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Florescent bulbs starting energy

  1. Jul 14, 2006 #1

    Pengwuino

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    Now Ive heard this so much that i just have to get a complete answer to it. Since they run on AC current, i wonder "Ok maybe this myth is right because i don't have a good enough understanding about alternatig current". The idea is that a flourescent bulb (sp?) uses more energy turning on then it will take to run for 30minutes to 1 hour. I figured i could try to guesstimate the OOM with DC current figuring "ok i dont think the estimation can be THAT far off using DC principles".

    When i did the math.... ok yah you need a power cable capable of 'hooking up' directly to a nuclear reactor :biggrin: . So im skeptical! I'm calling on hte experts! Does it require more energy to keep a flourescent bulb on for 30 minutes then it does to start the thing?
     
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  3. Jul 14, 2006 #2

    FredGarvin

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  4. Jul 14, 2006 #3

    Pengwuino

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    Yah that's what i've wondered about. I had looked them up and figured there must be a large jump in current... but THAT big? Obviously not...
     
  5. Jul 14, 2006 #4

    SGT

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    The starting power is much greater than the average one, but it lasts for only a short time, so the energy (power x time) is not great.
     
  6. Jul 14, 2006 #5

    russ_watters

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    Staff: Mentor

    I think this misconception may have come from the fact that HID lamps need to be heated for several minutes before lighting, and that does use a significant amount of power.
     
    Last edited: Jul 14, 2006
  7. Jul 14, 2006 #6

    FredGarvin

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    I don't like being next to one of our metal hailide lamps when they turn on. It kinda scares me.
     
  8. Jul 14, 2006 #7

    Pengwuino

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    So this is all officially a 'myth'?
     
  9. Jul 18, 2006 #8
    Basic estimation will get you far. Residential circuit-breakers typically max out at 15A; at 120V this means the instantaneous power consumption of all appliances on that circuit, can never exceed 1800W. Even a 100ms (~6 cycles) startup transient at the maximum possible current, which is clearly a strong upper bound, would consume 180J; a 20W fluorescent bulb would consume as much in 9 seconds of normal use. So there you go.

    The OP mentions "more energy turning on then it will take to run for 30minutes to 1 hour." In an hour, a 20W bulb consumes 72,000 Joules; it would take 40 seconds for any appliance to consume this much without tripping the breakers. This of course is grossly unresonable for a transient.
     
    Last edited: Jul 18, 2006
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