Electronics Question : Buzzing LED bulb on a dimmer

Factual Questions
1

Alright, so I have a dimmer in the kitchen, and I installed a shiny new Cree LED lightbulb that says on the package it is compatible with “most” dimmers.

It lights up just fine, but there’s a high frequency buzzing sound.

Now, thinking about how this thing must work, LEDs need DC power.

So the simplest setup I could think of was that they use a series of diodes (bridge rectifier) to make the 120 Volt 60 Hz wall current all positive.

Then, they take that 60 Hz signal and smooth it out with a capacitor and an inductor. Apparently, there are Capacitor-inductor high pass filters, wiki names a “T filter”, though I assume there are other arrangements listed in the back of an EE textbook. The reason for a capacitor - inductor filter instead of a capacitor -resistor filter is that resistors waste energy, which is bad if you want an efficient lightbulb.

They tuned the filter to get rid of that 60 Hz waveform, leaving a nice smooth signal at whatever the RMS voltage is. Then, they probably have all the LEDs in series so they don’t have to lower the voltage to the 3 volts or so the LEDs run on.

Now, the wall switch has a variable resistor somehow connected to a power control transistor - maybe an OP amp and an oscillator in there? This is going to output a signal that is not 60 Hz, because the power is getting chopped at some higher frequency.

This signal is what is getting through into the LED lightbulb, and the buzzing sound is from the coils in the lightbulb’s inductor vibrating.

Is this a reasonable guess as to how the electronics really work and why it makes a noise? Next week, I’m going to take apart a broken LED lightbulb to find out what the actual circuitry looks like, and I guess I’ll have to yank that dimmer as well.

Still, for the electronics experts on these forums (I know there are several), is this a plausible explanation?

2

No.

AC-powered LEDs use a switching regulator. The incoming AC is rectified to HV DC (there may be a power-factor correction circuit here), and than this DC is “chopped” with a transistor, producing variable-width pulses, that are then filtered through an inductor. The inductor acts as a energy storage device, smoothing out the pulses, and creating continuous low-volltage DC.

See:Switched-mode power supply - Wikipedia

3

Dimmer switches work by controlling a triac to switch only part of each AC half cycle. This creates a lot of electrical noise from the fast switching edges, and this can manifest itself audibly in your LED light bulb by exciting the inductors (magnetorestriction) and/or ceramic capacitors (piezoelectric effect).

The Cree LED driver circuit won’t have the luxury of a nice clean DC supply to work with, and some of the electrical noise from the dimmer will certainly find its way to the output flyback transformer. I agree that this is probably the major source of the acoustic noise.

This paper explains the issues involved in dimmable LED light bulb designs: http://electronicdesign.com/energy/dimmable-leds-have-bright-future

4

To expand a little bit on what beowulff and Fridgemagnet said,

Controllers today are designed to be efficient at the expense of everything else.

In the old days, light dimmer used power rheostats to decrease the amplitude of the 120 VAC voltage sine wave going to the lamps. They worked well, but were terribly inefficient, so today we use “phase-fired proportional control” to dim lights. These controllers use a solid state switch to delay the time of “turn on” for every half cycle in the voltage sine wave. They’re very efficient, but they can create a horrendous amount of EMI, both radiated and conducted. The high frequency content of the EM noise can cause the iron laminations in transformers & inductors located in the load to vibrate.

When LEDs are used for home lighting, flashlights, etc., a simple current-limiting resistor is usually not good enough. Modern LED lamps use a sophisticated power controller circuit that measures the LED voltage and current (and sometimes temperature) and supplies the optimum and most reliable power to the LEDs. A DC-to-DC converter is used to efficiently convert the raw incoming DC to a DC voltage appropriate for the controller and LEDs.

5

Good point on the ceramic caps. I forgot about those. And the filaments in incandescent lamps can also resonate.

It’s also worth mentioning that some dimmer controls are better than others. The better ones incorporate an LC filter to help “roll off” the sharp corners in the voltage waveform caused by the rapid turn on / turn off in each half cycle. These are more expensive, but well worth it IMO. According to Leviton, “A Radio Interference Filter is a built-in filter that prevent radio interference created by dimmers. Leviton filters are designed to prevent even the largest lighting loads from interfering with sensitive electronic equipment. Included in all Leviton Dimmers except the No. 700 model.”

6

That’s for sure. As a schoolchild, a long, long time ago, I had a Saturday job with an electronics workshop that, among other things, built an add-on light dimmer unit that fitted between the bulb and the bulb holder. It worked well, apart from that now your light bulb stuck out of the lampshade by an extra 4 inches, you’d have to reach past a hot light bulb to adjust the dimmer control, and you had a choice of having the light on or listening to the radio, but not both. I think they occasionally exploded too.

Anyhoo, here’s another link that mentions a tendency to use rubbery potting compound around LED bulb drive electronics, and suggests acoustic noise could be an issue across the board: http://www.edn.com/design/led/4369641/LED-bulbs-reveal-different-design-approaches