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I Wouldn't using AC to power a lamp result in flickering?

  1. Dec 2, 2017 #21


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  2. Dec 3, 2017 #22


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    Actually, it's upside-down. I did tests on several lamps, and I've completely forgotten how to use an o-scope. (It's been 30 years!) I ended up reversing the leads in an attempt to make sense of what was going on.

    Anyways, it was set to "AC", and I'm measuring the light output with a solar panel, so it "floated" down.

    With the o-scope set to "DC", and the probe connections corrected, it looks like this:


    None of the other lamps I tested bottomed out at zero volts. Though, they had interesting "noise" patterns, which I can't visually perceive.

    ps. And I just discovered that my $5 garage sale o-scope's second channel DOES work. Yippie!
  3. Dec 3, 2017 #23
    This effect caused us some head-scratching when a family member developed photo-sensitive epilepsy. Although UK's 50 Hz was not a photic-driver frequency, moving gaze through such flicker could trigger a fit given prior kindling.

    Trad incandescent filament bulbs were not a problem, but the more powerful variety have been phased out.
    CFLs, the replacement 'compact fluorescents', have a nasty habit of flickering at start-up, especially in the cold or towards 'end of life'.

    One three-lamp cluster over a stair-well was a real hazard; it was cooler than main house, the light was off until required, and it was in 'line of sight'. It was also hard to access, so failing lamps were left longer than we'd prefer...

    After some thought, I fitted two CFLs plus a 'rough service' incandescent. The latter, trading longer life for reduced efficiency, lit within one cycle, and masked the CFLs' start-up. After LED lamps improved enough to be trustworthy, I've begun replacing CFLs and 'rough service' incandescents alike...

    Along the same lines, I've replaced several 5-foot (~1500 mm) strip-lights with LED equivalents. Remember to replace 'blinky' starter with 'by-pass' supplied !! Now, they're 'Instant On', no darkening at ends, scant handling risk...
  4. Dec 5, 2017 #24
    You can get some vivid illustrations of the behaviour of lights if you look at city lights through binoculars or similar instruments at night at such a distance that the scene is dark, with the lights appearing as spots against a black background.
    When you move the instrument the lights describe lines determined by your movements. If you look at lights that glow continuously, such as flames and DC incandescents, they describe lines of relatively consistent width. Lights that plainly flicker, such as some discharge lights and LEDs appear as dotted or dashed lines.
    "Ordinary" AC incandescents produce continuous lines, but with distinct constrictions of intervals determined by how fast you move the instrument. Fluorescents vary depending on the details of the lamps, because the fluorescence tends to smear the discharge pulses out into lines.
    Stars don't blink unless you use a telescope that can show you pulsar behaviour (good luck with that one! :wink: )

    Sometimes one can get good effects with a camera. Using a digital camera, I once managed to capture some shots of a swarm of sunlit midges flying opposite a contrasting shadowed background, using fixed focus. Their wingbeats and motion described pulsed lines, in which I could count the beats, and calculate their frequency from the number of beats for the speed of the shots (roughly 1 kHz for that species!)

    As an exercise, playing with such a toy can be quite instructive.
  5. Dec 5, 2017 #25
    I lived and worked in Europe and did notice the lower frequency flicker and found it annoying. I was in my 50s and my eyes picked it up. Even LEDs fed by automotive DC circuits can flicker depending on how they're being driver. You notice it when your eyes scan by LED taillights on some cars.
  6. Dec 5, 2017 #26
    The "flicker " in AC bulbs is extremely difficult to detect visually since the difference between maximum and minimum is only a few percent.

    As for fluorescents, if they use a magnetic ballast then you have flicker (0-100% difference) at twice the mains frequency (100Hz in Europe, 120Hz in US). If its a newer fluorescent then it should have an electronic ballast and the flicker frequency is going to be 20-40kHz. Good luck trying to see that.

    As for LEDs, the frequency can vary. Most decent LED lamps will use a switching power supply which pushes the major artifacts into few hundred to few thousand Hz range. The filament style bulbs are the worst and will be as bad as magnetic ballasted fluorescents. Even a small capacitor though can improve the perception. Go for a cheap LED bulb/fixture and you will get exactly what you pay for.
  7. Dec 5, 2017 #27
    I am told that this was particularly a problem in the early days (even before my time), when some systems used 25 Hz.
  8. Dec 5, 2017 #28


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    I'm sure you're correct. This is interesting.

    I recall that 25 Hz at Niagara Falls continued until the 1990s. But not for lighting, but rather to power motors in the local mills. The motors were so rugged, that they needed no maintenance since 1895 other than a few drops of oil. There was no motivation to replace them with modern motors
  9. Dec 5, 2017 #29
    Now that is some impressive design!! To last almost a century without much maintenance is really amazing. If the original 25 Hz were no longer available, they would have had to scrap these great motors, or at the very least, do a major rebuild. Do you know what kind of mills they were powering (textiles, steel, feed, ???)?
  10. Dec 5, 2017 #30


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    I think it was textiles and flour mills. I used to have pictures of them but I can't find them. However, a 3 HP motor in 1895 was about 10 feet tall.

  11. Dec 5, 2017 #31


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  12. Dec 5, 2017 #32


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    I wonder if you were 18, if you could see the flicker. Got any teens in the house?
  13. Dec 5, 2017 #33


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    Nope. Just me.

    ps. Haven't had a teen in my house since my housewarming party, 30 years ago, when I was about 30. Damn 25 year olds invited their younger friends, who invited their younger friends, who invited their younger friends.......

    <Betty Davis voice>What. a. mess....... </Betty Davis voice>

    I found Cheez-whiz dripping down my walls the next morning.

    Never again........
  14. Dec 5, 2017 #34
    Yes I too affectionately remember doing an experiment just like that in the late sixties. The output light was an old 12V car side-light bulb . It needed to be fed with a constant DC bias which made it glow dim yellow and the audio input came from the wires that would have powered the 3 ohm speaker from an old valve radio. The receiver was a 6 inch diameter magnifying glass which threw a real image onto a phototransistor (OC71 type) with some amplifying circuitry feeding headphones. What staggered me was how undistorted the lower frequencies were from speech. When it came to music the higher frequencies got fuzzy but to this day I am still amazed that that car-bulb filament could cool down sufficiently in (I estimated) less than two thousands of a second so I could hear notes with frequencies of over 2000 hz. And I also was able to pick the signal up at night over 100 metres away , but at that distance keeping that tiny spot image focussed on the phototransistor became quite difficult I remember.
    Actually on thinking about it more now , I must have fed the filament with audio signal from a halfwave rectifier to stop the frequency doubling effect of 2 heat-ups per cycle.
    Last edited: Dec 5, 2017
  15. Dec 5, 2017 #35
    I think the silicon-iron alloys used then could have worked well at twice that frequency which would have been both good news and bad news. The good news would have been smaller lighter transformers and the bad news would have been a mains hum becoming an unpopular bumble-bee buzz everywhere.
  16. Dec 5, 2017 #36


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    The other bad news would be the series reactance of all transmission lines.

    The Russians did a study in the 70's. They concluded that if everything started from scratch, 100 Hz would be the optimum. But nobody cared about the study because there is no thought of starting from scratch.
  17. Dec 6, 2017 #37
  18. Dec 6, 2017 #38
    I remember reading an article back around 1962 or 63 in Electronics Illustrated magazine detailing how to build a light communications project using an ordinary incandescent flashlight (PR-2 bulb) buck boosted by an audio amplifier, My brother and I cobbled together a home built amplifier and tested it using a telescope and a cut open transistor as the photoelectric element. we were able to transmit recognisable music several hundred feet. I was very surprised at the frequency response of the system,
  19. Dec 9, 2017 #39
    I remember many years ago (60's) at a large engineering works that machine shops had to be wired in 3 phase when lit with fluorescent lights with no adjacent
    light on the same phase.This was to prevent a stroboscopic effect on rotating machinery which could make a fast rotating chuck on a lathe for instance appear stationary or slow moving.
  20. Dec 9, 2017 #40

    Mark Harder

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    I once wired a resistive optical sensor in series with a 1.5V battery and the microphone input of a cheap tape recorder to search for any interesting sounds ambient light might make. It was in a city, so no matter where I pointed the thing, all I could hear was the '60 cycle' (actually 120 Hz) hum. I thought that surely the filaments of incandescents would stay hot between the half-cycles and average out the oscillating current. But turning on the indoor lights created the loudest buzz of all. Replacing the optical sensor with a plain resistor eliminated the buzz, so the source of the hum was not currents induced by the home wiring.
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