Why does an LED bulb or dimmer hum?

Almost always because of a dimmer that does not suit LED. An old leading-edge dimmer sends a steep current surge into the lamp on every half cycle of the mains, and the electronics in the LED or in the dimmer itself vibrate along with it. A trailing-edge LED dimmer switches more gently and therefore hums little or not at all.

Know what you need? View all Accessories (40) →

What a dimmer actually does

A dimmer cuts a piece out of every half cycle of the mains voltage. That happens a hundred times a second. There are two ways to do it:

• Leading edge, also called phase cut or R-L. The dimmer leaves out the start of the wave and then switches on abruptly. This is the classic dimmer for incandescent bulbs and halogen with a wound transformer.
• Trailing edge, also called R-C. The dimmer lets the wave start gently and cuts off the end.

For an incandescent bulb it makes no difference: a filament is simply a resistor. An LED bulb has a small power supply with capacitors and coils. The abrupt switch-on of a leading-edge dimmer causes a current spike there, and parts that respond to that spike start to vibrate audibly. That is the hum.

Our own LED dimmers are all trailing-edge dimmers. On the universal dimmer from 3 to 100 watts it is printed on the front. That is exactly why it dims without buzzing or humming.

The four causes at a glance

1. An old dimmer. If it only hums when the dimmer is not at full, this is the first suspect. Look on the dimmer or behind the cover plate: if it says R-L, or only gives a maximum for incandescent bulbs, it is a leading-edge dimmer.

2. Too little load. Every dimmer has a minimum. An old halogen dimmer sometimes only starts at 20 watts or more, while four LED spotlights together draw less than 25 watts. Our 160 and 250 watt dimmers start at 0.5 watts, the universal one at 3 watts.

3. An old halogen transformer. If you have 12 volt spotlights on a transformer above the ceiling and only replaced the bulbs with LED, the transformer can start to buzz or flicker. It was made for a much higher load. That is covered in How do you replace halogen spotlights with LED?

4. The bulb itself. If one bulb hums and the rest do not, even without a dimmer, the problem is in that bulb's power supply. Swap it with another: if the noise moves with it, it is the bulb.

How to track it down

• Turn the dimmer up to full. If the hum stops, it is almost certainly the dimmer.
• Listen to where the sound comes from: the switch in the wall, the light, or above the ceiling.
• Add up the wattage of all the bulbs on the dimmer and compare it with the minimum on the dimmer.

The fix

With an old dimmer: replace it with a trailing-edge LED dimmer. Then use the trim screw to set the minimum level so the bulbs do not flicker at the lowest setting. For a few spotlights or a pendant light the 3 to 100 watt dimmer fits, for a larger group the dimmer up to 250 watts. Which size you need is in Which LED dimmer do you need for your spotlights?

With a bulb that hums by itself: replace it, and if it is on a dimmer choose a bulb sold as dimmable. A non-dimmable bulb on a dimmer hums, flickers or fails sooner.

When not to wait

A soft hum from a light is annoying but not dangerous. A crackling or hissing sound from a switch or a box is something else, especially if you also smell something or the cover plate feels warm. That can point to a loose connection. Switch off the circuit at the fuse box and have it checked.

Flickering rather than humming? The explanation is in Why does your LED bulb flicker?