How LED Works in AC Circuits: Drivers, Inrush, and Dimmer Selection

At the semiconductor level, an LED (Light Emitting Diode) generates photons through electroluminescence when electrons recombine with electron holes in a direct-bandgap PN junction. But on the jobsite or at the workbench, knowing how LED works means understanding it as a highly non-linear, capacitive DC load that you are trying to force onto a 120V/240V AC sine wave. If you treat an LED fixture like an incandescent bulb, you will trip breakers, burn out dimmers, and deal with relentless strobe-like flicker.

This guide bypasses the basic physics and focuses on the circuit-level realities of LED lighting: inrush current math, power factor penalties, thermal derating, and the exact dimmer-driver pairings required for a stable installation.

The PN Junction and the Constant Current Mandate

An LED’s voltage-current (V-I) curve is exponential, not linear. A typical high-power white LED has a forward voltage ($V_f$) of about 2.8V to 3.3V. Once you cross that threshold, a mere 0.1V increase in applied voltage can double the current flowing through the junction. If you drive an LED with a constant voltage source, minor fluctuations in your power supply or thermal runaway (where the LED heats up, its internal resistance drops, and it draws more current) will destroy the chip in seconds.

This is why commercial and high-end residential LED fixtures use Constant Current (CC) drivers. The driver’s job is to dynamically adjust its output voltage to maintain a rigid current (e.g., 350mA or 700mA) regardless of thermal shifts or line voltage sags. Cheap, direct-wire "AC-LED" bulbs bypass this by stringing dozens of tiny LEDs in series to match the AC peak voltage, but they suffer from severe 120Hz flicker and poor lifespan.

Circuit Impact Math: Inrush Current and Power Factor

Because LEDs require DC, the AC/DC driver contains a bridge rectifier and a bulk electrolytic capacitor to smooth the ripple. This creates two massive headaches for circuit sizing: inrush current and poor power factor.

The Inrush Multiplier

When you flip the switch, the AC voltage might be at its peak (170V for a 120V RMS line). The bulk capacitor looks like a dead short until it charges. According to the U.S. Department of Energy's SSL guidelines, LED driver inrush can be 100 to 300 times the steady-state operating current.

Power Factor (PF) Penalties

A purely resistive load (like a heater) has a PF of 1.0. Cheap LED drivers with passive rectification can have a PF as low as 0.5. This means to get 50W of real light output, the circuit must supply 100VA of apparent power. For large commercial runs, this triggers utility penalties. Always specify drivers with active Power Factor Correction (PFC) that guarantee a PF > 0.9 at 100% load.

Lumens, Watts, and Efficacy Equivalence

You cannot size an LED circuit using old incandescent wattage rules. The metric that matters is luminous efficacy (lumens per watt). While a 2015-era LED might have produced 80 lm/W, modern 2026 high-efficacy COB (Chip-on-Board) modules routinely exceed 180 lm/W, drastically reducing the required circuit ampacity.

Source context: Efficacy figures align with DesignLights Consortium (DLC) premium tier qualifications for modern solid-state lighting.

Dimmer Compatibility: Trailing Edge and Minimum Load

Flicker is the most common complaint in residential LED retrofits. It happens because standard incandescent dimmers use a TRIAC to chop the leading edge of the AC sine wave (Forward Phase). TRIACs require a minimum "holding current" to stay latched ON. Because LEDs draw so little current, the TRIAC misfires and drops out mid-cycle, causing the driver to rapidly cycle on and off.

The Fix: Trailing Edge and Dummy Loads

To fix flicker, you must match the dimmer topology to the driver’s input filter:

• Use Trailing Edge (ELV/Reverse Phase) Dimmers: These use MOSFETs instead of TRIACs to chop the trailing edge of the sine wave. They do not require a holding current and handle the capacitive input filters of LED drivers cleanly.
• Check the Minimum Load: Even ELV dimmers have a minimum wattage requirement to operate their internal logic. A dimmer rated for 15W minimum will strobe if you connect a single 9W LED bulb.
• The Dummy Load Solution: If your total fixture wattage is below the dimmer’s minimum, wire a bypass resistor (like the Lutron LUT-MLU) in parallel with the first fixture. It draws a few watts of purely resistive current to keep the dimmer’s logic stable.

Thermal Constraints and Enclosure Derating

LEDs do not project heat forward as infrared radiation; they conduct heat backward into the PCB. The maximum allowable junction temperature ($T_j$) is typically 85°C or 105°C. According to the Arrhenius equation, every 10°C increase in operating temperature above the 25°C baseline cuts the LED's lumen maintenance lifespan (L70) in half.

Enclosure Constraints: If you install an LED retrofit module into an older, airtight IC-rated recessed can, the ambient air inside the can can easily reach 65°C. If the driver is not rated for high-ambient environments, it will trigger thermal foldback—a protective circuit that intentionally dims the LEDs to reduce heat generation. Always verify the driver’s $T_a$ (ambient temperature) rating on the spec sheet. If $T_a$ max is 50°C, it cannot go inside a sealed recessed can without an external remote driver housing.

Decision Tree: Picking Your Driver and Dimmer

Stop guessing at the electrical supply counter. Use this exact decision path for a standard residential 120V dimmable LED run.