You replace an old filament bulb with an LED, and the light starts to shimmer. Or it flickers only when dimmed. Or it glows faintly after you have switched it off, which is unsettling the first time it happens.
None of it means the bulb is broken. LEDs behave differently from the bulbs our houses were wired for, and nearly every flicker has one of eight causes. Here they are in the order worth checking, cheapest first.
On this page
- Why LEDs flicker when filament bulbs did not
- Diagnose it in three questions
- The eight causes, in order
- Dimmers: leading edge, trailing edge, minimum load
- The light that glows when switched off
- When to stop and call an electrician
- Buying so it does not happen again
- In short
- Common questions
Why LEDs flicker when filament bulbs did not
An old 60W bulb was a lump of wire that glowed. It drew a large, steady current, and it was too hot and too slow to respond to small wobbles in the supply.
An LED is an electronic device with a small driver inside it. It draws a tiny current, it responds instantly, and it depends on that driver receiving a clean signal. Anything that chops up the supply — most often a dimmer — the LED will show you.
So a flicker is almost never the LED failing. It is the LED reporting something the old bulb was too crude to notice.
Diagnose it in three questions
| Question | Answer | Likely cause |
|---|---|---|
| Does it flicker only when dimmed ? | Yes | Non-dimmable bulb, or an incompatible dimmer |
| Does it flicker at full brightness too ? | Yes | Loose connection, or a cheap driver in the bulb |
| Does one bulb in a fitting flicker and the others not ? | Yes | That bulb, or that lampholder |
| Do all the lights in the room dip together ? | Yes | Load elsewhere in the house — often a shower or a kettle |
| Does it glow faintly when switched off ? | Yes | Illuminated switch, or two-way wiring |
Two minutes with that table will usually tell you which of the sections below you need.
The eight causes, in order
1. The bulb is not dimmable
The most common cause by a long way. An LED is only dimmable if the box says so, and a non-dimmable one on a dimmer will flicker, buzz or refuse to go below half. Cost to fix: the price of a bulb.
2. The dimmer is the wrong type
Most dimmers in British homes were installed for filament bulbs. They work by chopping the leading edge of the mains waveform, which LED drivers dislike. A modern trailing-edge dimmer marked as LED-compatible solves most flicker permanently. This is the single most effective fix in this guide. Decorative LED filament bulbs are covered in our filament bulb guide.
3. The load is below the dimmer's minimum
Dimmers have a minimum wattage, often 10W to 40W. Four 5W LEDs come to 20W, and on a dimmer with a 40W minimum they will stutter. Either add more bulbs to the circuit, fit a higher-wattage bulb, or change to a dimmer with a low minimum load.
4. A loose connection
If a bulb flickers at full brightness, suspect the connection before the electronics. Switch off at the wall, let it cool, and reseat the bulb — a quarter turn on a bayonet cap, half a turn on a screw. If reseating cures it, the lampholder was the problem.
5. A cheap driver in the bulb
Every LED has a driver. In inexpensive bulbs it is minimal, and a slight shimmer at low brightness is the result. If one bulb misbehaves and identical ones in the same fitting do not, the bulb is the fault.
6. Mixed brands in one fitting
Different drivers respond differently to the same dimmer. In a chandelier or a cluster, use the same bulb throughout: the E14 Candle Bulb comes in packs of four and ten for exactly this reason.
7. Load elsewhere in the house
If every light dips at the same moment, something large just started: an electric shower, a kettle, a washing machine heating. A brief dip is normal. Repeated, severe dipping is not, and belongs in the electrician section below.
8. A 12V fitting with an old transformer
Older 12V downlights use transformers sized for 50W halogen. Replace the lamps with 5W LEDs and the transformer is running far below its minimum, which flickers. The fix is an LED driver, not a bulb. Mains-voltage GU10 spotlights avoid the problem entirely.
Dimmers: leading edge, trailing edge, minimum load
| Leading edge | Trailing edge | |
|---|---|---|
| Designed for | Filament and halogen | LED and electronic loads |
| Typical minimum load | 40W or more | As low as 1 – 10W |
| With LEDs | Flicker, buzz, poor low end | Smooth to near zero |
| Noise | Often a faint hum | Silent |
If your dimmer is more than about ten years old, it is almost certainly leading edge. Swapping it is a like-for-like replacement in most cases, and it is the fix we recommend before anyone starts buying different bulbs.
There is a way to sidestep the whole question: buy fittings that dim themselves. The LED Floor Lamp dims by touch and switches colour temperature, the Rechargeable Wall Light dims by remote with no wiring at all, and the cordless range does the same on a shelf or a bedside table. None of them cares what is on your wall.
The light that glows when switched off
A faint glow from a switched-off LED is unnerving and almost always harmless. Two usual causes:
- An illuminated switch. The little neon in the rocker passes a tiny current through the circuit — enough to make an efficient LED glimmer.
- Two-way switching, or a long cable run. Capacitance between the wires induces a small current. The same physics, a different source.
The cure is a small CR capacitor fitted across the lampholder by an electrician, or a switch without the neon. Or, frankly, a different bulb: some tolerate it and some do not.
When to stop and call an electrician
Everything above is bulbs and switches. These are not:
- Lights that dim or brighten repeatedly and severely across several rooms.
- Flickering accompanied by a smell of hot plastic, a warm switch plate or a buzzing socket.
- Anything that trips the RCD.
- A fuse board with rewirable fuses rather than breakers.
Those are supply or wiring faults, and they are worth a registered electrician's hour. In England and Wales, new wiring in a bathroom or kitchen is notifiable under Part P in any case — the detail for bathrooms is in our guide to bathroom lighting zones.
Buying so it does not happen again
- Buy dimmable bulbs from the start in any room used in the evening. The E27 Filament Bulb is dimmable.
- Buy identical bulbs for one fitting, in one order.
- Fit a trailing-edge LED dimmer when you rewire anything.
- Check the cap before the shape — see bulb fittings explained.
- Keep 2700K and CRI 90+ across the room, so replacements match: our guide to colour temperature covers why.
In short
- Flickers only when dimmed: non-dimmable bulb, or the wrong dimmer.
- Flickers at full brightness: loose bulb, or a cheap driver.
- Old dimmers are leading edge. LEDs want trailing edge.
- Watch the minimum load: four 5W bulbs may not be enough.
- A faint glow when off is usually an illuminated switch.
- Whole-house dipping, heat or a smell: stop and call an electrician.
Common questions
Why do my LED lights flicker when dimmed ?
Either the bulb is not dimmable, or the dimmer is a leading-edge type made for filament bulbs. A trailing-edge dimmer marked LED-compatible cures most cases permanently.
Why do my LED lights flicker at full brightness ?
Usually a loose bulb or a tired lampholder. Switch off, let it cool and reseat the bulb. If it persists on one bulb only, that bulb's internal driver is the fault.
Can flickering LED lights be dangerous ?
Flicker caused by a dimmer or a bulb is not. Flicker with a warm switch plate, a smell of hot plastic, buzzing, or repeated severe dipping across several rooms points at wiring or supply, and needs an electrician.
Why does my LED bulb glow when switched off ?
An illuminated rocker switch or two-way wiring passes a tiny current through the circuit, and an efficient LED glimmers on it. It is harmless. A capacitor fitted by an electrician, or a switch without the neon, cures it.
What is the minimum load on a dimmer switch ?
Often 10W to 40W for older dimmers. Four 5W LEDs total 20W, which may fall below it and cause stuttering. LED-rated trailing-edge dimmers go much lower.
Do I need a special dimmer switch for LED lights ?
In practice, yes. A trailing-edge dimmer rated for LED loads gives a smooth range and no buzz; a leading-edge dimmer designed for halogen rarely does.
Why is only one bulb flickering in my chandelier ?
Either that bulb or that lampholder. Swap it with a bulb from another arm: if the flicker moves with the bulb it is the bulb, and if it stays in the same arm it is the holder.
Start with the switch
Before buying anything, find out whether your dimmer is leading edge. It answers most of this page, and it is the fix that lasts.
If you bought a fitting from us and it is behaving oddly, write to us at contact@thelightheritage.com with the bulb and the switch you are using. We answer every message ourselves.
Browse rechargeable and cordless lights, which dim without any switch at all, or the full range in ceiling lights and wall lights.
