Do switch mode power supplies flicker in time with mains?

[William Gothberg]

Do switch mode power supplies flicker in time with mains? Specifically LED power supplies in commercially available domestic lamps. By in time, I don't mean at the same 50/60Hz, but anchored to it. I.e. if you have several such lamps each with their own built in supply, will they all flicker in time, using the mains frequency to keep them in time, or will they be random, making the room overall not flicker due to them all being random? And is there any way I can test this? I tried taking photos of them, but my camera only goes as fast as 1/2000th of a second, which shows all the lights at the same brightness each time, I suspect the flicker is above 2000Hz.

[Adrian Caspersz]

Look at the infantile name of the poster, the crossposting, the stupid question.

before ye answer, it's hucker again...

[William Gothberg]

It's just a name.

To find more people who might know the answer.

It's a perfectly sensible question, I want to know if with a lot of similar LED lights, if the whole room will experience flicker.

So what?

[Mark Lloyd]

I once had an audio amplifier with a solar cell rather than a microphone for the input transducer. This made it possible to listen to light. The sun is steady, incandescent lights (AC powered) hum.

That was 40 years ago. Maybe something like that would work today.

[Andy Burns]

Certainly if you connect a solar cell to an oscilloscope, you can see the difference between

incandescent and LED GU10 lamps fed 240V mains

incandescent and LED MR16 lamps fed 12V from an electronic 'transformer'

a dimmer feeding either of the above.

[William Gothberg]

The trouble is I want to compare 2kHz+ from one light with 2kHz+ from a neighbouring light and see if they're in sync.

[Brian Gaff]

Well the answer as in many things these days is it depends. Some are very simple and do have a kind of pulsing taken from ripple on the mains. Others seem to not do this, indeed poking a phototransistor connected to an amplifier shows many different results. the same seems to go for CFLs as well. You would need to know what circuit they were using etc to figure out why. One particular led in a stood across the road has a 1khz whine when point the device at it but modulated onto a 100 hz buzz.

I often wonder if there is some jiggery pokery going on to drive leds hard for split seconds to make them brighter. Brian

[Clark W. Griswold]

Maybe use a dual trace oscilloscope?

Since this landed in alt.home.repair, I gotta ask.  Do you have single-phase or two-phase?

[William Gothberg]

Yes I know they have very bright pulsed LEDs in car lights so they can get pretend to look brighter than they really are. The average light output is the same, but the peak output is higher, which fools our eyes. Damn annoying if you have decent eyesight and can see the flicker. And also if you try to film it - I see TV programs about cars where the headlights are flashing as they aren't in time with the camera frames.

[William Gothberg]

Haven't got one unfortunately.

Single. I'm in the UK.

[William Gothberg]

Agreed. All I can detect (with my digital camera) is that one brand of LED light I have flickers about 5 times less (not sure if it's smother or faster) than the others. It's also the brand that lasts longer, probably better designed overall.

[Jon Fairbairn]

Try a longer exposure and move the light rapidly relative to the camera.

[William Gothberg]

Good idea.

I just tried it with the best lamps I have, which show a slight variation in brightness at exactly 100Hz, which must be seeping through from the mains. However the LEDs never go off, they just change brightness by 8%.

With the worst lamp, same 100Hz, but they actually go right on and off, with a duty cycle of 0.6.

Am I right in thinking these aren't SMPS at all?

[William Gothberg]

I was assuming it was higher frequency than that, from the SMPS. But I'm thinking they don't have one - see my other reply where I used the camera on a long exposure.

Who?

[William Gothberg]

Some brains (or eyes) seem to be faster than others. I can easily (and annoyingly) see flicker on CRT monitors below 90Hz, others don't even see the 50 or 60Hz ones. I can see flicker on 80% of car LED lights, others don't see any. Designers really ought to account for those of us with better eyesight.

[William Gothberg]

Perhaps he meant split phase, like in the USA - centre tapped 240V. Which could conceivably mean I could have some lights on each circuit, and if they were fed by half wave rectification, flickering at 50Hz, they could be out of time with each other and make the whole room flicker at 100Hz, filling in each other's gaps. Mind you the same can happen by just putting the bulb in the other way (in the UK bayonet cap fittings allow you to connect live/neutral the other way at random with bulbs).

[William Gothberg]

I wonder, if I fed the lamps with mains voltage DC, simply a bridge rectifier and a huge capacitor, they'd reduce their flicker. The cheap shit LED lamp I have that actually flashes at 100Hz would most likely get much brighter and burn out, so I'd have to adjust that, but the others which only flicker 8% would just get 4% brighter.

[whisky-dave]

No they'd probbaly blow up, don;t forget a bridge recifir would produce a voltage of at leat 330V and the power dissapated by each LED would also increase .

Doubt that.

[William Gothberg]

I thought about that, and the cheapest one, which seems to be just a bridge rectifier straight to the LEDs, would make them 65% brighter. But the others should only get 4% brighter. A switched mode supply fed by DC at the peak voltage of the mains, would still have its bulk capacitor at about the same voltage. It's already doing what I'm suggesting I do externally. They're rated at 85-260V, so I assume they're switched mode.

[Mark Lloyd]

[snip]

And you'd have to isolate the lights from each other.

Normally, neighboring lights would be powered from the same phase.