat Sink) |
| High-Efficacy LED (2026 spec) | 7W | 114.2 lm/W | Conductive (Requires Thermal Path) |
Driver Topologies, Power Factor, and Inrush Math
Because the P-N junction requires a precise, unidirectional current to prevent thermal runaway, LEDs cannot be connected directly to an AC mains supply. The driver is the critical intermediary that rectifies AC to DC and regulates the output. Drivers generally fall into two topologies: Constant Voltage (CV), typically 12V or 24V for LED strips, and Constant Current (CC), typically 350mA to 1050mA for high-power downlights and high bays.
When sizing branch circuits for commercial or multi-fixture residential LED arrays, continuous wattage is only half the story. You must calculate the circuit impact using Power Factor (PF) and inrush current.
A cheap, non-PFC (Power Factor Correction) LED driver might have a PF of 0.50. If the fixture draws 10W of Real Power (Watts), the Apparent Power (VA) drawn from the panel is calculated as:
VA = Watts / PF → 10W / 0.50 = 20VA
On a 15A, 120V branch circuit (1800VA capacity), you can only safely load 90 of these 10W fixtures, not 180, because the wiring must carry the 20VA apparent current, generating $I^2R$ heat in the conductors.
Inrush Current Constraints: LED drivers utilize input capacitors to smooth the rectified DC bus voltage. When AC voltage is applied at the peak of the sine wave, these empty capacitors act as a momentary short circuit. A 150W LED high bay fixture can exhibit an inrush spike of 45A for 200 microseconds. While a standard 15A C-curve breaker tolerates magnetic trips up to 10x rated current (150A), a B-curve breaker (tripping at 3-5x, or 45-75A) may nuisance-trip when switching on a bank of high-bay LEDs. Always consult the driver datasheet for the $I_{inrush}$ specification and use breakers with appropriate magnetic trip curves (usually C or D) for heavy LED loads.
Dimming Dynamics: Trailing Edge, Minimum Loads, and Flicker Fixes
Dimming LEDs is fundamentally different from dimming resistive incandescent loads. Standard leading-edge (TRIAC) dimmers chop the front of the AC sine wave. Because LED drivers have low capacitance and high reactance, this abrupt voltage chop causes the driver's internal circuitry to misinterpret the zero-crossing point, resulting in severe flickering or dropout.
For line-voltage phase dimming, you must use a trailing-edge (ELV/MOSFET) dimmer, which chops the back of the sine wave, allowing a softer voltage turn-off that the LED driver's bridge rectifier can process cleanly. Furthermore, modern smart switches and ELV dimmers have strict minimum load requirements.
A popular trailing-edge dimmer like the Lutron Diva C-L (DVCL-153P) has a maximum LED load of 150W, but a minimum load requirement of 15W (or roughly 2 to 3 standard LED bulbs). If you install this dimmer on a circuit with a single 9W LED vanity light, the driver will not draw enough holding current to keep the dimmer's internal TRIAC/MOSFET latched, causing the light to strobe or fail to turn on. Always sum your fixture wattages and verify they exceed the dimmer's minimum load threshold.
Which dimmer/driver for a specific fixture count? If you are wiring a kitchen with eight 12W recessed LED downlights (Total = 96W), select a trailing-edge dimmer rated for at least 150W LED, ensuring the total load easily clears the 15W minimum. If the fixtures are low-voltage (e.g., 24V tape lights under cabinets), bypass phase-cut dimming entirely. Use a 0-10V or PWM-compatible constant voltage driver paired with a low-voltage PWM dimmer module to achieve flicker-free 1% dimming without phase-angle math.
Why flicker happens and the fix: If a trailing-edge dimmer is correctly installed but ghost-flicker persists when the switch is off, it is usually caused by induced capacitance in long cable runs or a smart switch bleeding trickle current to power its internal Wi-Fi radio without a neutral wire. The fix is to install a dummy load (a 50k-ohm, 1/2W bleeder resistor) in parallel with the LED fixture at the junction box to absorb the stray leakage current.
Thermal Derating and Enclosure Constraints
A common misconception is that LEDs do not produce heat. While they do not emit infrared radiation forward (making the beam cool to the touch), the P-N junction itself generates significant conductive heat. If the junction temperature ($T_j$) exceeds the manufacturer's rated limit (typically 85°C to 105°C for commercial COB arrays), the LED experiences thermal droop—a reversible decrease in lumen output—and permanent acceleration of phosphor degradation.
According to EPA Energy Star guidelines, LED lifespan is rated using the L70 metric (the number of hours until lumen output degrades to 70% of initial output). Operating a fixture 10°C above its rated $T_j$ can halve its L70 lifespan, dropping it from 50,000 hours to 25,000 hours.
This physics reality dictates strict enclosure constraints:
- IC-Rated (Insulation Contact): Fixtures with this rating have internal thermal sensors and heavily finned housings designed to dissipate heat even when buried in R-38 cellulose or fiberglass batts. Never put a non-IC fixture in an insulated ceiling.
- Enclosed Fixture Ratings: Standard A19 LED bulbs are often not rated for fully enclosed glass globes. The trapped ambient air can push the internal driver's electrolytic capacitors past their 105°C boiling point, causing the bulb to fail in months rather than years. Always look for the "Suitable for Enclosed Fixtures" stamp on the bulb base for flush-mount or vapor-tight applications.
Frequently Asked Questions
How do LED lights work with standard AC mains voltage?
LEDs are inherently DC devices. To work on 120V or 230V AC mains, the bulb or fixture contains an internal driver circuit. This circuit first passes the AC through a bridge rectifier to convert it to pulsating DC. Next, a high-frequency switching regulator (usually a flyback or buck-converter topology) steps the high DC bus voltage down to the specific low voltage and constant current required by the LED chip array. In very cheap, low-wattage bulbs, a simple capacitive dropper circuit is used instead of a switching regulator, which sacrifices power factor and flicker performance to save component costs.
Why do my LED lights flicker when turned off on a smart switch?
This occurs primarily with "no-neutral" smart switches. These switches require a small amount of continuous current to flow through the lighting circuit to keep their internal Wi-Fi or Zigbee radios powered, even when the light is logically "off." With a high-impedance incandescent bulb, this trickle current does nothing. But with an LED driver, this tiny current slowly charges the driver's input capacitor. Once the capacitor reaches the driver's startup voltage threshold, the LED flashes briefly, discharging the capacitor, and the cycle repeats. The fix is either to upgrade to a smart switch that requires a neutral wire, or to wire a bypass capacitor/resistor across the live and neutral at the light fixture to divert the trickle current away from the LED driver.
How does temperature affect LED light output and circuit lifespan?
Temperature affects LEDs in two distinct ways. First, "thermal droop" causes an immediate, reversible drop in lumen output as the junction heats up; a fixture might output 1000 lumens at 25°C but only 850 lumens at 85°C. Second, prolonged high temperatures permanently degrade the cerium phosphor coating and the silicone encapsulant, causing the light to shift toward a harsh blue tint and permanently reducing total output. This is why high-quality commercial fixtures use massive aluminum heat sinks and remote-mount the driver electronics away from the LED heat source to maximize the L70 lifespan.
