LED lights flicker on a dimmer when the bulb and the dimmer don’t suit each other. Five causes cover almost every case: the bulb isn’t marked dimmable; the dimmer is the wrong type for it — leading-edge or trailing-edge; the total LED load is below the dimmer’s minimum; it is above the dimmer’s LED rating; or the dial goes lower than the bulbs can follow. The fix is to match them: dimmable bulbs, the dimmer type the bulb’s listing names, and a total wattage inside the dimmer’s LED range. Changing a dimmer is a job for a registered electrician.
Dimmers were designed for filament bulbs, and for decades they never had to think. A filament is just a wire getting hot: give it less power and it glows less, smoothly, all the way down. An LED bulb is a small electronic device, with a driver inside that turns the mains into something the LEDs can use — and that driver has to make sense of what the dimmer is doing to the supply. When bulb and dimmer understand each other, LEDs dim beautifully. When they don’t, you get the familiar trio: a flicker, a faint buzz, and a light that won’t go low or won’t quite go off. None of it is mysterious, and almost all of it is fixed by matching a few things before anyone touches a screwdriver.
Why do LED bulbs flicker on a dimmer switch?
UK mains electricity alternates 50 times a second. A conventional dimmer works by switching off part of every half-cycle — a hundred times a second — so that less power reaches the light. A filament holds enough heat to ride over the gaps, so you never see them. An LED has almost no such inertia. Its driver has to read the chopped supply and turn it into a steady, lower current, and if the driver wasn’t designed for that kind of chopping — or wasn’t designed to dim at all — the gaps leak through as flicker, as a hum, or as a bulb that drops out at the bottom of the dial.
That is why the same bulb can be perfect in one house and maddening in the next. The bulb usually isn’t faulty, and neither is the dimmer. They simply don’t match — and the questions below find out which part of the match has failed.
Leading-edge or trailing-edge: which dimmer do LED bulbs need?
There are two ways to chop the waveform, and they are the two dimmer types you will see on packaging. A leading-edge dimmer, often labelled TRIAC, cuts the start of each half-cycle. It is the older design, made for filament and mains halogen bulbs, and most older rotary dimmers in UK homes work this way. A trailing-edge dimmer cuts the end of each half-cycle instead. It was designed for electronic loads, tends to run quieter, and is the type most dimmers now sold for LEDs use. A third kind, the universal dimmer, can be set to either.
The tempting shortcut is “LEDs need trailing-edge”. Many do — but not all, and the only reliable answer is the one on the bulb’s own packaging or listing. Across our own bulbs the split is real:
| Bulb | Cap | Watts | Dimmer type the listing names | Six on one dimmer |
|---|---|---|---|---|
| Nimbus GU10 · CRI90 | GU10 | 3.7W | Leading-edge (TRIAC) | 22.2W |
| Nimbus Pro GU10 · CRI97 | GU10 | 6W | Leading-edge (TRIAC) | 36W |
| Kanto LED module & driver | Kanto downlight | 8W | Leading-edge (TRIAC) | 48W |
| Lumi G9 · frosted | G9 | 4W | Trailing-edge | 24W |
| Marlowe pear · 1800K | E27 | 6W | Trailing-edge | 36W |
So a ceiling of GU10s and a pendant full of G9 capsules may want different dimmers, even in the same room. Before buying either, find out what you already have. Most dimmers carry a model number on the plate or behind the knob, and looking it up tells you the type and its LED load range. If there’s nothing to go on, an electrician can check in minutes with the circuit isolated.
Nimbus Pro GU10 · CRI97
575 lumens from 6W, a 36° beam and colour rendering of 97 — the bulb for kitchens, art and anywhere colour matters. Its listing names a leading-edge (TRIAC) dimmer: exactly the line to check before a whole ceiling of them goes on one circuit. Five-year warranty.
How many LED bulbs can go on one dimmer?
Every dimmer has a working range — a minimum load and a maximum — and LEDs can fall foul of both ends.
The minimum is the one people miss. Halogen-era dimmers were built around loads of hundreds of watts. Six Nimbus GU10s add up to just 22.2W, and below a dimmer’s minimum load a circuit can flicker, flash at switch-on or refuse to dim smoothly. The fix is a dimmer whose stated LED range starts low enough for the bulbs you actually have — sized, in a downlit ceiling, from how many downlights you need.
The maximum is lower than the box suggests. Dimmers are often sold on their old filament rating, with a separate and much smaller figure for LEDs. Add up the wattage of every bulb on the circuit — the right-hand column above does it for six — and keep the total inside the LED figure with room to spare. A dimmer worked at its limit runs warm and is more likely to buzz.
Don’t mix bulbs on one dimmer. Different LED drivers respond to the same dimmer differently, and one awkward bulb can make a whole circuit misbehave. One bulb type per dimmer gives an even fade as well as an even colour — the same discipline that keeps a ceiling of downlights reading as one light.
And expect a floor. “Dimmable” rarely means dims to nothing: the Nimbus GU10’s listing, for example, gives its range as down to 25 per cent. A well-matched LED settles at a low glow rather than fading to black the way a filament did.
What do flicker, buzzing and ghosting each mean?
Each symptom points at a different part of the match, which makes them useful rather than just annoying.
| Symptom | Most likely cause | The fix |
|---|---|---|
| Flickers or pulses when dimmed | Wrong dimmer type for the bulb, or a load below the dimmer’s minimum | Match the dimmer type the bulb names; choose a dimmer whose LED range starts lower |
| Cuts out, or won’t come on, low on the dial | The dial goes lower than the bulbs can follow | Raise the dimmer’s minimum-level setting, which many LED dimmers have |
| Buzzes or hums | Dimmer and driver mismatched, or the dimmer near its maximum | The dimmer type the bulb names; less load on the circuit |
| Glows faintly when off | A tiny current through the dimmer or cable — enough for an LED | An LED-rated dimmer, or a load-correction device fitted by an electrician |
| Flickers at full brightness | Not a dimming problem: a failing bulb or dimmer, or a loose connection | Try a known-good bulb; if it persists, call an electrician |
Ghosting — the faint glow from a light that is switched off — is the one that surprises people, because a filament bulb never showed it. LEDs need so little current that the trickle some dimmers and long cable runs let through is enough to light them. It isn’t dangerous in itself, but it is a sign the dimmer wasn’t chosen for LEDs.
One warning about the last row. A flicker that comes with a buzz from the switch, a warm faceplate or any smell of burning is not a compatibility question: switch the circuit off at the consumer unit and call a registered electrician.
Marlowe Pear Bulb · 1800K
A clear-glass E27 pear at 1800K — candlelight-warm — giving 450 lumens from 6W. Its listing names a trailing-edge dimmer, and on one it takes a dining pendant down for the last course. It stands 140mm tall, so check the shade has room.
Can you use non-dimmable LED bulbs on a dimmer?
No — not even with the dimmer turned up full. A non-dimmable driver isn’t designed for a chopped supply at any setting; on a dimmer it may flicker, hum or fail early. If a circuit has a dimmer, every bulb on it should be marked dimmable, which is why our fundamentals guide puts “dimmable” on the list of things to check on every box. Smart bulbs are different again: they dim themselves from an app, and their makers generally say to run them from a plain switch rather than a dimmer.
If there’s no dimmer and you would rather not fit one, two routes need no new wiring. Step-dimming bulbs change brightness each time the existing switch is flicked off and on again, and lamps with their own dimmer — cordless lamps among them — put the control on the table rather than the wall.
Do I need an electrician to change a dimmer switch?
Changing a bulb is DIY; changing the switch is a job for a registered electrician. In England, replacing a light switch with a dimmer on an existing circuit is not notifiable under Part P in itself — new circuits, consumer-unit changes and work in the special locations around a bath or shower are — but it is still mains wiring that has to be isolated, done competently to BS 7671 and tested. A good electrician will also check the things this guide is about: that the dimmer type suits your bulbs, that the load sits inside its LED range, and that the back box is deep enough for the new dimmer.
In a bathroom, where a switch may go is itself regulated, and work in the zones is notifiable — our bathroom zones guide explains why. Wales, Scotland and Northern Ireland each have their own rules. And if you rent, a new dimmer is an alteration that needs your landlord’s consent first; our renters’ guide covers what you can do without it.
