Figuring out a dimmable filament LED light bulb

As also noted previously, I’ve done a lot of LED light bulb teardowns over the years, not even counting LED illumination sources that aren’t bulb-shaped, like touch-activated and motion-sensing panels:

Conventional
Zigbee-controlled
Wi-Fi-augmented
Bluetooth-enhanced
Multicolor
Three-way, and
Battery-backed

They’re consistently popular with the readers, which is admittedly part of my (and EDN’s) motivation to continue doing them. But I personally also find them fascinating; inevitably I come across at least a thing-or-few that surprise me and/or I learn something from each time.

Today’s teardown “victim” is a two-fer: it’s (traditionally) dimmable, and it’s got a historically atypical but increasingly common multi-LED structure inside it. I alluded to the latter attribute within that earlier three-way bulb dissection project:

…see this comparative image of a conventional (albeit dimmable) LED light bulb also in my teardown pile:

(By the way, notice the hint of what looks like a filament structure inside this dimmable LED bulb. You’re going to have to wait for a future teardown to find out more about that!)

That time is now. But I’m getting ahead of myself; let’s first revisit that earlier “(traditionally) dimmable” comment. Why exactly is it that dimmer switch-compatible LED light bulbs have historically been (and to some degree remain):

User complaint-rife
Rare, and
Notably pricier

than their non-dimmable counterparts?

Here’s a good summary of the situation:

Most dimmers installed today are designed to be used with high-power circuits to drive traditional filament lamps which were all quite uniform and dimmable by just a voltage change. LED lamps on the other hand are low-power and more complex. An LED bulb is a solid-state product that has built in circuitry (called a driver) that takes high-voltage AC input current and converts it to low-voltage DC current to drive the LEDs. Furthermore driver specifications are not uniform across the LED industry.

They are many different types of dimmers installed in homes and offices of various specifications (e.g. resistive; leading-edge and trailing-edge and electronic). So when using new LED lamps with existing dimmers there is a matching of old technology with new which can be challenging.

The driver in dimmable LED lamps may work with many types of dimmer but not all, for instance LED lamps tend to work better with trailing-edge dimmers rather than leading-edge dimmers, but an existing dimmer may have a minimum load that is too high for an LED lamp, e.g. A 60W filament lamp may use a dimmer that has a minimum load of 25W the replacement LED has a power rating of 6.5W – below the level required by the dimmer. Dedicated LED dimmers have a very low minimum power rating.

The dimming experience can be different with LED. Overall the LED dimming performance is regulated by the capability of the LED driver/chip and the compatibility of the dimming circuit. Since there are a huge number of possible combinations of lamps and dimmer, it is very difficult to produce an LED lamp that works in all dimming environments.

LED currently have a lower dimming range than a filament lamp – LEDs currently dim down to about 10% of the total light output whereas filaments may go down to 1-2%. Low-voltage transformers as used with MR16 12V spotlights also add to the complexity.

Some of the issues that may occur when a dimmer is incompatible with an LED lamp are:

Flickering – Lamps will flicker (can also occur if a non-dimmable lamp is used)
Drop-out – No light output at the end of the scale
Dead travel – When the dimmer is adjusted there is no matching change in light output (light may not dim to acceptable level)
Not smooth – Light output may not go from dim to bright linearly
Multiple lamps – issues may become apparent when multiple lamps are added
Damage or failure – LED driver, circuit or LED is damaged or fails.
Load below minimum – The power load of the LED lamp is below the minimum required by the dimmer
Mixed models- Different models of LED will likely have different drivers – since drivers behave differently this could result in dimming issues.

I’ve personally experienced several of these issues with the arrays of dimmable BR40 LED light bulbs in my residence’s hallways and rooms, which replaced incandescent predecessors. On that note, however, also notice the words “traditional filament lamps” in the previous website article excerpt. Hold that thought.

One more clarification, regarding “(traditionally)”, before proceeding. If you look back at my prior LED light bulb teardowns, you’ll find several “smart” bulbs documented as being dimmable (as well as capable of changing their color temperature and broader color output, generating various strobe patterns, varying their behavior at various times of day, and the like). This isn’t them. Those earlier bulbs, as their “smart” names imply, integrate networked intelligence that handles not only AC-to-DC conversion but also dimming and other functions, are powered by a consistent AC voltage input and are controlled by a smartphone app, an Amazon Echo, or the like. The “dumb” bulb we’re showcasing today is conversely intended to act just like its incandescent precursor, dependent on the varying voltage coming to it from premises power in combination with an in-between dimmer switch to determine the brightness lumens it outputs.

Let’s dive in, as-usual starting with some outer box shots. Today’s victim comes from an A21 form factor four-bulb package, with “soft white” (2700K, to be exact) color temperature and 100W incandescent-equivalent (15W actual) brightness (1600 lumens, to be precise). I bought ‘em from Walmart (“Great Value” is the store brand) back in early February on sale for $1.97, believe it or not (that said, they’re $15.97-for-four as I type these words in early November).

Front view first; particularly note the “Frosted Glass” mention. I didn’t. Again, hold that thought:

Left side:

Back (it came to me pre-dented, but the contents were thankfully still intact):

Now for the right side. In that earlier box-front shot you might have also noticed the three asterisks next to “Dimmable”. Per the earlier discussion in this writeup, here’s what right-side verbiage they reference:

May not be compatible with all dimmers. Dimming compatibility available at www.walmart.com.

Top:

and bottom:

Here are some “stock” images of a standalone bulb:

and its industry-standard E26 base:

Speaking of stock images, look how happy these two are with their new light bulb! (I digress):

And here’s our victim in real life, as usual accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes:

Top:

Various views of the base from the side, to show the various markings stamped in it:

And another of the bottom end, reminiscent of the earlier one (which I admittedly didn’t look at until after I took this one, therefore the similarity):

Now to get inside…and now for my confession regarding the earlier “frosted glass” comment. I hadn’t, as previously mentioned, in-advance noticed that particular portion of the packaging’s front-panel notation. And I’m pretty sure that every other LED light bulb I’ve taken apart to date has had a plastic globe. So, although I still distinctly remember having thought something along the lines of “gee, this sure feels like a legacy glass-globe incandescent bulb” when taking it out of the box, I banished the thought and proceeded forward, operating on my must-be-plastic presumption. Insufficient questioning of assumptions strikes again…

First, I tried clamping down on, and then twisting, the bulb base with a pair of pliers while holding onto the globe with my bare hands. Are you cringing yet? I sure am, thinking about it in retrospect. Thankfully, that attempt was unsuccessful (or, depending on your perspective, successful). As was my next “brilliant” idea, to take a hacksaw to the junction between globe and base. My guardian angel (unlike my brain) was obviously working overtime that day.

Third attempt: expose the globe to my heat gun on its highest temperature setting. The globe didn’t eventually begin softening/melting as plastic ones previously had, which I thought was strange at the time. Instead, it eventually exploded with a loud “pop”, partially shattering into shards all over my kitchen. Thank goodness I was wearing eyes-protecting glasses that day:

Putting the remainder in a thick plastic bag and tapping on it with a ball peen hammer completed the glass (yes, Brian, glass, not plastic) globe-removal task:

What do we have here? I’ve provided multiple side-view overviews to give you a fuller picture:

In contrast, here’s a multi-LED array picture taken from my very first LED light bulb teardown of a 60W (dimmable as well, as it turns out; I didn’t realize its distinctiveness at the time) device back in September 2016:

What we have here today is a set of six LED filaments, together comprising the illumination nexus of this light bulb. From the as-usual excellent Wikipedia summary:

A LED filament light bulb is a LED lamp which is designed to resemble a traditional incandescent light bulb with visible filaments for aesthetic and light distribution purposes, but with the high efficiency of light-emitting diodes (LEDs). It produces its light using LED filaments, which are series-connected strings of diodes that resemble in appearance the filaments of incandescent light bulbs. They are direct replacements for conventional clear (or frosted) incandescent bulbs, as they are made with the same envelope shapes, the same bases that fit the same sockets, and work at the same supply voltage. They may be used for their appearance, similar when lit to a clear incandescent bulb, or for their wide angle of light distribution, typically 300°. They are also more efficient than many other LED lamps.

Here’s more, complete with Wikipedia-sourced pictures:

The LED filament consists of multiple series-connected LEDs on a transparent substrate, referred to as chip-on-glass (COG). These transparent substrates are made of glass or sapphire materials. This transparency allows the emitted light to disperse evenly and uniformly without any interference. An even coating of yellow phosphor in a silicone resin binder material converts the blue light generated by the LEDs into light approximating white light of the desired colour temperature—typically 2700 K to match the warm white of an incandescent bulb.

Structure of a typical filament.

Closeup of a filament at 5% power, showing the individual LED light spots.

And now, some photos of my own, taken of the assemblage from various perspectives:

The various LED filaments’ conductors split off at the base and travel vertically in parallel, rejoining and “completing the circuit” via the transparent “tube” at the center.

So, here’s what baffles me, although I have some theories. Why on earth would Walmart and its bulb supplier go with a fairly exotic LED filament approach for bulbs that sold for only $0.50 each? I’d get it if the globes were clear, so that a customer (like those two happy folks you saw earlier) could fully enjoy the incandescent-reminiscent cosmetics:

But why with a bulb whose illumination source is obscured by a frosted globe?

Part of the reason, I suspect, is the aforementioned near-360° coverage of the approach versus a 180°-or-less spread of a traditional bulb with an array of LEDs spread out in only two horizontal dimensions (although again, the diffusion aspects of a near-360° frosted globe above the array would seemingly minimize any inherent LED filament advantage).

The other, bigger, reason, I suspect, is two-fold and related. Although the LED filament structure itself may be more expensive to manufacture (albeit less so over time thanks to high-volume manufacturing efficiencies) the complexity, therefore cost, not to mention size of the circuitry driving those LED filaments can be less, as a total-cost counterbalance. I suspect, too, that this circuitry simplification also makes LED filament-based bulbs inherently more dimmer-friendly.

Here’s a pictorial representation of what I’m talking about:

And here’s more from Wikipedia, further bolstering my hypothesis:

A benefit of the filament design is potentially higher efficiency due to the use of more LED emitters with lower driving currents…The power supply in a clear bulb must be very small to fit into the base of the lamp. The large number of LEDs (typically 28 per filament) simplifies the power supply compared to other LED lamps, as the voltage per blue LED is between 2.48 and 3.7 volts DC. Some types may additionally use red LEDs (1.63 V to 2.03 V). Two filaments with a mix of red and blue is thus close to 110 V, and four are close to 220–240 V, compared to the mains AC voltage reduction to between 3 V and 12 V needed for other LED lamps.

Then there’s this, explaining (among other things) the eventual explosion after my bulb’s glass (did I mention that?) globe’s lengthy exposure to high heat:

The lifespan of LED emitters is reduced by high operating temperatures. LED filament bulbs have many smaller, lower-power LED chips than other types, avoiding the need for a heatsink, but they must still pay attention to thermal management; multiple heat-dissipation paths are needed for reliable operation. The lamp may contain a high-thermal-conductivity gas (helium) blend to better conduct heat from the LED filament to the glass bulb. The LED filaments can be arranged to optimize heat dissipation. The life expectancy of the LED chips correlates to the junction temperature (Tj); light output falls faster with time at higher junction temperatures. Achieving a 30,000 hour life expectancy while maintaining 90% luminous flux requires the junction temperature to be maintained below 85 °C. Also worth noting is that LED filaments can burn out quickly if the controlled gas fill is ever lost for any reason.

So, there you have it. The base of this bulb, like that of the charging base for the rechargeable electric toothbrush in a recent teardown, is “potted”, as you can probably tell from some of the photos I took, so I’m not going to bother trying to pry it open (the lingering shards of sharp-edge glass are admittedly also a deterrent). But as the earlier conceptual diagram suggests, the circuitry inside it is likely pretty elementary. I hope you’ve found this teardown exercise “illuminating” (hardy har har) and I welcome your thoughts in the comments!

—Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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