An LED (Light Emitting Diode) works by electroluminescence: when forward-biased, electrons recombine with electron holes across a semiconductor p-n junction, releasing energy as photons. But on a workbench or in a junction box, an LED is not just a light source; it is a highly non-linear, temperature-sensitive current device. You cannot apply a constant voltage directly to a raw LED without a current-limiting mechanism, or it will draw excessive current, overheat, and fail catastrophically. To use them in AC mains circuits, we rely on constant-current drivers, which introduces complex power factor, inrush, and dimming challenges.
The Semiconductor Junction and Thermal Constraints
At the silicon level, a white LED is typically a blue indium gallium nitride (InGaN) chip coated with a yellow cerium-doped YAG phosphor. The forward voltage ($V_f$) of a standard white LED sits between 2.8V and 3.4V. Because the voltage-current curve is exponential, a tiny 0.1V increase in forward voltage can double the current draw. This is why U.S. Department of Energy Solid-State Lighting guidelines mandate constant-current drivers rather than constant-voltage power supplies for high-power lighting.
Unlike incandescent bulbs that radiate heat forward as infrared, LEDs conduct heat backward through their substrate. The critical metric is the junction temperature ($T_j$). Most commercial LEDs are rated for a maximum $T_j$ of 85°C to 105°C. For every 10°C rise in junction temperature above the 25°C testing baseline, lumen output drops by 5% to 8%, and the operational lifespan is effectively halved.
Heat and Enclosure Constraints
When installing high-output LEDs (like COB downlights or high-bay fixtures), the enclosure must act as a heatsink. Raw LEDs must be mounted to an MCPCB (Metal Core Printed Circuit Board) with a dielectric thermal interface layer, which is then bonded to an aluminum extrusion. If you install a high-wattage LED retrofit into an enclosed, non-IC-rated fixture without adequate convective airflow, the ambient temperature inside the housing will exceed the driver’s thermal shutdown threshold (usually around 75°C to 85°C), causing the fixture to blink or shut off entirely.
When selecting fixtures, looking at raw lumens is a trap. You must evaluate luminous efficacy (lm/W), which tells you how much electrical power is converted to light versus waste heat. The table below provides real-world equivalence data with efficacy context to help you calculate both the lighting output and the thermal load imposed on your enclosure.
| Fixture Type | Nominal Watts | Efficacy (lm/W) | Total Lumens | Thermal Load (BTU/hr) |
|---|---|---|---|---|
| Incandescent A19 | 60W | 14 | 840 | 204 |
| Halogen PAR38 | 90W | 16 | 1440 | 307 |
| Standard LED A19 | 9W | 88 | 800 | 30 |
| High-Efficacy LED A19 | 7.5W | 115 | 860 | 25 |
| Commercial COB Downlight | 24W | 105 | 2520 | 81 |
Circuit Impact Math: Drivers, Power Factor, and Inrush
An LED driver converts AC mains into a regulated DC current. However, the input stage of these drivers profoundly impacts the branch circuit, specifically regarding Power Factor (PF) and inrush current.
Power Factor and Branch Circuit Sizing
Power Factor is the ratio of Real Power (Watts) to Apparent Power (Volt-Amps, VA). Cheap, non-power-factor-corrected (non-PFC) LED drivers often have a PF of 0.5 to 0.6. High-quality commercial drivers achieve a PF of 0.9 or higher.
Why does this matter for circuit sizing? Breakers and wire ampacity are limited by current (Amps), which is dictated by Apparent Power, not Real Power.
The Math: If you connect 50W of LED panels with a PF of 0.5 to a 120V circuit, the real power is 50W, but the apparent power is 100VA. The current draw is $100VA / 120V = 0.83A$, not the 0.41A you would expect from the wattage alone. On a standard 15A breaker (derated to 12A for continuous lighting loads per NEC Article 210.20), a 0.5 PF driver cuts your maximum fixture count in half compared to a 0.95 PF driver.
Inrush Current and Breaker Tripping
LED drivers use a bulk electrolytic capacitor on the DC bus to smooth out AC ripple. When you flip the switch, this capacitor acts as a dead short for the first few hundred microseconds, drawing massive inrush current. A single 150W commercial driver can pull 30A to 50A of inrush current for 200µs.
The Fix: Limit high-wattage drivers to 6–8 per 20A breaker, use a zero-crossing solid-state contactor to stagger the switching, or specify drivers with built-in NTC thermistors for inrush limiting.
Dimmer Compatibility, Fixture Counts, and Flicker Fixes
Dimming an LED is not as simple as lowering the voltage. Because the driver is actively regulating current, reducing the RMS voltage often just causes the driver to drop out or flicker. According to Lighting Design Lab guidelines, successful dimming requires matching the dimmer’s phase-cut topology to the driver’s input circuitry.
Which Dimmer and Driver for Your Fixture Count?
There are two primary phase-cut dimming topologies for AC mains:
- Leading-Edge (TRIAC): The traditional standard for incandescent bulbs. It chops off the beginning of the AC sine wave. These require a minimum load (typically 10W to 40W) to keep the internal TRIAC latched in the "on" state.
- Trailing-Edge (ELV / MOSFET): Chops off the end of the AC sine wave. These use electronic switches (MOSFETs/IGBTs) that do not require a minimum load to latch, making them vastly superior for low-wattage LED circuits.
Sizing the Dimmer: If you are installing six 12W LED downlights (72W total real power) on a leading-edge dimmer rated for 600W incandescent, it will likely fail. The 600W rating drops to roughly 150W for LED loads due to inrush and thermal limits, but more importantly, 72W might fall below the dimmer's specific minimum hold-current threshold if the PF is poor. Always choose a trailing-edge (ELV) dimmer like the Lutron DVELV-300P for LED circuits, and verify that the total LED wattage exceeds the dimmer's stated minimum load (often 2W to 5W for modern ELV models).
Why Flicker Happens and How to Fix It
Flicker in LED circuits is almost always a mismatch between the dimmer's leakage current and the driver's minimum operating threshold. Here is the diagnostic decision path:
- Symptom: Flickering only at the lowest dimmer setting.
Cause: The phase-cut angle is too deep, and the driver's internal logic is browning out on every half-cycle.
Fix: Adjust the dimmer's low-end trim potentiometer up slightly, or configure the driver's dip-switches to a higher minimum output current. - Symptom: Strobe effect or flashing when the dimmer is turned completely OFF.
Cause: A leading-edge TRIAC dimmer is leaking a tiny amount of current (a few milliamps) through its internal snubber circuit. This slowly charges the LED driver's bulk capacitor until it has enough energy to flash the LED, discharge, and repeat.
Fix: Install a bypass capacitor/resistor module (like the Lutron LUT-MLC) directly across the Line and Load at the first fixture. This provides a dedicated path for the leakage current, preventing the capacitor from charging. - Symptom: Constant high-frequency ripple flicker (visible on smartphone cameras).
Cause: The LED driver has poor output filtering (high ripple current) or is being operated on a cheap, incompatible smart switch utilizing PWM on the AC line.
Fix: Replace the driver with a flicker-free model specifying <5% output ripple, or switch to a 0-10V DC dimming topology if the fixture supports it, completely bypassing AC phase-cut issues.
Understanding the LED as a non-linear semiconductor rather than a simple resistive bulb is the key to designing reliable lighting circuits. By calculating apparent power, managing thermal junction limits, and matching trailing-edge dimmers to low-wattage loads, you eliminate the vast majority of field failures and callbacks.
