How LED Dimming Works, and Why It Sometimes Flickers

A filament dimmed because you reduced the power reaching it. An LED dims either by being switched on and off faster than the eye can follow, or by having its drive current reduced — and neither of those is what a dimmer built for filaments produces. That mismatch is the source of nearly every complaint about dimmed LEDs: flicker, buzz, a narrow usable range, and lamps that refuse to light at all at the bottom of the dial.

None of what follows involves replacing a wall dimmer. That’s fixed wiring, and it’s a qualified electrician’s job in most jurisdictions.

Why filaments were easy

An incandescent bulb is a resistor that glows. Reduce the average voltage across it and the filament runs cooler, so it emits less light — and, because a cooler filament glows oranger, it also shifted colour as it dimmed. The filament’s thermal inertia smoothed out anything jerky in the supply, so even a crude dimmer produced a smooth, silent, full-range result down to a dull red glow. Almost every property of that behaviour was a happy accident of the physics, and LEDs inherit none of it.

What the dimmer actually does

The traditional domestic dimmer doesn’t reduce voltage. It chops the mains waveform: each half-cycle, it stays off for part of the cycle and conducts for the rest, so the average power delivered drops. This is phase-cut dimming, and it comes in two flavours.

Leading-edge dimmers, the older type, switch on partway through each half-cycle, cutting off the beginning. They’re built around a triac, they’re cheap, and they were designed for the large, forgiving loads filament bulbs represent. The switch-on is abrupt: fine for a filament, rough on electronics.

Trailing-edge dimmers conduct from the start of the half-cycle and switch off partway through. The switching is gentler, they generate less electrical noise, and they suit LED drivers much better. They cost more.

There’s also 0–10 V and DALI control in commercial installations, where a separate low-voltage signal tells the driver how bright to be rather than mangling its supply — cleaner, and the reason professional installations behave better.

What the LED lamp does

An LED emits light in proportion to the current through it, and it has essentially no thermal inertia — it responds instantly. Every mains LED lamp therefore contains a driver: electronics that take the mains supply and produce a controlled current for the diodes.

Dimming has to happen inside that driver, by one of two methods:

Pulse-width modulation. The driver switches the diodes fully on and fully off very rapidly, varying the proportion of time they’re on. Half brightness means on half the time. Because the switching is far faster than vision can resolve, it reads as a steady, dimmer light. PWM holds colour consistently across the range, because the diodes are always running at their design current when on. Its weakness is that a low switching frequency, or a deep dim level, can produce visible or camera-visible flicker.

Constant-current reduction. The driver simply supplies less current. No switching artefacts — but LEDs shift colour slightly as drive current changes, and at very low currents the output becomes unstable and inconsistent between diodes.

Many drivers combine the two: current reduction over the upper range, PWM below it.

Why it goes wrong

Now the mismatch is visible. A phase-cut dimmer is delivering a chopped waveform to a driver that wants a clean supply, and asking it to infer a brightness level from how much of the waveform survives.

Flicker happens when the driver can’t hold a steady output from a heavily chopped input. Its internal smoothing was sized for a full waveform; take most of it away and the output ripples at mains frequency or its harmonics. Visible flicker is the obvious version; sub-visible flicker still shows up on phone cameras and is reported by some people as eye strain.

Buzz and hum come from two places: the dimmer’s inductive components responding to the abrupt switching of a leading-edge cut, and components inside the lamp’s driver vibrating at switching frequency. Both are mismatch symptoms rather than faults in either device individually.

Minimum load. A phase-cut dimmer needs a certain current flowing to operate its own control circuitry — a figure sized for filament bulbs. An LED draws a small fraction of what a filament did, so a fitting that was fine before conversion may present too little load afterwards. Symptoms: lamps glowing faintly when switched off, flicker at the bottom of the range, or erratic behaviour.

A narrow usable range. The bottom of the dial does nothing, then the lamps jump to a moderate brightness. The driver has a minimum input below which it can’t run at all, and that threshold arrives well above the dimmer’s minimum setting.

Uneven behaviour between lamps. Several nominally identical lamps on one dimmer reaching their minimum threshold at slightly different points, so at low settings some are lit and some aren’t. Mixing lamp types on one circuit makes this worse.

No colour shift. Not a fault, but a disappointment: dimming an ordinary LED gives you less light of the same colour, where a filament went orange. Since low brightness and low colour temperature together are what reads as cosy — the argument is in what colour temperature actually means — a dimmed LED room can feel dim rather than warm. Dim-to-warm lamps reintroduce the shift deliberately.

Where smart control fits

A smart bulb sidesteps the phase-cut problem entirely, because the dimming instruction arrives over a radio and the driver receives a clean, undimmed mains supply. It does its own PWM at a frequency it chose, across a range it defines — which is why smart bulbs generally dim more smoothly and further than the same lamp on a wall dimmer.

The catch: a smart bulb must not be installed on a phase-cut dimmer. Its driver expects full mains, the dimmer expects a dimmable load, and the combination produces flicker, erratic behaviour, or damage. Replacing the dimmer with a plain switch, or with a smart module designed for the job, is fixed-wiring work for an electrician either way.

Smart bulbs also need power to be reachable, so the wall switch has to stay on. A household that switches lights off at the wall finds the automation unavailable, which is the most common practical disappointment with them.

What to actually do

1. Check the lamp is marked dimmable. A non-dimmable LED on a dimmer will flicker or fail, and there’s no fixing it at the dimmer end.
2. Find out which dimmer you have. Trailing-edge suits LEDs; leading-edge is the usual cause of buzz and limited range.
3. Check the dimmer’s minimum load against the total wattage of the LED lamps — converting a fitting from filament to LED often drops below it.
4. Use identical lamps on one dimmer, bought together, so thresholds match.
5. Consult a manufacturer’s compatibility list if one exists for either device; this is one area where they’re genuinely useful.
6. Consider dim-to-warm lamps if the goal is a room that gets cosier rather than just darker.
7. Never put a smart bulb on a phase-cut dimmer.
8. Any dimmer or switch replacement is a qualified electrician’s job, subject to local wiring regulations.