Dimming and Lighting Controls: Why Good Fixtures Still Flicker
Dimming and Lighting Controls: Why Good Fixtures Still Flicker
It is handover day. The joinery is right, the stone is right, and the fixtures are exactly the ones you specified.
The client turns the dimmer down to show someone how the room looks in the evening.
About a third of the way down, the lights stutter. Then they settle, then stutter again. Something behind the wall plate is buzzing faintly. Pushed lower, everything drops out at once.
The client looks at you.
Nothing in that room is broken. The fixtures are good, the wiring is correct, the electrician did the job properly. What has happened is that the dimmer on the wall and the drivers in the fittings were built to work in two different ways, and nobody checked whether they would get along.
It is the most common way a good lighting scheme fails, it is invisible on a drawing, and it costs nothing to prevent at design stage. It just requires knowing what to ask for.
What a Dimmer Actually Does
Worth understanding, because the whole problem follows from it and it is simpler than it sounds.
A dimmer does not reduce the voltage. It chops it up.
Mains electricity arrives as a wave, rising and falling a hundred times a second. A dimmer switches the supply off for part of each wave and lets the rest through. Let more through and the light is brighter. Let less through and it dims. Your eye cannot follow switching that fast, so it reads the average.
There are two places you can make the cut, and this is where everything goes wrong.
Leading edge dimming chops the front of each wave. This is the old technology, and it is what is sitting in most existing walls. It was designed for halogen and incandescent lamps.
Trailing edge dimming chops the back of each wave. This is the newer approach, and it is what modern LED drivers are built for.
Why the old kind fails with LEDs
Here is the part that explains almost every flickering room.
A leading edge dimmer uses a switching component called a TRIAC, and a TRIAC has one awkward requirement: once it switches on, it needs a certain amount of current still flowing through it to stay on. Technical literature describes this as a minimum holding current. Drop below it and the TRIAC resets itself and turns off early.
Halogen lamps drew plenty of current, so this was never a problem for decades.
An LED draws a fraction of that. So the TRIAC keeps losing the current it needs, keeps dropping out, and keeps switching back on. That stutter is the flicker you see.
A trailing edge dimmer works differently. It uses a MOSFET with zero crossing detection instead of a TRIAC, and it has no holding current requirement at all. Which is why most LED drivers behave properly on one and badly on the other.
So the most common failure in the world is simply this: an old leading edge dimmer left in the wall, now being asked to dim LEDs that were built for trailing edge control. Every component is working exactly as designed. They were just designed for different things.
The Second Cause: Too Few Watts on the Circuit
Now the version that catches out even good installers, because it happens when everything appears to have been done correctly.
Someone replaces six halogen downlights with six good quality dimmable LEDs. Same positions, same circuit, correct dimmer. And the room flickers.
The reason is that a dimmer needs the lights to draw a certain amount of power in order to work at all. It takes a reference current from the circuit to regulate itself, so every phase cut dimmer has a minimum load below which its own electronics become unstable. Older dimmers commonly specify somewhere around 40 to 60 watts.
Now look at what swapping halogen for LED does to a circuit:
| Before | After | Minimum load 40W | Minimum load 60W |
|---|---|---|---|
| 5 x 50W halogen = 250W | 5 x 7W LED = 35W | Below minimum | Below minimum |
| 6 x 50W halogen = 300W | 6 x 6W LED = 36W | Below minimum | Below minimum |
| 8 x 35W halogen = 280W | 8 x 5W LED = 40W | Just meets it | Below minimum |
| 10 x 50W halogen = 500W | 10 x 7W LED = 70W | Fine | Fine |
The old circuit sat comfortably above any threshold. The new one falls under it.
That is the whole explanation. Nothing is faulty. The dimmer is simply starved of the current it needs to regulate itself, and it misbehaves accordingly. LEDs are so efficient that they can accidentally fall below the floor the dimmer was built around.
The fixes, roughly in order of preference:
Replace the dimmer with a trailing edge unit specified for low LED loads. Modern LED dimmers have far lower minimum loads.
Add a bypass or dummy load module, which puts a fixed resistive load on the circuit to bring the total above the threshold. It works, but it wastes energy and treats the symptom.
Move to driver level dimming, using 0-10V or DALI wired at the fitting rather than at the switch, which removes the phase cut dimmer from the equation entirely.
The Four Ways to Dim
Everything above concerns dimming at the wall, by chopping the mains. There are other approaches, and on anything beyond a simple domestic circuit they are usually better.
The distinction that matters: phase cut dims the power itself, while the other three send a separate instruction to the driver and let the driver do the dimming. That is why they are more reliable, and why they need a control cable run at first fix.
| Method | How it works | Best for |
|---|---|---|
| Phase cut (leading or trailing edge) | Cuts part of the AC waveform at the wall dimmer | Residential, retrofit, simple circuits |
| 0-10V | A separate low voltage control signal varies brightness | Commercial, straightforward architectural work |
| DALI | Digital addressing, each fitting individually controllable | Larger projects, scene control, anything needing flexibility |
| Wireless (Casambi, Zigbee and similar) | Control by radio, no control cabling | Retrofit where running control cable is impossible |
Phase cut is cheap, familiar and requires no extra wiring. It is also the most likely to have compatibility problems, and it dims a whole circuit rather than individual fittings.
0-10V is reliable and widely supported, but it needs a control cable run alongside the power, which has to be decided at first fix.
DALI allows every fitting to be addressed individually, so scenes can be built and rebuilt in software rather than by rewiring. It costs more and requires commissioning, and it is the right answer when a space needs to do several different things.
Wireless removes the cabling problem entirely, which makes it the practical answer in retrofit work where opening walls is not an option.
Three Specification Details That Catch People Out
Dim to off versus dim to ten percent. A driver described as 0-10V dim to off will go to zero on the control signal alone. A driver described as 0-10V dim to 10% cannot switch off from the dimming signal and needs a separate relay or switch to do it. Miss this and the client has lights that will not turn off from the wall plate.
DALI will not dim a TRIAC driver. They are entirely different systems. A DALI controller connected to a phase cut driver does not dim at all, and it is a surprisingly common specification error on mixed projects.
"Dimmable" is not a specification. Confirm the fitting is explicitly dimmable, then check whether the manufacturer publishes an approved dimmer list. Good manufacturers do. If they do, use it, because it is the difference between a scheme that works and an afternoon of swapping dimmers on site.
What All This Buys You
Worth pausing on the payoff, because everything above is effort and it should be spent for a reason.
A room has more than one job. A kitchen at breakfast and the same kitchen during a dinner party are different rooms. A living room at four in the afternoon and at eleven at night need different things. A hotel lobby at check-in and at midnight are not the same space.
Without dimming you get one setting, and one setting is always a compromise between conditions that are not alike. Too bright for the evening, too dim for the afternoon, and usually both.
But the bigger prize is what dimming does to layers. Ambient, task and accent light on separate dimmable circuits can be balanced against each other: ambient pulled right back, one accent brought up, task switched off entirely. That is the difference between a room that is lit and a room that has been tuned.
Which is why the most useful thing to specify in a project is rarely a better fixture. It is separate circuits and the ability to dim each one independently.
What to Specify, and When
New residential build. Separate circuits per layer, trailing edge dimmers specified from the outset, drivers checked against the dimmer. Consider running control cable to key areas even if you use phase cut initially, since it costs little at first fix and impossible afterwards.
Residential retrofit. Replace leading edge dimmers with trailing edge units rated for low LED loads. Check total circuit wattage against the dimmer's minimum. Where cabling cannot be run, wireless is the honest solution.
Commercial or larger projects. DALI or 0-10V, decided at the earliest stage, because the protocol determines the driver on every fitting in the scheme.
Anywhere that will be filmed. Confirm flicker performance explicitly. Some dimming methods introduce flicker that is invisible to the eye and clearly visible on camera.
And regardless of project type: decide the protocol before the fixture schedule. The control decision constrains the driver, and the driver constrains the fitting. Doing it the other way round is how projects end up with fixtures that cannot be dimmed the way the design intended.
Checklist
Design stage
- Each lighting layer on its own dimmable circuit
- Control protocol chosen before the fixture schedule
- Control cabling run at first fix where 0-10V or DALI is intended
- Scenes agreed with the client, not left to be worked out later
- Number of scenes kept small enough that people actually use them
Specification
- Every fitting confirmed dimmable, not assumed
- Manufacturer's approved dimmer list obtained and followed
- Trailing edge specified for phase cut LED circuits
- Total circuit load checked against the dimmer's minimum load
- Dim to off versus dim to 10% confirmed on 0-10V drivers
- Flicker performance specified where anything will be filmed
Retrofit
- Existing dimmers identified as leading or trailing edge
- New circuit wattage calculated and compared to the old
- Bypass module considered only if the dimmer cannot be replaced
- Wireless considered where control cable cannot be run
Five Mistakes
1. Leaving a leading edge dimmer in place during an LED upgrade. The most common cause of flicker and buzz, and every component involved is working correctly.
2. Ignoring minimum load. Replacing 300W of halogen with 36W of LED can drop a circuit below the threshold the dimmer needs to regulate itself.
3. Choosing fixtures before choosing the control protocol. The protocol determines the driver, which determines the fitting. Reversing that order limits your options.
4. Assuming "dimmable" is enough. Check the manufacturer's approved dimmer list. If there is one, it exists for a reason.
5. Putting every layer on one dimmer. Dimming everything together is far less useful than it sounds. Separate circuits are what make scenes possible.
Frequently Asked Questions
Why do my LED lights flicker when dimmed? Almost always a mismatch between the dimmer and the driver. The most common version is a leading edge dimmer, designed for halogen and incandescent lamps, now driving LED fixtures built for trailing edge control. Leading edge dimmers use a TRIAC, which requires a minimum holding current to stay switched on. If the current falls below that threshold the TRIAC resets and turns off early, producing visible flicker or complete failure. Trailing edge dimmers use a MOSFET with zero crossing detection instead and have no holding current requirement, which is why most LED drivers perform well on them.
What is minimum load, and why does it matter for LEDs? Every phase cut dimmer draws reference current from the load in order to regulate itself, so each one has a minimum wattage below which it becomes unstable. Legacy dimmers commonly specify around 40 to 60 watts. The problem appears on retrofits: six 50 watt halogens draw 300 watts, comfortably above any threshold, while six 6 watt LEDs draw 36 watts and fall below it. Nothing is faulty, the dimmer is simply starved. The fixes are to replace the dimmer with a modern unit rated for low LED loads, add a bypass or dummy load module, or move to driver level dimming with 0-10V or DALI.
What is the difference between leading edge and trailing edge dimming? Both cut part of the AC waveform, but at opposite ends of each half cycle. Leading edge cuts the beginning, using a TRIAC, and was designed for resistive loads like incandescent and halogen lamps. Trailing edge cuts the end, using a MOSFET with zero crossing detection, which removes the holding current requirement that causes most LED flicker. For LED loads, trailing edge is the correct choice in nearly every case, and a leading edge dimmer driving a trailing edge optimised driver will very often flicker, buzz or fail.
Should I use DALI or 0-10V? It depends on the project. 0-10V is simpler and reliable, varying brightness through a low voltage control signal, and suits straightforward commercial and architectural work. DALI addresses each fitting individually, so scenes can be built and changed in software rather than by rewiring, which makes it the right answer for larger projects or anywhere the space needs to serve several different functions. Both require control cabling run alongside the power, so the decision has to be made at first fix. Note that a DALI controller will not dim a phase cut driver at all, since they are entirely different systems.
What does dim to off versus dim to 10 percent mean? It describes whether a driver can be switched off using the dimming signal alone. A 0-10V dim to off driver goes to zero on the control signal. A 0-10V dim to 10% driver cannot, and needs a separate relay or switch to actually turn the lights off. It is a detail buried in datasheets and easy to miss, and missing it means delivering a scheme where the lights dim right down but never go out from the wall plate.
Can I dim any LED fitting? No, and "dimmable" on the packaging is not sufficient on its own. Confirm the fitting is explicitly dimmable, then check whether the manufacturer publishes an approved dimmer list, because good manufacturers do and it reflects actual testing. Following that list is the difference between a scheme that works on commissioning and an afternoon spent swapping dimmers on site to find one that behaves.
Notice
This article is published by Archlior for general information and educational purposes. It is written for architects, interior designers and specifiers as an introduction to dimming and lighting controls.
It is not professional lighting design advice, electrical engineering advice, or a statement of regulatory compliance for any project.
Minimum load figures, dimmer and driver compatibility, and protocol behaviour vary considerably between products and manufacturers. The figures given here reflect commonly published values and industry practice rather than any standard, and the worked examples are illustrations rather than predictions for a specific installation. All compatibility must be confirmed against the manufacturer's published data and approved dimmer lists for the specific products being used.
All electrical work must be carried out by a suitably qualified electrician working to the regulations in force locally.
Archlior accepts no liability for any loss, cost or damage arising from reliance on the information in this article. Readers act on it at their own discretion and remain responsible for compliance with all applicable regulations.
Last reviewed: August 2026.