How a phase-cut dimmer chops the AC waveform to control luminaire brightness
Standard 230V/120V sinusoidal AC at 50 or 60 Hz. This is the full, unmodified power waveform that feeds into the dimmer.
The TRIAC semiconductor chops each half-cycle of the sine wave. It delays firing by a "phase angle" — the larger the angle, the more of the wave is removed, reducing delivered power.
A phase-cut compatible driver detects the conduction angle and converts it to the appropriate constant-current DC level. This ensures smooth, flicker-free dimming.
The LED module receives regulated DC current proportional to the dim level. Less current = less photon emission = dimmer light.
| Property | Leading-Edge (Forward Phase) | Trailing-Edge (Reverse Phase) |
|---|---|---|
| Cut Position | Chops the start of each half-cycle | Chops the end of each half-cycle |
| Switch Type | TRIAC / SCR | MOSFET / IGBT transistor |
| Best For | Incandescent, halogen, magnetic transformers | LED drivers, electronic transformers |
| LED Compatibility | May cause flicker / buzz — needs compatible driver | Smoother dimming, less EMI, preferred for LEDs |
| Minimum Load | Often requires higher minimum load | Works well with low-wattage LED loads |
| Cost | Lower cost, widely available | Higher cost, better performance |