If you are asking what is a TRIAC dimmer, the direct answer is that it is a solid-state, forward phase-cut switch that dims lights by chopping off the leading edge of the AC sine wave. TRIAC (Triode for Alternating Current) dimmers are the legacy standard for incandescent and halogen bulbs. While they are cheap and ubiquitous, pairing them with modern LED fixtures requires careful math regarding minimum load, inrush current, and power factor to avoid strobe-like flickering, dropped circuits, or melted switch yokes.
The Physics of a TRIAC: Forward Phase-Cut Explained
Inside a standard wall dimmer, a TRIAC acts as a high-speed electronic switch. On a 60Hz AC line, the voltage crosses zero 120 times per second. A TRIAC waits for a specific delay after each zero-crossing, then 'fires' (turns on) and conducts current until the next zero-crossing, where it naturally turns off. By delaying the turn-on point, the dimmer chops off the front (leading edge) of the sine wave, reducing the total RMS voltage delivered to the load.
This works flawlessly with resistive loads like incandescent filaments. However, LED drivers are capacitive or inductive switching power supplies. According to the Lighting Controls Association, when a TRIAC chops a sine wave feeding a capacitive LED driver, the sudden voltage step at the firing angle causes massive current spikes. If the driver's input filter isn't designed for leading-edge phase-cut, this results in audible buzzing, thermal stress on the driver capacitors, and visible flicker.
Lumens, Watts, and the LED Efficacy Reality Check
When sizing a dimmer, many DIYers look at the lumen output of the bulb and guess the wattage. This is a mistake. You must look at the driver's actual input wattage and its efficacy (lumens per watt). High-efficacy LEDs draw so little real power that they often fail to meet the TRIAC's minimum load requirement.
| Fixture Type | Incandescent Equiv. | Actual LED Watts | Output (Lumens) | Efficacy (lm/W) | TRIAC Load Impact |
|---|---|---|---|---|---|
| Standard A19 Bulb | 60W | 9W | 800 | 88 | Low: May fall below min-load threshold |
| High-Efficacy A19 | 60W | 5.5W | 800 | 145 | Critical: Will cause drop-out flicker on legacy TRIACs |
| 6-inch Downlight | 65W BR30 | 11W | 680 | 61 | Moderate: Safe if 4+ fixtures are on one switch |
| Under-Cabinet Tape | N/A | 24W | 1800 | 75 | High: Easily meets min-load, watch inrush current |
Efficacy Context: A 145 lm/W bulb is highly efficient, but that efficiency is the enemy of legacy dimming. If you put four 5.5W bulbs on a circuit, your total load is 22W. Many older TRIACs require a 40W minimum load to keep the internal semiconductor latched. The Department of Energy's Solid-State Lighting program notes that this mismatch is the primary cause of residential LED dimming failures.
Circuit Impact Math: Inrush Current and Power Factor
To properly size a dimmer for a multi-fixture circuit, you cannot just add up the wattages on the box. You must calculate apparent power (VA) and account for inrush current.
1. Power Factor (PF) and Apparent Power
TRIACs and breakers must handle the total current flowing through the wire, which is dictated by apparent power (Volt-Amps), not just real power (Watts). The formula is:
VA = Watts / Power Factor
If you have ten 15W LED downlights with a poor power factor of 0.65:
- Real Power = 150W
- Apparent Power = 150W / 0.65 = 230VA
- Current Draw = 230VA / 120V = 1.91 Amps (Not the 1.25 Amps you'd expect from 150W).
Always size your dimmer and 14 AWG branch circuit wiring based on the VA rating, not the wattage.
2. The Inrush Current Multiplier
When you flip a dimmer on, the LED driver's input capacitors are completely discharged and look like a dead short for the first few milliseconds. Inrush current can be 100x to 200x the steady-state current. Ten fixtures pulling 1.91A steady-state can generate a combined inrush spike of 1,500+ Amps for a fraction of a millisecond. Standard incandescent TRIACs will fuse their internal silicon under this stress. You must use a dimmer specifically rated for 'LED Inrush' (often labeled with a specific LED wattage limit, like 150W LED / 600W Incandescent).
Why LEDs Flicker on TRIACs (And the Exact Fixes)
If your LEDs are strobing, buzzing, or shutting off when dimmed low, you are experiencing one of two specific failure modes:
Cause: TRIACs require a minimum 'holding current' (usually 20mA to 50mA) to stay latched on. At low dim levels, high-efficacy LEDs draw less current than this threshold. The TRIAC turns off prematurely, misfires on the next half-cycle, and creates a 120Hz strobe effect.
Fix: Install a bypass resistor (like the Lutron LUT-MLC) across the Line and Load terminals at the first fixture. This provides a dummy 8mA-12mA load to keep the TRIAC latched without generating noticeable heat.
Cause: The LED driver's internal EMI filter is ringing (oscillating) when the TRIAC fires, causing the driver's internal logic to reset or misinterpret the phase angle.
Fix: The driver is incompatible with leading-edge phase cut. Swap the TRIAC for an ELV (Electronic Low Voltage) trailing-edge dimmer, which chops the back of the sine wave and ramps down gently, eliminating the harsh voltage step.
Heat, Enclosures, and Minimum Load Constraints
Dimmers dissipate heat. A TRIAC operating at 400W will generate roughly 4W to 6W of waste heat inside the wall box. When you install dimmers in a multi-gang box (e.g., three switches side-by-side), the heat pools and can trigger the dimmer's internal thermal shutdown.
Enclosure Derating Rules:
- Single Gang Box: A 150W LED-rated dimmer can run at its full 150W capacity.
- Multi-Gang Box (No fins removed): Derate the maximum LED load by 25%. A 150W dimmer is now limited to 112W of LED load.
- Multi-Gang Box (Side fins removed): Derate the maximum LED load by 50%. That same 150W dimmer is now limited to just 75W of LED load.
If your calculated LED load exceeds the derated limit, you must upgrade to a higher-capacity dimmer (like a 250W LED model) and derate that, or split the fixtures across two separate switch loops.
The Decision Matrix: Pick Your Dimmer and Driver
Stop guessing. Use this decision tree to select the exact hardware for your lighting circuit based on your fixture count and driver specs.
| Circuit Condition | Total LED Wattage | Driver Power Factor (PF) | Required Dimmer Type | Concrete Hardware Pick |
|---|---|---|---|---|
| 1-2 High Efficacy Bulbs | < 15W | > 0.9 | ELV (Trailing Edge) + Bypass Resistor | Lutron DVELV-300P + LUT-MLC Resistor |
| Standard Residential Room (4-10 bulbs) | 15W - 120W | > 0.85 | LED-Rated Forward Phase (TRIAC) | Lutron Diva LED+ (DVCL-153P) |
| High-Density Downlights / Tape | 120W - 250W | > 0.9 | Heavy Duty LED TRIAC (No derating) | Leviton Decora Smart DW6HD-1BZ |
| Commercial / Poor PF Drivers | Any | < 0.7 | 0-10V Dimming (Requires neutral and low-voltage control wire) | Lutron NTSTV-DV + 0-10V LED Driver |
The Default Recommendation
If you are wiring a standard residential room with 4 to 8 integrated LED downlights or screw-in bulbs, and you have verified the driver PF is 0.85 or higher, the default pick is the Lutron Diva LED+ (Model DVCL-153P). It features an adjustable low-end trim potentiometer that lets you manually set the drop-out threshold, bypassing the most common flicker issues without needing external resistors. Pair it with LED drivers that explicitly state 'TRIAC/Forward Phase Compatible' on the spec sheet, such as the Hatch Lighting LTP12 series, to guarantee smooth 1% dimming.
