At the silicon level, a Light Emitting Diode (LED) is a P-N junction that emits photons when forward-biased. But on the workbench or inside a residential junction box, asking what's a LED really means dealing with a constant-current load driven by a switching power supply. Unlike legacy incandescent bulbs, which act as simple resistive loads, modern LED fixtures are non-linear, current-driven devices. This fundamental difference dictates everything from your breaker sizing to your dimmer selection.
The Core Circuit: Driver Math, Power Factor, and Inrush
An LED chip cannot be connected directly to a 120V AC mains supply; it requires a driver to rectify the AC to DC and regulate the current. This driver is a switch-mode power supply (SMPS), and it introduces two major circuit impacts that trip up DIYers and even some journeyman electricians: Power Factor (PF) and inrush current.
Power Factor (PF): A cheap, non-corrected LED driver might have a PF of 0.5. If you have a 10W LED fixture with a 0.5 PF, it draws 20 Volt-Amps (VA) of apparent power from the transformer, even though it only consumes 10W of real power. On a residential 15A branch circuit, this rarely matters. But in a commercial 277V lighting run with 150 fixtures, a low PF will severely undersize your feeder wire and cause voltage drop. Always specify drivers with a PF > 0.9 for commercial or heavy-residential loads.
Inrush Current: When you flip the switch, the bulk electrolytic capacitors inside the LED driver are completely discharged. For the first few milliseconds, they act like a dead short. A standard 15W LED downlight draws about 0.12A at steady state (120V nominal), but its inrush current can spike to 15A or more—over 100 times the running current.
Lumens, Watts, and Efficacy in Modern Fixtures
When replacing legacy fixtures, you must size the new LED based on lumen output, not wattage. However, looking at lumens without understanding efficacy (lumens per watt, or lm/W) and driver losses leads to undersized circuits or overheated cans. Modern 2026 chip-on-board (COB) and mid-power SMD LEDs routinely hit 180 lm/W at the bare-chip level, but once you account for optical losses, thermal droop, and driver inefficiency, the system efficacy drops.
The table below maps standard lumen targets to real-world 2026 LED wattages, including the hidden driver loss factor that generates heat inside your enclosure.
| Target Output (Lumens) | Incandescent Eq. | Modern LED Wattage | System Efficacy (lm/W) | Driver Loss Factor |
|---|---|---|---|---|
| 450 lm | 40W | 3.8W | 118 lm/W | 12% |
| 800 lm | 60W | 6.5W | 123 lm/W | 10% |
| 1100 lm | 75W | 8.8W | 125 lm/W | 9% |
| 1600 lm | 100W | 12.5W | 128 lm/W | 8% |
| 2600 lm | 150W | 19.5W | 133 lm/W | 7% |
Source context: System efficacy data aligns with current DOE Solid-State Lighting performance benchmarks, factoring in real-world thermal droop at 85°C junction temperatures.
Dimmer Compatibility: Trailing Edge and Minimum Load
Because an LED is a solid-state diode, it does not possess the thermal inertia of a glowing tungsten filament. If you chop the AC sine wave using a legacy leading-edge TRIAC dimmer, the LED will turn off instantly every time the voltage crosses zero, resulting in severe flickering or audible buzzing from the driver's inductors.
Which dimmer/driver for this fixture count?
For standard residential 120V retrofits, you must use a trailing-edge (ELV) dimmer, such as the Lutron MACL-153M or Leviton DDMX. These use MOSFETs or IGBTs to chop the back half of the sine wave, providing a smoother turn-off that the LED driver's input capacitors can filter cleanly.
The Minimum Load Trap:
Every dimmer has a maximum rating (e.g., 150W for LED) and a minimum load requirement (often 10W to 15W). If you install a trailing-edge dimmer to control three 2W under-cabinet LED pucks (6W total), the total load falls below the dimmer's minimum threshold. The dimmer's internal circuitry will fail to latch properly, causing the lights to 'ghost' (glow faintly when off) or strobe at low dimming levels. Always sum your fixture wattages and verify they exceed the dimmer's minimum load spec.
Why flicker happens at low levels (and the fix):
Flicker at the bottom 10% of the dimming range usually happens because the TRIAC or MOSFET holding current drops below the driver's bleed requirement. The driver attempts to shut down, the voltage rebounds, and it fires again in a rapid loop.
The Fix: Install a bypass capacitor (like the Lutron LUT-MLC) across the line and neutral of the first fixture on the run. This provides the necessary bleed current to keep the dimmer's electronics stable at low loads. For commercial builds, bypass phase-cut dimming entirely and use a 0-10V DC dimming signal paired with a compatible constant-current driver.
Thermal Constraints and Enclosure Derating
A common misconception is that LEDs don't produce heat. While they don't project infrared (IR) heat forward like a halogen bulb, the P-N junction and the driver circuit generate significant conductive heat. If the junction temperature ($T_j$) exceeds the manufacturer's rating (typically 85°C to 105°C), the fixture will suffer from 'thermal droop'—a permanent shift in color rendering index (CRI) and a severe reduction in lumen output.
The real point of failure in an LED fixture is rarely the diode itself; it is the electrolytic capacitors inside the driver. According to the Arrhenius equation, the operational lifespan of an electrolytic capacitor is halved for every 10°C rise in ambient temperature. A driver rated for 50,000 hours at 45°C might fail in less than 12,000 hours if baked at 75°C.
Enclosure Constraints:
When installing high-output LEDs (15W+) in IC-rated (Insulation Contact) recessed cans, the enclosure traps heat. You must ensure the fixture's thermal management system (usually an aluminum heat sink bonded to the COB array with thermal interface material) is rated for enclosed spaces. If you are building custom fixtures, never pot a driver in a sealed plastic junction box without calculating the thermal resistance ($\theta_{JA}$). For enclosed architectural slots, always specify drivers with a 'Type TL' or enclosed-fixture rating, which use higher-temperature polymer capacitors or derate their maximum output current to protect the internal components.
