Two dimmer technologies dominate the UK market: leading edge (the older type, designed for incandescent and halogen bulbs) and trailing edge (designed for LED bulbs and electronic transformers). Pick the wrong one and you get buzzing, flickering, a poor dim range, and reduced bulb life. Pick the right one and the lights just work.

This guide explains what each technology does, which UK lamp types each one supports, how to identify what you currently have, and when to switch from one to the other.

Quick answer

How a leading-edge dimmer works

A leading-edge dimmer uses a TRIAC (triode for alternating current) to chop off the beginning of each AC half-cycle. The lamp sees a sudden voltage rise from zero up to wherever the TRIAC fires, then the rest of the half-cycle as normal. The earlier the TRIAC fires, the more energy reaches the lamp and the brighter the light.

This abrupt, sharp-edged waveform is fine for the simple resistive load of an incandescent bulb or a halogen lamp – they do not care how the voltage arrives, only how much average power they receive. It was the standard UK dimmer technology for decades, when nearly every domestic lamp was incandescent or halogen.

Leading-edge dimmers are also compatible with magnetic low-voltage transformers (the old wire-wound transformers used with 12 V MR16 halogens). The transformer’s inductance smooths the sharp edge before it reaches the lamp.

Where leading-edge falls down: the sharp leading edge excites the electronic drivers inside LED bulbs, producing audible buzz, visible flicker, and a dim range that does not go very low. Most leading-edge dimmers also have a relatively high minimum load (around 25–40 W of incandescent equivalent), which is more than a single small LED bulb provides.

How a trailing-edge dimmer works

A trailing-edge dimmer uses a MOSFET (metal-oxide-semiconductor field-effect transistor) to chop off the end of each AC half-cycle. The lamp sees the natural rise of the AC waveform from zero up to its peak, then a controlled, gentle cut-off before the half-cycle would have ended naturally. The later the MOSFET cuts off, the more energy reaches the lamp.

This soft-edged waveform suits the capacitive loads inside modern LED drivers and electronic transformers. The LED driver sees a gradual voltage change rather than a sudden step, so it stays quiet and dims smoothly down to very low brightness.

Trailing-edge dimmers typically have a much lower minimum load than leading-edge (often 3–10 W), which makes them suitable for small LED installations – even a single 5 W bulb on its own circuit.

Where trailing-edge falls down: they are not compatible with magnetic low-voltage transformers for halogen MR16 setups – a leading-edge dimmer is needed there. They are also slightly more expensive than leading-edge because of the additional electronics inside.

UK lamp compatibility at a glance

Manufacturer compatibility data on the LED bulb packaging trumps the table above – some LEDs are specifically rated for leading-edge, others only trailing-edge. Always check the bulb manufacturer’s recommendation if you have a mixed or unusual installation.

How to identify your existing dimmer

Pull the dimmer out of its back box (with the circuit isolated and proved dead) and look at the rating sticker or the engraved markings:

• Markings “TRIAC”, “LE”, “leading edge”, or “for incandescent / halogen only” – leading-edge dimmer.
• Markings “MOSFET”, “TE”, “RC”, “trailing edge”, or “for LED / electronic transformer” – trailing-edge dimmer.
• Markings “universal”, “MLV/ELV”, “all load types”, or with separate LE / TE selector switches – universal dimmer.
• Marked only with an incandescent wattage (e.g. “60–400 W”) and silent on LED – assume leading-edge.

Some dimmers have a small switch or selector behind the dial that lets you change between LE and TE modes manually – useful when retrofitting a single dimmer position from halogen to LED without buying new hardware.

A note on universal dimmers

Modern universal dimmers detect the load type when first powered up and switch internally between leading-edge and trailing-edge modes. They handle mixed-bulb retrofits, future bulb changes, and uncertain installations gracefully. The trade-off is cost (typically 30–50% more than a single-mode dimmer) and slightly more electronics to go wrong over the dimmer’s lifetime.

For a known LED-only circuit a dedicated trailing-edge dimmer is the cleaner choice. For a mixed circuit or a position where the bulbs will likely change later, universal is the more forgiving option.

When to switch from leading edge to trailing edge

The decision usually comes down to one of these scenarios:

• Replacing halogens with LEDs. Switch the dimmer at the same time as the bulbs. A leading-edge dimmer kept “because the halogen worked fine” almost always produces buzz on the new LEDs.
• Adding LEDs to a previously all-halogen circuit. Either swap the dimmer to trailing-edge (and accept that the halogens go too, eventually) or fit a universal dimmer.
• Existing LED circuit producing buzz, flicker, or a poor dim range. Almost always solved by switching the dimmer to trailing-edge. See our guide on dimmer buzzing for the full diagnostic process.
• New install, all LED. Start with trailing-edge from day one – there is no reason to fit a leading-edge dimmer on a new LED circuit.

Smart trailing-edge dimmer options

For app-controlled or voice-controlled dimming with the same trailing-edge technology, the SM323 range covers all three major smart-home protocols:

• SM323 Zigbee Dimmer – Zigbee mesh; Home Assistant, Philips Hue Bridge, SmartThings, Hubitat
• SM323-WF WiFi Dimmer – direct WiFi; Tuya / Smart Life, Alexa, Google Home
• SM323-MT Matter Dimmer – Matter over WiFi or Thread; Apple Home, Alexa, Google Home

All three are trailing-edge, LED-rated, no-neutral, and fit a standard UK back box.