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I have just had some 230V GU10 LED downlights installed in my living room (by a professional) to replace a traditional pendant fitting.

At the same time, we added a (mechanical) dimmer, however it gives off a very audible buzz from the switch and the light fittings on anything but full.

The light fittings and bulbs are all rated as dimmable, and the dimmer is specifically designed for mains-voltage LEDs, I'm told that this is an unfortunate by-product of using a mechanical dimmer; every dimmer switch I've ever owned has given off an audible hum, but not to this extent and never from the light fittings as well as the switch. There is a second adjustment fine-tune pot on the back of the dimmer, however this is for adjusting in case of flicker, and does nothing but "shift" the hum to different brightnesses.

So, my question is twofold: is the above valid? Are there alternatives, like some kind of switch with 3 positions (e.g. off/low/hi) that may not give off the same hum?

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Yes, there are digital dimmers. Lutron being one of several manufacturers: lutron.com/en-US/Products/Pages/StandAloneControls/… – DA01 Jul 30 '13 at 21:50
I'd ask the professional who installed the lights. – mike Jul 30 '13 at 23:46
I've had a few dimmers that hummed, then I replaced them! I'm in the US using 110v but I was always taught a humming dimmer switch was not a good thing. – Jason Jul 31 '13 at 0:14
up vote 1 down vote accepted

As far as I know humming is produced mainly in the core of inductions (coils, transformers) built into dimmers and voltage converters.

Unfortunately all of these 230V LEDs have their own voltage converters, with dimmeable LED's usually sensing the phase cut angle and adjusting the LED chip current accordingly. So you probably may not overcome the humming inside the LED itself. Maybe the only way to have humming free LED light is to use LED bulbs without any current shaping electronics (e.g. 350 or 700mA LEDs) and matching PWM dimmers.

As far as the dimmer is concerned, there are products from Osram, Tridonic and others that are fully solid state electronics, well encapsulated and shouldn't produce humm by themselves. They work on bus systems (DALI, 10V, KNX/EIB) as well as on tap switches.

Regarding off/lo/high ... Ledon produces a LED bulb with "double click" feature, where you use an ordinary on/off switch to switch between off, 30% and 100% ... I saw it in operation recently - very sexy ... other manufacturers are following - search the net for "double click light"

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