When Cree came out with their 3-way bulb a couple of weeks ago, I thought, big deal. They already make a 60W- and a 100W-equivalent LED bulb, so a 3-way just sort of smushes them all into the same bulb.
But then I read how Cree said that it was the second most requested LED product, and I remembered my intense dislike of phase-cut dimming switches. Perhaps 3-way lights could be another work around for dimming, as was the dim-on-rapid-switch approach (which so far I haven’t seen take off), whereas 3-ways are apparently already popular with some consumers. Hmmm.
So I got my hands on a Cree 3-way ($22, Home Depot; AFAIK not eligible for rebates) and switched it on, noticing the very obvious visual difference between the three light levels: 320 lm (30W-equivalent); 820 lm (60W-equivalent); and 1620 lm (100W-equivalent). Cree says that in the old incandescent 3-way bulbs, the common 30W/70W/100W was not too useful because, while the human eye can easily see a difference between 30W of incandescent light and 70W, the difference between 70W and 100W is barely discernible. Because of the way the incandescent bulb and the lamp fixtures switch works, incandescent bulbs are constrained to the brightest light being the sum of the two lower lights.
Here’s a brief summary of how the switch works: A 3-way incandescent bulb has two separate filaments, most commonly a 30W and a 70W filaments. The base of the bulb has three different contact points: Ground to the threads (same as a regular bulb) the hot connection down at the center of the base(Also like a regular bulb, and another hot connection, insulated from the first, that’s a ring around the first hot connection at the base of the bulb. (See photo.)
So the switch position on a 3-way lamp will either power one filament or the other, or both together, or all off. For a 30W/70W/100W bulb the brightness levels are the lowest level, 30W, the brighter filament, 70W, or the sum of both, 100W.
In the world of incandescent light, you’re stuck with the brightest light being the sum of the two filaments. But how does it work with LEDs? Because LEDs are current-controlled devices, Cree’s bulb works by varying the current through the LEDs in three levels.
Because 3-way bulbs enable dimming without having to use a phase-cut dimming switch — which is a noisy mess — the Cree LED 3-way bulb can do without the circuitry required to smooth and filter the output of a dimming switch.
(Never operate this or any other bulb with its cover off; It has exposed line voltage.)
With the top off, you can see that the bulb’s LEDs and heat sink is the same as the Cree 100W-equivalent LED bulb. Regardless of the switch position, all of the LEDs are illuminated with the same current — there’s no switching in banks of LEDs to mimic the way that the incandescent bulb switches in different filaments. I’m guessing the board uses the same chip family as we saw in the 100W bulb teardown, which is possibly a custom version of the STMicro 656X power controller family. This time the cryptic marking is “E2 4LE”; the 100W-equivalent bulb’s IC read “E2 3zE.”
As you would expect from a PWM-driven bulb, the color temperature and CRI stays quite constant regardless of the light output of the LEDs. (Measured with MK350 handheld spectrometer.)
|Switch position||Lux||CCT||CRI||Watts||Power Factor|
The measured power factor is quite high for the bulb, especially at the brighter outputs. If high power factor is important to you, note that 3-way bulbs have an advantage that they can maintain a high power factor at all times, unlike phase-cut dimmers where the power factor goes way down when the bulb is dimmed.
The pc board for the 3-way bulb is quite similar to the 100W-equivalent version, in keeping with Cree’s game plan of standardizing on one basic hardware platform.
So I’ve made an about-face, from not being able to see the benefits of a 3-way bulb to becoming a grudging supporter. I personally don’t use dimming on my home lights, but for those who need dimming and don’t want to go the phase-cut dimmer switch route, with all of its noise and poor performance, the 3-way is a good alternative.
Oh, and maybe you thought you don’t have a 3-way lamp fixture in your house? If you have a common table lamp, and have ever wondered why you have to switch the darn thing twice to get it to go on or off, you have a 3-way fixture. The second “click” is the switch looking for the 3-way contact.
I also saw 3-ways from Philips, Osram Sylvania, and TCP being demonstrated at LightFair this month; the market may be heating up.