Few lighting upgrades feel more disappointing than installing sleek new LED bulbs on a dimmer, sliding the control down, and watching them flicker, stutter, or hum like a tired refrigerator. The bulbs are usually not defective. The problem is almost always a mismatch between how the dimmer was designed to work and how modern LEDs actually draw power. Once you understand that mismatch, most flicker and buzz problems become straightforward to diagnose and fix.
Why old dimmers and new bulbs disagree
Dimmers do not lower the voltage the way a faucet lowers water flow. Instead, they switch the power on and off many times per second and vary how much of each cycle the current is allowed to pass. This is called phase cutting. The dimmers in most older homes were designed for incandescent bulbs, which behaved like simple, hungry resistors. An incandescent bulb of 60 watts pulled a steady, heavy load that gave the dimmer plenty of current to work with, so it dimmed smoothly all the way down.
An LED that produces the same brightness might draw only eight or nine watts. From the dimmer’s point of view, that is almost nothing. The circuitry inside the dimmer can lose track of such a light load, especially near the bottom of the range, and the result is visible flicker, sudden drop-outs, or a narrow usable band where only the top third of the slider does anything. The bulb is faithfully following the erratic signal it is being handed.
Leading edge versus trailing edge
Phase-cutting dimmers come in two families, and the distinction matters far more with LEDs than it ever did with incandescents. Leading-edge dimmers, the older and cheaper type, cut the power at the start of each electrical cycle. They were built for high-wattage resistive and magnetic loads and often struggle with the small, sensitive electronics inside an LED. Trailing-edge dimmers cut power at the end of each cycle, produce a gentler transition, and were on .