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Jun 15, 2023

Teardown: LED Light Bulb Theory vs. Reality

Duane Benson | Aug 25, 2023 The typical household LED light bulb is advertised with a 25,000 hour lifespan, which would be more than 20 years at 3 or so hours of use per day. That sounds great, but it

Duane Benson | Aug 25, 2023

The typical household LED light bulb is advertised with a 25,000 hour lifespan, which would be more than 20 years at 3 or so hours of use per day. That sounds great, but it seems that a good number of them end up flickering excessively or not powering on at all after 30 to 60 days of use. Granted, I’ve had plenty of incandescent light bulbs (ILBs) burn out in the same time frame. But ILBs come at a much lower cost with a promise of only 2,000 hours, so I have higher expectations for LED bulbs, especially now that ILBs are no longer available.

Starting August 1, 2023, the manufacture or sale of most incandescent light bulbs is no longer legal in the US. The law specifically addresses any light bulb that produces less than 45 lumens per Watt. Given that your garden variety household light bulb delivers on the order of 10–12 lumens per W and halogen tops out at around 20 lumens per W, it is effectively an incandescent ban. There are exceptions for appliances and a few other harsh environment installations and—have no fear about your own new old stock—usage has not been banned. If you’ve got them, you can use as many as you want.

In this article I will share a teardown of common LED light bulbs to figure out why they so often last just 30–60 days, despite the promise of a decades-long lifespan.

The promise of LED lighting is twofold. First is the power efficiency. I have 26 bulbs on the upper floor of my house. I typically utilize 60 W or equivalent in all the fixtures for a total of 1.56 kW. Once I have replaced all of them with LED bulbs of the same brightness, I can turn on all 24 of those lights, should I so choose, for the same power as just the one multi-bulb fixture in my big bathroom. That’s less than 3 Amps of 120-V power running through my wires lighting my entire upper floor. As an engineering-minded person, I call that a lot of headroom for safety.

Personally, I’m a fan of LED lighting for general illumination purposes. I feel safer about the load I’m putting on my home wiring, and I’m pretty happy about the reduced power use. The flexibility of having a wide variety of color temperatures is nice as well. I enjoy soft white over the dinner table and bright light in places I work. Of course this amount of choice does bring up a “first world problem” of too many options. In addition to soft white and daylight, Home Depot offers an additional seven light types in the humble Edison base—and that’s not even counting the smart bulbs.

The second alleged advantage is the lifespan of LED bulbs. At $4.00 per LED bulb, I expect the installed performance to at least come close to the advertised performance. I understand solid-state electronics quite well, including LEDs, so I can see the theoretical 30-year lifespan seeming valid. However, I also understand the economics of designing for mass scale, profitability, and the reality of manufacturing. That gives me some theories about the high number of premature failures I have experienced. In my estimation, it comes down to saving a few pennies on critical parts—most likely capacitors.

While LEDs are incredibly efficient compared to an incandescent bulb, they still lose about 60% of their input power to heat. That means a 9 W (60-W IC equivalent) is 5.2-W heater. That doesn’t sound like much, but all of that heat is generated in the enclosed base and LED mounting substrate. The ILB has to contend with a much larger heat load, but the critical materials are the filament and glass envelope, which the industry has had some 140 years to refine.

For this teardown, I disassembled three LED E26 (Edison-base) lamps, all with less than 60 days installed time. None were run under continuous-duty conditions. One is a standard bulb rated for use in fully enclosed fixtures. The other two are indoor floods that are marked as not suitable for fully enclosed fixtures.

The standard-sized bulb places its electronics and LEDs on an aluminum substrate PC board. The base portion of the lamp is lined with aluminum to act as an additional heat sink area. For safety, that base portion is located inside a plastic shell. The main electrolytic capacitor is suspended under the PC board in the otherwise empty cavity of the lamp base. This arrangement is where I discovered my first issue, with the manufacturing assembly quality control.

Electronics and LED on metal clad PCB substrate with aluminum internal shell for additional heat sink area.

In this bulb, the circuit board is tilted slightly. The tilt was not an artifact of my disassembly, but is the way the bulb came from the factory. This only allows for heat transfer away from the PCB in two small contact areas. Compare this to the image below, which shows a properly seated PCB from a different bulb.

Properly seated metal clad PCB.

The circuit in both of the above bulbs is quite similar. The resistor (R3) is a 10-ohm, 5% resistor to limit inrush current. All of the bulbs I opened up had this resistor. In some cases it was located in line with the wire connecting hot on the bulb base to the PCB. BR is a bridge rectifier and the IC and D1 are part of the current limiting and voltage regulating circuit.

Bulb number two is an indoor flood light used in an overhead can fixture. The bulb is not rated for use in completely enclosed fixtures, but should be fine for an uncapped can, as it was used. The larger-sized lamp envelope allows for a different mechanical layout. In the flood lamp, the LED board is separate from the power supply circuit and is mounted to an aluminum disk at the widest point in the lamp structure. The circuit board is separated and located near the base within the lamp plastic envelope.

Metal clad LED PCB on 3 ¼” (82-mm) heat spreader with heat sink grease. Note LED D6 with burn artifacts.

With one of the LEDs, D6 in the image above, showing signs of catastrophic failure, I thought I had my cause. However, D6 was only part of the story. I could see signs of thermal discoloration on the PCB even at a first glance.

Top of power supply PC board shows signs of overheating.

The bottom side of the PC board looked fine, other than revealing assembly work that I would classify as a bit shoddy. The circuit is very similar to the standard-sized lamp above. The resister on the right (in heat-shrink tubing) is the inrush current limit. I later removed the heat shrink and found it marked as 10 ohms, but it tested at 26. BD1 is a bridge rectifier and the rest would be regulation.

Bottom side of flood light power supply board.

The top side, in addition to the discoloration, exposed the main capacitor with a tell-tale bulging top. Compare the inset the 130-degree C capacitor (left), from another lamp, with an intact case, to the bulging 105-degree C capacitor (right) from this non-functional lamp.

Flood lamp power supply PCB showing thermal discoloration and comparison of good vs. bad electrolytic capacitor.

I opened up another flood lamp of the same brand to take a look at its power board and capacitor. This bulb was still functioning properly. However, the board also showed some discoloration and the capacitor appeared to be just starting to bulge.

I would love to see an improvement of the quality of assembly work in all home electronics. Crooked parts, too much or too little solder, messes of flux, and cold joints are far too common in the electronics I’ve opened. Unfortunately, that seems to be what we have these days. Other than that, higher-temperature parts or parts that meet spec’d tolerances would go a long way toward minimizing premature failures. There just is not a lot of room for thermal management in an E26 Edison base, and many of the fixtures only make heat more of a problem.

Most new LED home lighting fixtures come with the LEDs spread out on a large metal plate and the electronics in locations that allow for much better convection. I have yet to see a built-in LED fixture of this type fail. If that type of installation does, in fact, have a 20-year life, it becomes practical to install it and forget about it, which would eliminate the need for replaceable bulbs altogether. The long-term solution may be to dispense with the Edison base except in specialized installations or legacy support.

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