LED dimmer switches: which one to buy and why your LEDs flicker
Some links below are affiliate links: if you buy through them I may earn a small commission, at no extra cost to you. As an Amazon Associate I earn from qualifying purchases. How LightGuru makes money
Mike's quick answer
Fit a trailing-edge dimmer made for LEDs, check its LED rating covers the number of bulbs on the circuit, use dimmable bulbs of one make and model, and set the dimmer's minimum level so the lights stay steady at the bottom. Most flicker and buzz on dimmed LEDs comes from an old dimmer made for halogen and filament bulbs.
Most dimmer switches in UK homes went in when every bulb was halogen or filament. Swap those bulbs for LEDs, even ones marked dimmable, and the same dimmer can make them flicker, buzz, cut out before they’re properly dim, or glow faintly when they should be off.
The bulbs usually aren’t the problem. Six 50 W halogens in a kitchen ceiling drew 300 W; the six LED GU10s that replace them draw about 30 W, and an old dimmer wasn’t built for that. Whether LEDs dim well comes down to four things: the type of dimmer, whether it’s rated for the number of LEDs on it, the bulbs, and a minimum setting most people never touch.
Why LEDs flicker on old dimmers
A mains dimmer doesn’t turn the voltage down. It switches the power off for part of every half-cycle of the mains, 100 times a second, and the longer the off part, the dimmer the light.
Older dimmers are leading-edge: they cut the start of each half-cycle, using a triac that switches on with a jolt. A filament or a wire-wound halogen transformer doesn’t mind. The small electronic driver inside an LED bulb does, and you get flicker, a buzz from the switch or the bulbs, and drivers that wear out sooner.
Then there’s the minimum load. Older dimmers often need 40 W to 60 W on the circuit (Varilight’s 1-gang V-Dim, a standard leading-edge dimmer, is rated 60 to 500 W and isn’t sold for LEDs at all), and five LED GU10s draw about 25 W. Below its minimum a dimmer can’t hold a steady output, so the lights flicker or drop out at the bottom.
Leading edge or trailing edge
A trailing-edge dimmer cuts the end of each half-cycle instead, and switches smoothly. It’s gentler on LED drivers and quieter, and LED versions work with as little as one bulb. That’s what “LED dimmer” means on most UK packaging, but check the spec.
| Leading edge | Trailing edge | |
|---|---|---|
| How it dims | Cuts the start of each half-cycle | Cuts the end of each half-cycle |
| Made for | Filament, mains halogen, wire-wound transformers | LED drivers, electronic transformers |
| Minimum load | Often 40–60 W on older models | About 0–10 W on LED models |
| Noise with LEDs | Can buzz | Silent or close to it |
| With dimmable LEDs | Flicker and drop-out are common | Smooth, if the bulbs suit it |
Yellow = the better option on that line.
There are exceptions. LED drivers labelled “TRIAC dimmable” are made for leading-edge dimming and need a dimmer to match: Varilight lists them as unsuitable for its V-Pro and points you to its V-Com dimmers instead. And a few lamps dim better with leading-edge control, which is why the V-Pro has a simulated leading-edge mode for “certain LEDs”. For ordinary dimmable bulbs and downlights, start with trailing edge.
How many LED bulbs can a dimmer take?
Every LED dimmer has a minimum load and a maximum. For LEDs the maximum is often a number of lamps as well as watts, because each driver draws a surge of current as it switches on and the dimmer has to survive all of them at once.
Here are real ratings from the makers’ spec sheets:
| Dimmer | LED rating | Minimum | Type |
|---|---|---|---|
| Varilight V-Pro (rotary) | 1 to 10 dimmable LEDs, up to 120 W, per gang | One lamp | Trailing edge, programmable |
| BG Nexus 881 | 5–100 W LED | 5 W | Trailing edge |
| Lightwave Smart Series | Up to 15 LED lamps, 100 W, per gang | 10 W, or 5 W with a neutral | Smart dimmer |
| Varilight V-Dim | Not for LEDs | 60 W (rated 60–500 W) | Leading edge |
Add up the real LED wattage (the figure on the bulb, not the “equivalent to 50 W”) and count the bulbs. Six 5 W GU10s are 30 W and six lamps: inside a V-Pro’s limits and above the BG’s 5 W minimum. On multi-gang plates the rating is per gang, and some makers rate each gang lower than a single dimmer, so check the leaflet.
Dimmable bulbs and compatibility lists
Every bulb on a dimmer has to be marked dimmable. A non-dimmable LED on a dimmer can flicker even at full brightness, and its driver is likely to fail early.
Dimmable only means the driver will tolerate a dimmer; how smoothly and how low it dims depends on the pairing. That’s why the big lamp brands, Philips among them, publish compatibility lists giving, for each dimmer they’ve tested, how many lamps work and how low they’ll go. A listed dimmer takes the guessing out.
Keep to one make and model of bulb on each dimmer. Different bulbs dim at different rates, and the odd one out is usually the first to flicker. Don’t mix halogens and LEDs on one dimmer either: Varilight says its dimmers aren’t designed for mixed lighting types on one circuit.
For living rooms and bedrooms, I like bulbs that warm up as they dim. Philips WarmGlow goes from 2700K at full to about 2200K at the bottom, as filament bulbs did, so a dimmed room looks cosy rather than grey. My other dimmable picks are in the best LED bulbs guide.
Setting the minimum level
Most good LED dimmers let you set the bottom of the range, and it’s the step most people skip. It moves the bottom of the knob to the lowest level your bulbs hold steadily, so fully down is a steady glow rather than a shimmer or a blackout, and the lights always come back on cleanly.
On a Varilight V-Pro rotary, turn the dimmer down low, then click it off and on three times, about a second apart. The lights step up and down to show it’s in set-up mode. Turn the knob fully clockwise, then slowly down and up until you find the lowest level that’s steady, and leave it there; after about three seconds the setting saves. The same V-Pro instructions cover lowering the maximum, if lights flicker at full, and two alternative modes to try if one type of bulb still flickers. Push-button and multi-way dimmers do it differently, so keep the leaflet.
Integrated downlights and drivers
Integrated LED downlights have their driver built in, or in a small box on a short cable, and the dimmer is talking to that driver. A fitting has room for a better driver than a GU10 bulb can hold, which is why quality integrated downlights dim smoothly to around 5%, some lower. GU10 or integrated downlights compares the two.
Check the fitting is sold as dimmable (some ranges have non-dimmable versions that look identical), use the fitting maker’s compatibility list for how many fittings each dimmer will handle, and keep one model per dimmer. If you didn’t choose the downlights, read the driver label on one before buying a dimmer; it often says which dimmer type suits it.
Old 12 V downlights are a different case. Halogen-era MR16 fittings have a small transformer above each one, made for 35 W or 50 W halogens, and on a dimmer those transformers often struggle with a 5 W LED. Replacing the transformers with LED drivers, or the fittings with mains GU10s, is the usual cure. Light bulb fittings explained shows how to tell MR16 from GU10.
Dimming LED strip
LED strip runs at 12 V or 24 V from a driver, which gives you two ways to dim it.
The first is a mains-dimmable driver wired after the wall dimmer, and the two have to suit each other. Lightwave rates its smart dimmers for strip only through a 24 V TRIAC-dimmable driver, one per channel, with limits on length and wattage (Lightwave’s load guide), while Varilight says TRIAC-dimmable drivers aren’t for its V-Pro. Size the driver with about 20% to spare.
The second keeps a non-dimmable driver and adds a small controller between the driver and the strip. It dims by PWM (pulse-width modulation), switching the strip on and off hundreds or thousands of times a second and varying how long each “on” lasts; it’s how most smart and colour-changing strip works. Cheap controllers that switch slowly can show flicker on a phone camera, so pay a little more. More in LED strip lighting for kitchens.
Two-way and multi-way dimming
Lights with a switch at each end, on stairs and in long rooms, are on a two-way circuit. You can fit a dimmer at one end and keep an ordinary two-way switch at the other, but only one dimmer: Varilight’s instructions say simply, “Replace only one switch with a dimmer.” The dimmer end sets the level, and the other end switches the lights on and off at that level.
To dim from both ends you need a multi-way system: a master dimmer and a companion unit at each other position, on the existing cable. Varilight’s V-Pro Multiway takes up to two companion controllers, so three positions, with no more than 50 m of cable from the dimmer to the furthest one. Ordinary switches can’t be mixed in, so every position gets changed. Varilight’s V-Pro Smart also takes up to two companions, and a smart dimmer can be worked from an app, a voice assistant or a remote, so you may not need one at every position. More on stairs in the hallway and stairs lighting guide.
Smart bulbs or a smart dimmer?
| Smart bulbs, plain switch | Smart dimmer, normal LEDs | |
|---|---|---|
| Fitting | Swap the bulbs | Replace the wall switch |
| Works with | Smart bulbs only | Dimmable bulbs and integrated downlights |
| Colour and white tuning | Yes, on many | No, dimming only |
| The wall switch | Must stay on | Works as a normal dimmer |
| Cost | About £10–£50 a bulb | One dimmer for the whole circuit |
Yellow = the better option on that line.
Smart bulbs and smart dimmers don’t mix. A smart bulb dims itself and needs full power all the time: on a dimmer it will flicker or misbehave, and switched off at the wall the app can’t reach it. So smart bulbs go on a plain switch that stays on, dimmed from the app, a voice assistant or a battery remote such as the Hue Dimmer Switch. If people keep switching them off at the wall, Philips’s Hue wall switch module (about £45, needs a Hue Bridge) is wired in behind the existing switch so the bulbs stay powered.
A smart dimmer replaces the wall switch and dims ordinary dimmable LEDs or integrated downlights, so it’s the cleaner route for a ceiling of downlights, though you give up colour changing.
The catch is the neutral wire. Many UK switch boxes have none, so a smart dimmer powers itself by passing a tiny current through the lights. That brings the minimum load back, and it’s why LEDs sometimes glow faintly when “off”. Lightwave’s Smart Series dimmers need 10 W per gang, or 5 W with a neutral connected; Varilight’s V-Pro Smart needs no neutral and handles up to 120 W of dimmable LEDs. If lights glow or flash when they’re off, a small bypass (load-correction) device wired in at one of the lights usually soaks up the stray current; Varilight sells one for its V-Pro range. For kitchen scenes, see smart kitchen lighting scenes.
Troubleshooting checklist
Work down this list before you buy anything:
- Flicker or buzz only when dimmed, fine at full: the dimmer doesn’t suit LEDs, or it’s over its lamp limit. Fit a trailing-edge LED dimmer and count the lamps.
- Flicker only at the very bottom: set the dimmer’s minimum level.
- Lights go out before they’re properly dim: raise the minimum setting, or check the load isn’t below the dimmer’s minimum.
- One bulb flickers or dims differently: it’s a different make, model or batch. Replace it with the same model as the rest.
- Lights won’t dim at all, or flash: a non-dimmable bulb or fitting is on the circuit, or the dimmer is in the wrong mode.
- Glow or flashing when off: usually a no-neutral smart dimmer or an illuminated switch. A bypass device fixes it.
- Buzz from the switch: a leading-edge or overloaded dimmer. A hum from the ceiling points to a driver or an old transformer.
- A plate that’s hot rather than warm, a burning smell, or flicker when the kettle or oven switches on: switch off and call an electrician.
The flicker and buzz guide goes through each cause in more detail.
What I'd buy
Affiliate linksWho can fit a dimmer switch?
In England, swapping a light switch for a dimmer isn’t notifiable building work. Approved Document P makes three kinds of electrical work notifiable: a new circuit, a replacement consumer unit, and additions or alterations to circuits in a special location such as a bathroom. Replacements, repairs and maintenance aren’t notifiable anywhere. The work still has to be safe and meet the wiring regulations, BS 7671, and the rules differ elsewhere in the UK.
Safe isolation is the part that matters. Switch the lighting circuit off at the consumer unit, make sure nobody can switch it back on while you’re working, and check the switch is dead with a proper voltage tester before you touch a wire. Don’t rely on the lights being off: a switch is live whether the light is on or not.
Check the back box is deep enough: Varilight asks for at least 25 mm behind its classic plates and 35 mm behind its screwless and Ultraflat plates. A metal plate must be connected to earth at its earth terminal, so if there’s no earth wire in the box, as in some older houses, don’t fit a metal one. And photograph which wire went where before you take the old switch off.
I’d call an electrician if there’s no earth in the box, the wiring isn’t what you expected, the switch is for bathroom lights, you want a new switch position, a smart dimmer needs a neutral that isn’t there, or you find scorched insulation or loose connections. As Electrical Safety First puts it, if you’re not sure, don’t. NICEIC lists registered electricians.
FAQs
Why do my LED lights flicker on a dimmer switch?
Usually the dimmer was made for halogen or filament bulbs: it’s leading-edge, or its minimum load is more than the LEDs draw. A trailing-edge LED dimmer with dimmable bulbs normally cures it, and setting its minimum level stops flicker at the bottom.
What is a trailing edge dimmer?
One that cuts the end of each half-cycle of the mains rather than the start. It’s smoother and quieter with LED drivers and has a much lower minimum load.
How many LED bulbs can I put on a dimmer?
As many as its LED rating allows, which usually gives a number of lamps as well as watts: a Varilight V-Pro takes 1 to 10 dimmable LEDs up to 120 W per gang. If the lamp maker’s compatibility list gives a lower figure, use that.
Why is my LED dimmer buzzing?
Usually it’s a leading-edge model or has too many lamps on it. A trailing-edge LED dimmer inside its lamp limit should be close to silent. A hum from the ceiling points to a driver or an old 12 V transformer.
Can I use non-dimmable LED bulbs with a dimmer switch?
No, not even at full brightness. They’re likely to flicker and fail early. Fit dimmable bulbs, or swap the dimmer for a plain switch.
Do I need an electrician to change a dimmer switch?
Not by law in England, as replacing a switch isn’t notifiable, but the circuit must be safely isolated and tested dead first. If you’re not confident with mains wiring, there’s no earth in the box, or it’s for a bathroom, use a registered electrician.
SaveSave it for later
Pin this guide to one of your Pinterest boards so it's there when you start the job.
