Why is the LED bulb wattage limit in my lamps so much lower than the Incandescent limit?

Question

My lamp specifies that the bulb limit is "60w Incandescent / 10w LED"

LEDs are more efficient and give off less heat. Shouldn't I be able to put in a higher wattage LED bulb? (regardless of the brightness difference)

Answer 1

One possibility is the socket heat limit.

An incandescent bulb produces all of its heat in the middle of the bulb and away from the socket. The socket itself gets some heat, but not that much. If it gets 1/6 of the total heat then that's 10W.

On the other hand, many LED bulbs produce all of their heat in the socket. All (or nearly all) have their driver circuitry in the socket, and many also have the LEDs sitting right on top of the driver circuitry. (Some have the LEDs in the middle of the bulb for more even light dissipation and/or a more incandescent-like appearance.) So that's as much as 10W.

If the socket is designed to handle 10W of heat dissipation then 60W incandescent/10W LED is a reasonable combination.

Or it could be lazy engineers/cheap manufacturer that didn't want to really test things out to see how large an LED bulb would be safe.

Answer 2

LEDs and the light bulbs that use them don't like to get hot. Running hot significantly shortens their lifetime. So I suggest that the power rating for the fitting is what they exist it to be able to dissipate without the LED bulb getting too hot. It's normally the driver circuit that fails, and this isn't very well cooled by convection in most fittings.

I'd consider this more of a guideline for LEDs, unlike incandescent bulbs with their much greater heat load that leads to an actual hazard. But I'd still follow it, because I want to get my money's (and carbon footprint's) worth out of the bulbs, and the rating is normally plenty for a single source of light.

Answer 3

LED lights (the whole assemblies, not just the diodes) are producing less parasitic heat than incadescent lightbulbs for sure.

On the other hand LEDs are much more sensitive to temperatures.

Even though the new LED fits in the fixture and does not breach the limits for either the current rating and wattage (rated not to melt the insulation and the fixture structure) the heat produced by the LED might exceed temperature limit for the LED itself.

In more detail - LEDs are based on interface between two different semiconductors or semiconductor-metal interface. The material combination is selected the way that atoms in the interface get excited and deexcited regularly. As they deexcite they emmit radiation (Quantum meachanics!) in the visible (or infrared, or UV,...) range.

Another huge difference between incadescent lights and LEDs is in voltage. Incadescend lights can be built for any voltage. 1.5V? no problem; 1kV? no problem either. The voltage drop over LED interface is from milivolts to volts. There must be either low-voltage ("high" curent) circuitry or driver circuits to drop the voltage down from 230V to the proper one. This is based on pulse-width-modulation and is driven by "high-power" transistors. These also generate parasitic heat and they are also made of finely-structured semiconductors.

None of the semiconductors are "pure" metals or stoichiometric compounds - in other words, their microstructure is not stable. Increasing temperature "helps" diffusion and when the alloying elements near the interface diffund away and across the interface, the properties are gone for good. The device may look unharmed to naked eye, it may look unharmed to microscope, it will be blown and you can indentify it with electron microscope with additional detectors (focused ion beam, energy dispersion spectroscopy).

Answer 4

My guess is they tested it with a 10W LED, to comply with some (perhaps new?) UL requirement, the test succeeded, and they couldn't be bothered testing higher wattage LEDs. They may not have to regression-test everything, sometimes just establish safe guidelines. 10W was safe. More testing costs money. Done.

I don't think the lamp manufacturer is trying to ensure the longevity of your LED bulbs. It's a good theory but I'm skeptical.

I routinely put LEDs far in excess of their incandescent equivalent into old light fixtures. I monitor them carefully for heat but it's ridiculous, there is essentially zero heat compared to the old bulbs. Sometimes the LED bulb life is compromised, sometimes drastically. I don't care, I buy cheap bulbs and I love the light.

Answer 5

It can be boiled down this simple:

• The incandescent bulb limit is fire safety related. The bulb itself can safely self-heat to a very high temperature, because it is made of glass and metal and a tiny amount of inorganic glue between them. E.g. 250C is pretty much OK. On the other hand, the materials that the fixture is made of, including but not limited to wiring, as well as the expected materials around (e.g. wood) are not this much tolerant. Assuming some model of heat exchange, one can estimate the maximum power where the bulb is e.g. 200C and the surrounding stuff neither ignites nor degrades way too quickly.

• The LED bulb limit is related to the LED bulb itself. LEDs, as well as the accompanying electronics, are much more sensitive to the heat they generate than the incandescent bulb. LED bulbs in a fixture are also first to suffer from overheating, long before the materials around start to degrade from the heat. The limit could be placed around 60C at the bulb surface. Assuming a similar heat exchange model, one can get much lower power limit, generally 1/10 from the incandescent one.