Dimming is where LED lighting most often disappoints people, and it is almost always a compatibility problem between two components that were designed decades apart. The good news is that the fault is systematic and can be diagnosed in a fixed order. The bad news is that the fix usually involves changing the dimmer, which means an electrician.
Why dimming an LED is harder than dimming a filament
Dimming an incandescent lamp is almost embarrassingly simple. Reduce the average voltage, the filament runs cooler, it emits less light. The lamp is a resistor; it does not care how the voltage was reduced, and the thermal inertia of the filament smooths out anything the dimmer does to the waveform.
An LED has no thermal inertia and is not a resistor. It is a diode with a sharp knee in its current-voltage curve: below the forward voltage it conducts almost nothing, above it conduction rises steeply. You cannot dim it by lowering voltage in any controlled way. The driver must instead regulate current, and it must do so while being fed a mains waveform that a dimmer has been chopping into pieces.
So dimming an LED lamp is really a negotiation: the dimmer distorts the supply in order to signal “less”, and the driver has to both power itself from that distorted supply and interpret the distortion as a brightness command. When the two do not agree, you get flicker, buzz, drop-out or dead travel.
Leading edge, trailing edge and the rest
Leading-edge (TRIAC / forward phase)
The old standard, designed for incandescent and magnetic transformers. It switches on partway through each half-cycle, cutting off the leading part of the waveform. Cheap, robust, and it produces a sharp switching edge full of harmonics — which LED drivers dislike, and which makes lamps buzz.
Leading-edge dimmers also need a minimum load, typically 40–60 W, to hold the TRIAC conducting. Six LED lamps at 10 W present 60 W to an incandescent-era dimmer’s expectations only on paper; the actual current draw is far lower than the TRIAC expects, and below its holding current it will drop out and re-trigger, which you see as flicker.
Trailing-edge (ELV / reverse phase)
Switches off partway through each half-cycle, cutting the trailing part. The turn-on is a smooth zero-crossing rather than an abrupt step, which is gentler on driver electronics, quieter, and gives a noticeably smoother dimming curve. Minimum loads are much lower, often under 10 W. This is the right choice for almost every LED retrofit.
0–10 V and DALI
Used in commercial installations. A separate low-voltage control pair tells the driver what level to run at, so the mains supply is never chopped at all. The result is the smoothest dimming available and full addressability under DALI, at the cost of extra wiring. If you are wiring a commercial space from scratch, wire for DALI.
Mains-free wireless
Smart lamps dim themselves on command from a radio link, with the mains supply left intact. Excellent dimming quality; the trade-off is standby power, a dependency on a hub or network, and the need to leave the wall switch permanently on.
The short answer
If your LEDs flicker when dimmed, you almost certainly have a leading-edge dimmer. Replacing it with a trailing-edge, LED-rated model fixes the great majority of domestic dimming complaints.
Minimum load and maximum load
Every dimmer has a load range. Below the minimum it cannot regulate; above the maximum it overheats.
The minimum is the problem with LEDs. A dimmer specified 40–400 W for incandescent might be specified 10–100 W for LED, and the LED figure is the one that matters. Four 5 W lamps are 20 W and will work; two are 10 W and are marginal; one is 5 W and will flicker.
The maximum is the problem in commercial work, and it is not the one you expect. LED drivers draw a large inrush current at switch-on as their input capacitors charge — briefly, tens of amps per lamp. A dimmer rated for 400 W of LED may still fail if forty lamps switch on simultaneously, because the inrush exceeds what its switching device can survive. Manufacturers publish a maximum number of drivers alongside the wattage rating for this reason. Count the lamps, not just the watts.
Cheating the minimum load
Dummy-load modules exist that sit at the fitting and burn a few watts to bring the circuit above a dimmer’s minimum. They work. They also throw away part of the saving you just installed, and they run warm. Treat them as a last resort for a dimmer that genuinely cannot be replaced.
Flicker: the kind you see and the kind you do not
All mains lighting flickers to some extent, because the supply reverses 100 or 120 times a second. An incandescent filament smooths this almost entirely. An LED, having no thermal mass, reproduces whatever its driver gives it.
Two measures describe it. Percent flicker is the modulation depth: the difference between peak and trough as a proportion of the total. Flicker index weights the waveform shape and is the better measure, though less often published.
Why it matters, even when invisible:
- Below about 90 Hz flicker is directly visible to most people and is a stroboscopic hazard around rotating machinery.
- Between 90 and 165 Hz it is generally invisible but is associated in the literature with headaches, eyestrain and, in susceptible individuals, migraine and photosensitive epilepsy.
- Above about 200 Hz the biological effects largely disappear, but the stroboscopic effect visible in fast movement — the “phantom array” you see when you move your eyes quickly past a lamp — persists to well over 1 kHz.
- On camera, flicker produces rolling bands, which matters in any space that will be filmed or used for video calls.
Cheap drivers dim by pulse-width modulation at low frequency, which produces deep modulation at 100–200 Hz. Better drivers use constant-current reduction, or PWM above 2 kHz. IEEE 1789 gives the recommended limits; in practice, ask for flicker below 5% at all dimming levels for offices, classrooms and healthcare, and below 10% for general use.
You do not need an oscilloscope for a first check. Point a phone camera at the lamp in video mode and look for banding, or wave a pencil rapidly under it and look for a stroboscopic multiple-image effect. Both tests fail to detect good lamps and reliably detect bad ones, which is the useful direction.
A fault-finding sequence
Work through this in order. Each step eliminates a class of cause.
- Does the lamp flicker when not dimmed? If yes, the dimmer is innocent. Suspect a loose neutral, a shared neutral on a multi-way circuit, or a failing driver. Test the lamp in a non-dimmed fitting.
- Identify the dimmer. Take the plate off and read the model number. Search it with the word “trailing”. If it is leading-edge and older than about 2015, you have found the problem.
- Add up the load. Total lamp wattage against the dimmer’s LED minimum and maximum. Also count drivers against any stated maximum number.
- Check the compatibility list. Both lamp and dimmer manufacturers publish them. If your combination is not listed, that is a finding, not an oversight.
- Try one lamp alone. If a single lamp behaves and six do not, suspect inrush or cumulative driver interaction. If one lamp misbehaves and six are fine, it is minimum load.
- Try a different lamp brand. The cheapest diagnostic available. Two lamps of one brand and two of another on the same dimmer will separate lamp problems from circuit problems in five minutes.
- Check the trim adjustment. Most LED-rated dimmers have a low-end trim, sometimes a tiny screw, sometimes a button sequence in the manual. Setting it correctly removes dead travel at the bottom of the dial and stops drop-out.
- Check for a neutral. Two-wire dimmers without a neutral trickle current through the load to power themselves, which causes low-level glow and flicker with low-wattage LEDs. Fitting a dimmer with a neutral connection resolves it where the wiring allows.
- Measure the supply. Persistent problems across a whole building point at the supply rather than the fittings. Log the voltage for a day.
Specifying dimming that works first time
For a new installation, the sequence is straightforward:
- Choose the lamp or luminaire first, on light quality grounds. Do not let the dimmer dictate the lighting.
- Read the driver’s compatibility list and pick a dimmer from it. This is the entire trick.
- Prefer trailing-edge for residential and small commercial, 0–10 V or DALI for anything larger or anything that will be recommissioned later.
- Size for inrush, not just wattage, and derate by 20% for headroom.
- Specify dim-to-warm in living and hospitality spaces. Reproducing the incandescent warm shift is most of what people mean when they say dimming “feels right”.
- Specify flicker limits in writing for offices, schools and healthcare. If it is not in the specification, you will not get it.
- Buy one of everything and test it before ordering two hundred. A sample circuit costs an afternoon and saves a re-fit.
We will do this part for you
Every lamp in the uBRITE range ships with a published dimmer compatibility list, and our retrofit service tests every dimmer through its full range before the fitters leave. If you are specifying yourself and want the list for a particular lamp, ask us — we will send it.
Symptom-to-cause quick reference
| Symptom | Most likely cause | First thing to try |
|---|---|---|
| Flicker at low levels only | Below dimmer minimum load | Adjust low-end trim; add lamps; change dimmer |
| Flicker across whole range | Leading-edge dimmer | Replace with trailing-edge LED-rated |
| Audible buzz from the lamp | Phase-cut harmonics exciting driver components | Trailing-edge dimmer; different lamp |
| Audible buzz from the dimmer | Inductive load interaction, or overload | Check load rating and driver count |
| Dead travel at bottom of dial | Trim not set for LED load | Set the low-end trim per the manual |
| Lamps drop out then restart | TRIAC losing holding current | Increase load or change dimmer type |
| Faint glow when switched off | Two-wire dimmer or illuminated switch leakage | Bypass capacitor at fitting; neutral-wired dimmer |
| One lamp brighter than the rest | Colour/flux binning, not dimming | Batch-matched replacement set |
| Banding on camera | Low-frequency PWM in driver | Specify flicker < 5%; change luminaire |
Common questions
Sometimes, badly. An old leading-edge dimmer will often light the lamps but flicker at the low end, buzz, and shorten driver life. A trailing-edge LED-rated dimmer costs about $25 and fixes nearly all of it.
That is a supply symptom, not a lighting one. A large inductive load starting causes a brief voltage sag that the driver reproduces as a dip. Persistent, visible dips across a building are worth reporting to your utility — they can indicate a loose service neutral, which is a safety issue.
For most people, invisible flicker is a comfort issue rather than a hazard — headaches and eyestrain after long exposure. For people with photosensitive epilepsy, flicker between roughly 3 and 70 Hz can trigger seizures, and for anyone working near rotating machinery the stroboscopic effect is a genuine safety risk. Both are reasons to specify low-flicker drivers in workplaces.
