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Comment by perlgeek | Hacker News Reader

Just the other day I idly wondered how LED "light bulbs" regulate down their 230V (here in central Europe, at least) to the ~2V needed per LED.

Although it's from a different starting point (AC, not DC), after reading the article it seems they could use an AC->DC converter and then a charge pump.

Is that what they actually use? Or is there something easier/cleverer when starting from AC?

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LED bulbs usually use the most basic of power supplies, the capacitive dropper[0], the downside is that they tend to die easily.

Also, the LED diodes themselves will often be multiple in series (a string), or series-parallel (several strings in parallel) depending on the bulb, that end up needing more than just 2V, anywhere from 12V to 60V or so per string of diodes.

Sometimes the high power diodes being used are themselves a series chain of diodes on a singular piece of silicon encased in a blob of phosphor, so that the diode package ends up needing 12V or so. These are often referred to as 'COB' diodes.

(apologies for the RAS syndrome, but saying 'LE diodes' or just 'LED' to refer to the individual light elements when talking about 'LED bulbs' is too confusing otherwise)

[0] https://en.wikipedia.org/wiki/Capacitive_power_supply

How about. "Also, the LED elements will often be in series (a string)"

RAS syndrome is not a bad thing for exactly the reason you've just mentioned! redundancy is common in language, it does actually serve a useful purpose and this particular redundancy confuses no-one.

Capacitive droppers are the most reliable for LED bulbs in my experience, because they are very simple and there's not much to go wrong. It's usually the IC-based ones that die first.

Just to add a data point, I have the opposite experience based on just a few bulbs of different brands I disassembled over the years.

What has failed in them?

In my experience, it's usually the capacitor that pops, but my experience is mostly with GU10 form factor bulbs, which have the downside of being very compact and fitted in fittings that don't have good heat reduction properties resulting in everything running hot, even in a LED bulb that should be cool. Even the cheapest LEDs last longer than standard halogen GU10 bulbs though, so there's that.

Capacitor usually blows - they don't like operating really hot and heat removal inside those small form factors is a serious challenge. Also, many times, the whole power converter is encased in a semi-insulative rubber, so that just compounds the problem.

They probably have a switching power supply, but if you want really simple and low parts count, you'd do a transfomer down to something nicer, have a full bridge rectifier, a capacitor, a current limiting resistor, and then put all the leds in the bulb in series; that way you don't need a big ratio for the transfomer... although maybe that's not a big deal, and you may prefer to not need a higher voltage capacitor, or a more failure tolerant parallel wired LEDs. You could have a linear regulator in there too, but careful component choices may allow that to be omitted.

But switch mode power supplies are probably more efficient and highly miniturized, better adapted to different line voltages, etc. All around a better choice. Some sort of smarts are needed to work well with dimmers as well.

Transformer is probably a lot more expensive than what they use.

Nowadays most basic LED bulbs just put the LEDs in series to get the combined voltage drop close to the rectified and filtered mains voltage, and a linear regulator takes care of the rest. This is cheaper and simpler than other methods mentioned by other users (switching or capacitive dropper). Here's a nice video with a schematic and even a way to hack them to lower the power: https://youtu.be/5HTa2jVi_rc (feel free to skip around, no need to watch the whole video to understand it)

For household LED bulbs almost always Flyback.

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