Dimmable LED Bulbs

Why an LED has to be made to dim

An old incandescent bulb dimmed naturally, because turning the power down simply cooled the filament. An LED works differently: it runs from a small driver inside the bulb, and that driver has to be designed to respond to a dimmer. A bulb without one either ignores the dimmer, flickers, buzzes or fails early. That is why dimmable is a separate specification rather than a feature every LED has, and why it is worth choosing deliberately rather than hoping. The mark to look for is simply the word dimmable on the product; where it is missing, assume the bulb will not dim, however similar it looks to one that does.

The dimmer matters as much as the bulb

Most problems with dimmed LEDs come from the switch, not the bulb. Older dimmers were built for the heavy load of incandescent bulbs and expect a minimum wattage that a few LEDs never reach, so they flicker at the bottom of the range or will not dim smoothly at all. A modern trailing-edge dimmer, rated for LED loads, is the reliable pairing. If a room still flickers with dimmable bulbs fitted, the dimmer is the first thing to change. The same is true when a room is changed from a few old bulbs to many small LEDs: the total load can fall below what the dimmer needs to work, and a dimmer rated for a low minimum load solves it without changing a single bulb.

How low it will go

Every dimmable LED has a range, and none of them fades all the way to nothing the way an old filament bulb did. The best dim smoothly to a small fraction of their full output; others step down, or cut out before the bottom of the dial. If very low light matters, such as a bedside lamp or a dining room in the evening, a dimmer with an adjustable minimum setting lets you tune the bottom of the range to the bulbs you have. It is set once, with the bulbs fitted, and then left alone. Without that adjustment, the safest course is to test the lowest setting on the dimmer before relying on it, because the point at which a bulb cuts out varies from range to range and is rarely printed.

Matching bulbs on one circuit

Bulbs on the same dimmer should be the same, ideally from the same range. Different drivers respond to a dimmer differently, so a mixed set will dim unevenly, with one bulb still bright while its neighbour has faded. In a room of downlights this is obvious at once. If one bulb on a circuit fails, replacing it with the same product keeps the set behaving as one; replacing it with a different dimmable LED is where odd behaviour usually starts.

Caps, shapes and filament

Dimmable LEDs come in all the everyday caps: E27 and E14 screw, B22 bayonet, GU10 spotlight and G9 capsule. GLS and candle shapes suit shades and chandeliers, spotlights suit downlights, and capsules suit the small G9 fittings in many pendants and wall lights. Filament LEDs dim beautifully and are the natural choice where the bulb is on show, because at low power they glow in a way that recalls an old bulb.

Warm white for dimming

Most dimmed light is evening light, which is why warm white makes up most of this range. A warm white bulb turned down reads as cosy; a cool white one turned down can look grey and flat, because an LED keeps its colour as it dims rather than growing warmer the way a filament did. If you want the light to warm as it dims, look for a bulb described as warm-dimming or with an adjustable colour temperature; otherwise choose warm white for rooms you dim.

On an ordinary switch

A dimmable bulb works perfectly well on a normal on-off switch and simply never uses the feature, so there is no harm in buying dimmable for a fitting that might go on a dimmer later. The reverse is not true: a non-dimmable bulb on a dimmer is the most common cause of flicker. When in doubt, dimmable is the safer choice.