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LED Bulb that Stays Lit Despite Power Outtage

  1. May 26, 2015 #1

    Drakkith

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    Random question, folks. A friend of mine lost power in his house this past evening, yet his front porch light, an LED bulb, is still shining bright. Any idea why this might be? Do they sell bulbs with some sort of backup battery built in?

    Edit: Apparently the light on one of his power strips (the kind with a light-up switch) is still on, so we assume there is still some power coming in, just not enough to do anything other than light up a bulb or two.

    Thanks.
     
    Last edited: May 26, 2015
  2. jcsd
  3. May 26, 2015 #2
    There are many possible reasons for this. Some of the more likely:

    First, there are lots of outdoor lights with built in batteries. Many have solar collectors as well.

    He has an automatic generator that switches on as the power goes out. (Often the entire house is not on the generator, just a few circuits.)

    He has a UPS for his computer and the LED is plugged into that.

    The LED has some "glow in the dark" plastic in it.

    There's some magnetic induction somewhere causing enough current to keep the LED lit. (Nearby high voltage power lines can cause this sort of thing. Of course that assumes they are working even if your local distribution lines are not.)
     
  4. May 26, 2015 #3

    Drakkith

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    Hi Jeff. I'm guessing there's still a small amount of power trickling in to his house from the lines. I'm certain he's not running a generator, and I'm pretty sure he's not using any solar power. The LED definitely isn't glowing in the dark either (he posted a picture to facebook using his cell phone and the light is very clearly working). I can't say anything about magnetic induction, as I don't know how close any power lines are. That's an interesting possibility though.
     
  5. May 26, 2015 #4
    i also face this sort of problem but I investigated and found that the low power in circuit was due to some current in neutral wire as my main switch was only on phase wires which were disconnected due to fues, but as neutral was connected to main and thus carried some current( say that is going to be grounded).
     
  6. May 26, 2015 #5

    Merlin3189

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    If so, I'm pretty sure that it can't be from the supply company. They are normally obliged to disconnect the circuit, if they can't maintain it within a few % of nominal voltage and frequency.

    In UK there is the new phenomenon of "feed in" customers, which could impact this and I have no idea how the electricity companies deal with it. I had assumed that FI generators were synchronised to the general supply and would automatically shut down if the general supply failed, but I really don't know.
    If Drakkith's neighbours had feed in generators which did not shut down, maybe he is getting power from them? I would have thought though, that in parallel with the rest of the substation area any FI power would be too small to produce a usable voltage.

    In general, if there were some source of low voltage coming in on the supply cable, I'd have thought it would be noticeable on other devices as well. It would be a brownout rather than a blackout. Even though an LED is a low power device, it still expects to have to receive its input voltage at the full nominal value.
    But if the LED drivers are switchmode they can probably function correctly with very reduced supply voltage. I'm not sure what the general level is, but just looking at one I see it is specified to work 100V - 240V and I'd guess it would still deliver some output below that.
     
  7. May 26, 2015 #6

    jim hardy

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    I'd guess he's got troubles in his fusebox.

    Take an incandescent lamp to several outlets around the house and plug it in.
    Low voltage will make the lamp glow dim.

    In US scheme (center tapped single phase),
    typically one side of the incoming power feed opens at a fuse or a loose connection.
    That puts out all the 240 volt appliances and half the outlets, but the supposedly "dead" side can get some power through a stove or water heater's heating elements.

    An open neutral wire from a loose connection unbalances the voltage on the two sides of the house wiring system. The symptoms are puzzling at first.
    That might have blown out all the incandescent lights that were on at the time.
    But the LED bulb is probably happy with 240 volts, or it makes near full brightness at reduced voltage. The eye is logarithmic in perceiving brightness so one wouldn't notice unless looking for it.....
    You'll find open neutral trouble with the incandescent lamp test, if that's what it is -
    in some outlets it will glow dim and in others it will give a brief bright light then burn out with a flash after a few seconds. Wear safety glasses just in case - some Chinese incandescent lamps dont have the internal current limiters of good ol' US made GE bulbs. (Don't let me start on subject of politically driven technology )

    old jim
     
  8. May 26, 2015 #7

    russ_watters

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    So...how exactly does he know he lost power?

    Power does not "trickle in" (caveat: brown-out, but that's rare): You either have it or you don't. My guess is he lost one of the legs of his service, but not the other one. It happened to my girlfriend last year. Basically, half of her house still had power and the other didn't. Not a huge deal, just don't try to run any of your 240V appliances if you have a dead leg.
     
  9. May 26, 2015 #8

    Drakkith

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    Everything else in his house, and the rest of the street/block, is without power. Stormy weather to blame.
     
  10. May 27, 2015 #9
    If anyone on the grid has a power generator and doesn't know to disconnect his house from the power grid while his generator in on, his power can feed everything on the circuit. This not only provides a small amount of power to everyone on the circuit but makes it dangerous for the linemen working to restore power.
     
  11. May 28, 2015 #10
    Since the power strip is on, I will presume it is plugged in to an uninterruptable power supply.

    If that is not the case, he probably lit it with his psychic powers. His personality is so bright LEDs just light up because he's around. :olduhh:

    Sorry, but this is starting to sound like an urban legend in the making. An unusual situation is presented. A number of reasonable explanations are given. Each is shot down in turn by information that should have been forthcoming earlier. "Scientists" are proven mystified.

    I don't buy it. :headbang:
     
  12. May 28, 2015 #11

    jim hardy

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    They sell fixtures with such a feature...
    http://www.brownlee.com/catalog/listproducts.php?category=BBU
     
    Last edited: May 28, 2015
  13. May 28, 2015 #12

    russ_watters

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    Yeah; is it mean to ask if this guy knows the difference between a power strip and a UPS?
     
  14. May 28, 2015 #13

    Drakkith

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    I'm not quite sure what you mean. The only ones turned down were the ones that simply don't apply. I know he's not using a generator or solar panels for example.

    Interesting. I didn't even know they made such things. Thanks, Jim.

    It's a power strip. I saw the picture.

    Perhaps I shouldn't have asked this question since I'm not him. My mistake.
     
  15. May 28, 2015 #14
    No problem. I'm not blaming you. But his posting it on his Facebook page makes me a little suspicious of him. I no longer believe my own eyes when viewing stuff on the internet.
     
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