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Question about Ballasts

  1. Dec 26, 2011 #1
    Are these things rated like other inductors? I've tried to find this answer on the forums and on the web, but I've found nothing to get me closer to this answer. I forget how I came to this number, but it seemed like it came out to 9450Henrys. That seems high and not ordinary.

    The post I found that most closely resembled my question was this: https://www.physicsforums.com/showthread.php?t=326951&highlight=ballast , but it was over two years old.

    What I have is rated at 0.27A and 120V at 60Hz. It is for a typical 18" bulb.

    Any insight will have great appreciation.

    Edit: Wrong
  2. jcsd
  3. Dec 26, 2011 #2

    jim hardy

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    ballasts are a clever device

    basically a stepup transformer with a core that's best described as "leaky".

    when current starts to flow it diverts some of the magnetic flux around the high voltage winding, reducing the voltage - ie flux "leaks" around HV winding
    and a balance is established between the voltage and current in HV winding.
    it's designed to produce about right voltage and current for the lamp. that's why they are lamp specific.
  4. Dec 26, 2011 #3
    If I am to understand, when there is a current, the magnetic field within the coil causes the core to become magnetic, and when the current raises or drops, the now magnetized core reacts and fluctuates as needed?
  5. Dec 27, 2011 #4
  6. Dec 27, 2011 #5

    jim hardy

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    indeed the magnetic circuit reacts as needed to regulate current. it's the physical dimensions that make it do so.

    aha - it IS designed!
  7. Dec 28, 2011 #6
    Thank you for your input. It sounds like it acts like an inductor, but I think my calculation was wrong because 10KH seems like a lot.
  8. Dec 28, 2011 #7

    jim hardy

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    indeed, it acts like combination of a variable transformer and inductor.
    so its equivalent circuit includes feedback.

    have you ever messed with DC generators? it's analogous to a DC differential compound machine.

    i dont know the words to paint that simple picture right now
    if they come i will post them.

    but i think you're almost there - keep on reading and working it in your head.

    old jim
  9. Dec 28, 2011 #8
    You ballast is probably around 300mH.

    The simplest ballast is just a two-wire inductor. You use it to cancel out the negative incremental impedance of gas discharge lamps so they won't blow up/burn out. You could use a resistor but they would get hot and waste energy. Capacitor ballasts have even used and are more efficient than magnetic ballasts but are less practical in size, cost and power factor.
  10. Dec 31, 2011 #9
    old jim,

    I've never had the fortunate opportunity to be able to experiment with DC generators. Maybe I should find an old car alternator in the garage? I've never heard of a differential compound machine, but I will do some searches into it.

    Thanks for the encouragement. It's been 8 or so years since I've done any study with E&M, but it is all coming back like a dream.

    That sounds like a more reasonable number, but I am still having some trouble with it.

    THe ballast that I took apart looked as if it was one coil wrapped around about 3600 times. Are you saying that there is a second coil that completes its own circuit?

    I would like to try using a resistor just for testing purposes, but I would rather like to avoid any resistors. I will do a look around for a capacitor ballast and see what kind of info I can find.

    Thanks for your suggestions.


    I've done some more tests on this thing and it's got me in a loop. I guess the current drops from the rated .27 A to 87.4 mA after start. I would think this would mean the voltage would rise from 120 to 360, but I measured it at 60 V, so I am now thinking the lamp doesn't use the rated 15 W, but somewhere around 5 W.

    Does this observation sound correct?
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