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Comment by derefr | Hacker News Reader

It’s not like these bulbs are flickering in a way you can perceive just by slowing it down by 2/3/4x. They’re flickering at 10000+ Hz. The reason cameras perceive the flicker is that they take discrete samples, with an (electronic or optical) shutter in between. This creates a beat frequency with things that also flicker, in effect “tuning into” a HF flicker and “lowering it” into the visible range, the same way a superheterodyne radio tunes into a HF carrier wave and brings it down into the audible range.

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The 10khz+ flicker only comes about if the bulbs have a proper switching power supply (and if that switching power supply itself runs at 10khz+).

But for the 'real cheap' bulbs, they likely (due to being "real cheap") have either a half wave or full wave rectifier (i.e., no switching PSU) which results in the LED's having a flicker at power line frequency (either 50hz, 60hz, 100hz, or 120hz depending upon which combination of line frequency and full/half wave rectifier is present).

Now that incandescent bulbs are almost gone, I wonder if it will become common to have wires carrying DC in the ceiling instead of every bulb having to implement the AC to DC conversion as cheaply as possible.

Then we can also use this for our video camera’s and wireless access points. Maybe we can add network to those cables. Let’s call it “power over ethernet” ;)

Ubiquiti has got you covered:

https://unifi-led.ui.com/

I have DC powered under-cabinet lights which are definitely flickering at some fairly high rate.

Imperceptible if you are just looking at it, but in a dark room if you move something quickly in front of it you can see the strobe effect.

But where is the DC coming from? If it is a cheap power source (a rectifier plugged or wired into AC), the DC into the lights could be intermittent, and so cause flicker. For true continuous DC, you need a good rectifier or get the current directly from batteries.

With renewable energy and local batteries, it would make sense to have DC wiring. It used to only be that off-grid systems had batteries (usually lead-acid), and they always just use an inverter. But now with battery packs such as the Tesla Powerwall, even grid-connected houses have DC storage.

Unfortunately, Powerwalls and other similar products are made with built-in inverters and connect only to AC, there is no DC tap. And there aren't any standards around DC wiring and small appliances, so it isn't likely to get traction.

I'm mentioned before (https://news.ycombinator.com/item?id=21109247) that I've seen a DC installation at a friend's house, he used 12V cable lights and DC bulbs, so it seems to work.

The real cheap filament style bulbs are a rectifier, and nothing but.

I have 960 fps video of some LED bulbs in my house. It's very apparent that some bulbs do not flicker (perceptibly), some flicker at 120Hz, and some flicker at 60Hz. At night I can sweep my eyes across the bulbs and see the dashed lines resulting from them turning off and on.

Yeah mine do that, you can see it in slo-mo video mode.

Yep ... it's the digital (meaning on-off) version of the slow moving line you see if you point your camera at a TV (or computer) screen - the speed that it happens at is the sum and the difference of the two frequencies (and perhaps at the sum/differences of their harmonics).

In the CATV industry, we used to send 110 analog channels across roughly 800MHz of spectrum (54MHz to 860MHz). Other than background and ingress noise, a major source of noise (effectively) was distortion - non-linearities in the amplifiers that kept the signal at a reasonable level. Composite Second Order (CSO) and Composite Triple Beat (CTB) distortions were two values that equipment transporting these signals would generally call out - they are analogous to the summing and differencing of two signals and three signals (respectively) as described above for the lightbulb, but imagine doing that with 110 different frequencies simultaneously.

Filament LED bulbs usually flicker at 120Hz (with high duty cycle) and Christmas LEDs at 60Hz (with a short duty cycle). Other LEDs are, indeed, high frequency like you describe and are perceptually effectively constant.

The filament in a filament bulb doesn't fully cool down when the voltage crosses to zero, so that smooths them out a lot.

Sorry...I just noticed you are talking about filament LED and not old fashioned analog filament bulbs.

I guess you are right about the beat frequency bit, but I still assert that the cheap LED's are cycling on and off 60 times a second (or 30.... math is too hard early in the morning). That is a bit low for my taste.

Take a battery, photo-resistor and a capacitor and hook up to headphones in series and you will be able to hear your bulbs flickering.

No, as reported by other posters, most bulbs that flicker do it at 120 Hz or 60 Hz. This is easily confirmed by eyesight alone because 60 or 120 Hz can be perceived when an object illuminated by the bulb (eg. your hand) moves quickly in front of a dark background.

Some bad LED bulbs flicker so slowly you don't even need to video it, you can just tell when you walk into the room

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