Both types dim by removing part of each mains cycle. They differ in which part, and that difference decides how an LED driver behaves.
Leading edge
Also called TRIAC or forward-phase dimming. It stays off at the start of each half cycle and switches on partway through, so the waveform begins with an abrupt vertical step.
This was the standard for decades because it suits resistive loads — filament bulbs and wirewound transformers. A filament has thermal inertia and does not care about the sharp switching edge.
That sharp edge is exactly the problem for electronics. It produces a current spike and electrical noise, which is why leading-edge dimmers buzz on LED loads and why the LED driver can be knocked out of regulation by it.
Trailing edge
Also called MOSFET or reverse-phase dimming. It switches on smoothly at the start of each half cycle and off partway through.
Because the turn-on is gradual, there is no inrush spike and far less noise. That suits capacitive and electronic loads — which is what an LED driver is.
Trailing-edge dimmers are quieter, usually dim more smoothly at the low end, and are what modern LED-rated dimmers use.
| Leading edge | Trailing edge | |
|---|---|---|
| Also called | TRIAC, forward phase | MOSFET, reverse phase |
| Cuts | Start of each half cycle | End of each half cycle |
| Designed for | Filament, wirewound transformer | Electronic drivers, LED |
| With LEDs | Buzzing, flicker, poor low end | Smooth, quiet |
| Audible noise | Common | Rare |
| Minimum load | Often high (40 W+) | Usually low (1–10 W) |
| Typical age | Pre-LED installations | Modern LED-rated |
How to tell which one you have
Look at the front. Modern LED-rated dimmers usually say so, and many say "trailing edge" or show an LED symbol.
Listen. A dimmer that buzzes audibly on an LED load, especially in the middle of its range, is almost certainly leading edge.
Check the age. Anything installed before LEDs were mainstream is leading edge unless it was replaced.
Check the minimum load. A high minimum — 40 W or more — points to leading edge. Modern LED dimmers commonly go down to a few watts.
Look for markings inside. Some are stamped with a symbol: a rising ramp for leading edge, a falling ramp for trailing edge.
Which to fit
Fit trailing edge for LEDs. It is the right answer in nearly every domestic case and it fits the same backbox as the dimmer you are removing.
The exceptions are narrow: wirewound (magnetic) transformers for low-voltage halogen genuinely want leading edge, and a small number of LED drivers are specified as leading-edge only. If a driver's datasheet says leading edge, believe it.
Universal dimmers handle both, sometimes with a switch or a learning routine. They are a reasonable choice when the load is mixed or unknown, though a dedicated trailing-edge unit usually gives a slightly better low end.
Fitting one is not the whole job
Two other things decide whether the result is smooth:
Total load against the dimmer's minimum. LED loads are tiny and easy to fall below. See minimum load for an LED dimmer.
All bulbs the same model. Different drivers respond differently to the same chopped waveform, and two brands that each work alone can flicker together.
If flicker persists after fitting a trailing-edge dimmer with an adequate load and matched dimmable bulbs, the bulbs themselves are the remaining suspect — work through why LEDs flicker when dimmed and dimmer compatibility.