Dimming LED bulbs: the right dimmer, and what to do about flickering or humming

Which LED bulbs can you dim?

Only the bulbs marked “dimmable”. An LED bulb contains a small power supply, the driver, which converts the mains voltage into the current for the LEDs. Only a driver designed for it understands what a dimmer does. The European ecodesign rules oblige manufacturers to state on the packaging if a bulb is not dimmable or only works with certain dimmers.

A non-dimmable LED bulb on a dimmer is no small matter. Signify, the manufacturer of Philips Hue, warns that it can cause flickering, humming and even damage to the bulb and the switch.

With fixtures that have built-in LED, the driver determines how you dim. There are four possibilities: with an ordinary wall dimmer (phase dimming), with a control signal such as DALI or 1–10 V, wirelessly via an app, or not at all. It is in the product details, usually as “dimmable (trailing edge)”, “triac dimmable” or “DALI”.

How does an ordinary dimmer work?

The mains voltage is an alternating voltage of 50 Hz: a hundred half-waves per second. A wall dimmer cuts a piece off each half-wave. An incandescent bulb thus receives less energy and glows less brightly. An LED bulb responds differently: the driver measures how much has been cut off and translates that into a lower current through the LEDs.

That is where the difficulty lies. How well that translation works depends on the combination. The American Pacific Northwest National Laboratory studied this for the US Department of Energy and found that dimming problems with LED are usually caused by a poor combination of bulb and dimmer, not by a poor bulb. What a manufacturer says about the dimming range is a possibility that you only achieve with the right dimmer.

Trailing edge or leading edge: what is the difference?

There are two ways to cut a piece off the wave. With leading-edge dimming, the dimmer only switches on some time into each half-wave, cutting off the beginning. With trailing-edge dimming, it switches on straight away and off again later in the wave, cutting off the end.

Why is trailing edge usually the better fit? Many LED drivers do not draw their current evenly, but in short peaks. According to the research by the Pacific Northwest National Laboratory, these recurring current peaks in LED bulbs are often five to ten times higher than the average current. A leading-edge dimmer switches on abruptly in the middle of the wave and so delivers exactly the current surge that such drivers tolerate poorly. You hear that as humming and see it as unsteady dimming.

It is not a law. Some bulbs work better with leading edge, and the manufacturer then says so in the product details. Follow that advice, and be wary of general claims such as “works with all dimmers”.

Minimum and maximum load: the most common mistake

Every dimmer has a minimum and a maximum load in watts. Below the minimum, the dimmer does not get enough current to work properly itself. Old dimmers were made for incandescent and halogen bulbs. In the American research, the minimum load of most installed dimmers was around 40 W. Connect two 5 W LED bulbs to one and you are far below the minimum.

LED dimmers are designed for this. The Belgian manufacturer Niko gives a range of 4 to 200 W for its rotary dimmer for LED bulbs (310-04901), with an adjustable minimum to avoid blinking or going out at the lowest setting and an adjustable start level so that difficult LED bulbs also come on. That is the kind of information you are looking for.

The maximum works differently too. Because of those current peaks, an LED bulb loads the dimmer more than its wattage suggests. Manufacturers therefore state a separate LED maximum, lower than the maximum for incandescent bulbs. If it is not stated, the American report advises dividing the incandescent maximum by five to ten: a 600 W dimmer would then still control 60 to 120 W of LED bulbs.

Flickering, afterglow, humming: cause and solution

Most dimming problems can be recognised by what you see or hear. The table follows the symptoms described in the American research, with the solution that most often works at home.

Afterglow needs explaining. A two-wire dimmer is in series with the bulb and has no connection of its own to the neutral wire. To power its electronics, it lets a very small current flow through the bulb even in the off position. A filament does not notice it, but an LED driver sometimes does: it starts working and the bulb glows faintly or blinks in the dark. A dimmer connected to the neutral wire, a three-wire dimmer, does not have this problem. Check whether there is a blue wire in the switch box before you buy a dimmer.

Flicker you do not see, but do notice

Some LED bulbs flicker so fast that you do not consciously see it. You notice it as a stroboscopic effect: a spinning fan or a moving hand seems to stutter, or stripes run through a video. Since 1 September 2021 the EU has had a limit for visible flicker (Pst LM at most 1.0) and for the stroboscopic effect (SVM at most 0.9, and at most 0.4 since 1 September 2024).

Those limits apply at full power. There is no European limit for dimmed light. A bulb that burns steadily at 100 per cent may therefore flicker at 20 per cent. Manufacturers that measure it state the values in their technical data. If you are buying bulbs for a room where you often sit with dimmed light, it is a question worth asking.

Can you dim an LED bulb from two places?

Not with every dimmer. An ordinary rotary dimmer only works in a single-pole circuit, with one switching point. Niko says so literally for the LED dimmer mentioned above: not suitable for two-way or intermediate switching. If you want to dim from two places in a hallway or a living room with two doors, you need a dimmer with a secondary control unit, or a wireless solution where the second control is a wireless push button. Have an electrician look at the circuit: with two-way switching there are more wires in the box than with an ordinary switch.

Dimming without a classic dimmer

You do not always need to fit a dimmer in the wall. There are four other ways.

• Bulbs that dim with the ordinary switch. Lucide sells filament bulbs with “3 StepDim”: they dim in three steps with the existing light switch and remember the last setting. There are also ceiling lights that you dim with an ordinary switch.
• Smart bulbs. A bulb with Wi-Fi, Bluetooth or Zigbee dims itself, controlled with an app, a wireless button or your voice. Never put such a bulb behind a wall dimmer: it needs a constant voltage. For the driver of its smart 48 V track system, Lucide expressly states that it is dimmable via the app and not compatible with an ordinary wall dimmer.
• LED strips. You dim an LED strip via the power supply or via a controller between the power supply and the strip. An ordinary wall dimmer only works with a power supply specified as “triac dimmable”. More in choosing an LED strip.
• Control signals. For larger installations, a system such as DALI, in which the dimmer sends a digital signal to each driver, is more reliable than phase dimming. It does need an extra control wire or a wireless variant.

Dim-to-warm: dimming like an incandescent bulb

An incandescent bulb became warmer in colour when dimmed. An ordinary LED bulb does not: when dimmed it only gets weaker, and therefore looks cold in the evening. Dim-to-warm bulbs do: Lucide states 3000 K at full power and 2200 K dimmed for its dim-to-warm GU10 spotlight, with a compatible dimmer. How to choose the colour temperature further is explained in choosing a colour temperature.