Almost always, neither part is broken. The dimmer was designed for a load a hundred times bigger than the one you have connected to it.
The root of it
A dimmer does not reduce voltage. It chops the mains waveform, switching off for part of each cycle so less energy reaches the lamp.
With an incandescent filament this worked beautifully. The filament is a big, simple resistive load with thermal inertia — it cannot cool fast enough to follow the chopping, so it glows smoothly.
An LED is neither big nor simple nor slow. A 9 W LED draws roughly a fifteenth of the current a 100 W filament did, and its driver responds to changes almost instantly. Feed it a chopped waveform and the driver may lose its footing between cycles, restart, or pass the chop straight through to the chip.
The result is visible flicker, buzzing, a limited dimming range, or bulbs that jump to full brightness at the bottom of the dial.
The four causes, in the order worth checking
1. The dimmer is an old leading-edge type. Most dimmers installed before LEDs were common are leading-edge (TRIAC), designed for resistive filament loads. LEDs generally want trailing-edge dimming, which switches off later in the cycle and is gentler on an electronic driver. This is the most common cause and the most reliable fix. See trailing edge vs leading edge.
2. The total load is below the dimmer's minimum. Nearly every dimmer specifies a minimum load — often 10 W, 20 W or 40 W. Three 6 W LEDs total 18 W and may fall under it. Below the minimum, the dimmer cannot regulate reliably and the symptom is flicker at the low end. See minimum load for an LED dimmer.
3. The bulbs are not dimmable. A non-dimmable LED on a dimmer will flicker, buzz, dim in steps, or fail early. It is printed on the box and it is not a suggestion.
4. Mixed bulbs on one dimmer. Different drivers respond differently to the same chopped waveform. Two brands on one circuit can each be fine alone and flicker together. Use one model throughout a dimmed circuit.
| Symptom | Most likely cause |
|---|---|
| Flicker at low brightness only | Below minimum load |
| Flicker across the whole range | Leading-edge dimmer, or non-dimmable bulbs |
| Buzzing from the dimmer | Leading-edge on an electronic load |
| Buzzing from the bulb | Driver struggling with the waveform |
| Will not go below about 30% | Normal dimmer range mismatch |
| Jumps to full at the bottom | Driver restarting |
| Only after adding one bulb | Mixed models |
The fix order
Start with the dimmer. Replacing a leading-edge dimmer with a modern LED-rated trailing-edge one solves the majority of cases. It is a like-for-like swap in the same backbox, and LED-rated dimmers are specified with low minimum loads.
Then check the numbers. Add up the wattage on the circuit and compare it to the new dimmer's stated minimum and maximum. Note that some manufacturers de-rate heavily for LED loads — a dimmer rated 400 W for filament may be rated 100 W or less for LED, and the LED figure is the one that applies.
Then check the bulbs are all dimmable and all the same model.
Then check the manufacturer's compatibility list. Most dimmer makers publish one, and most bulb makers name compatible dimmers. Two products that appear on each other's lists have been tested together.
When the dimmer is not the problem
If the light also flickers with the dimmer at full, or if it flickers on a plain switch, the dimmer is innocent. That is a bulb-quality or supply problem — start with flicker-free LED bulbs and the phone camera test in it.
The alternative worth considering
If you are replacing the dimmer anyway, a smart switch does the same job with better LED behavior, because it was designed for electronic loads from the start rather than adapted to them. The trade-offs against smart bulbs are in smart bulb vs smart switch.