Which dimmer switch works with LED bulbs?

Dimmers do not reduce voltage, they chop the mains waveform and pass only part of each half cycle. A filament lamp is a simple resistor that does not care how ugly the chopped waveform is. An LED lamp contains a switching power supply that very much does, and the mismatch between the two is the whole of the LED dimming problem.

Step by step

1. Understand leading edge.A leading edge, or TRIAC, dimmer switches on part-way through each half cycle, cutting off the front. It is cheap, robust, and was designed for incandescent lamps and wire-wound transformers. Its abrupt switch-on is what makes LED drivers buzz and flicker.
2. Understand trailing edge.A trailing edge dimmer conducts from the start of the half cycle and switches off part way through, using a MOSFET rather than a TRIAC. The turn-off is far gentler, which suits electronic drivers. It is quieter, smoother and the right default for LED.
3. Check the minimum load.This is the figure that catches people. A dimmer designed for filament lamps often needs 40W or more to operate stably. Four 5W LED bulbs is 20W, below the minimum, so the dimmer misbehaves — flicker, ghosting, refusing to strike. LED-rated dimmers typically go down to 1W to 10W minimum.
4. Check the maximum LED load, not the maximum wattage.A dimmer marked 250W may only be rated for 100W of LED, because LED drivers present a difficult load with high inrush. Manufacturers publish a separate LED rating and it is always lower. Use that figure.
5. Use lamps marked dimmable.A non-dimmable LED on a dimmer will flicker, strobe, buzz, fail early or simply not light. There is no adjustment that fixes it. "Dimmable" on the box is not marketing — it means the driver is designed to interpret a chopped waveform.
6. Prefer one brand of lamp per circuit.Different drivers respond differently to the same waveform. Mixing makes matched cases go out of step and produces visible stepping and flicker. It is the cheapest thing to get right and the one people ignore.
7. Check for a trim adjustment.Many good LED dimmers have a small potentiometer or a push-and-hold setup mode to set the minimum brightness, which is how you eliminate the dead zone at the bottom of the range. Read the instructions — this feature solves most residual complaints.
8. Handle two-way and multi-gang correctly.Only one dimmer per lamp circuit, ever. On a multi-gang dimmer plate, the per-gang rating is usually much lower than the plate total because of heat, so read the derating table.

Tips

• Universal dimmers that auto-detect the load type exist and are a good default when you do not know what you are driving.
• Low voltage downlights on a transformer need a dimmer matched to the transformer type — leading edge for wire-wound, trailing edge for electronic. Integrated LED fittings usually specify a compatible dimmer list.
• If you are choosing new lamps and a new dimmer together, buy one lamp and try it first. It costs a few pounds and saves a full set of the wrong thing.

Common mistakes

• Keeping the old filament dimmer and just changing the bulbs — The old dimmer's minimum load is typically far above what the LEDs draw, and its leading-edge waveform is exactly what LED drivers handle worst. It is the single most common cause of LED dimming complaints.
• Judging capacity by the dimmer's headline wattage — A 400W dimmer might be rated 60W for LED. The LED rating is the one that governs and it is usually in the small print.
• Mixing lamp brands or models on one dimmer — Each driver responds slightly differently, so some lamps strike before others and the set steps rather than fading. Nothing is faulty; they are just not matched.

If it doesn't work

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