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Flourescent Light Transformers = Ballasts

Discussion in 'Electronic Basics' started by BretCahill, Sep 24, 2003.

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  1. BretCahill

    BretCahill Guest

    How did the high voltage transformers in flourescent lights get the name
    "ballasts?"


    Bret Cahill
     

  2. Letsee here. My Websters New World dictionary defines "ballast" as
    "anything giving stability and firmness to character, human relations, etc."

    So it sounds like adding a ballast to a system will make it more stable.

    In terms of fluorescent (by the way, it is not spelled FLOURescent, though a
    very common misspelling) lamps the "high voltage transformer" you speak of
    is really performing a ballasting function more so than a voltage
    transformation function.

    Simple transformers merely change voltages, they do nothing in terms of
    limiting current. An electric arc however will not respond properly to a
    constant voltage. The more the arc conducts current the more conductive the
    plasma becomes, thus the lower the resistance of the plasma, and thus the
    lamp draws more current from a constant voltage supply. The situation soon
    degenerates to a point where either the lamp explodes or a circuit breaker
    or something trips to interrupt the current (at least in extreme cases).


    So in the case of fluorescent lamps the "high voltage transformer" you speak
    of really is a ballast. That is, it makes the system more stable by
    introducing a substantial series inductive reactance (something not
    exactly/precisely true of regular voltage transformers) which limits the
    current through the lamp.
     
  3. Since a fluorescent lamps don't really need ballast, (I have had several
    that did not have ballast boxes) It's probably to reduce the voltage from
    208 to 110 volts. I have only seen ballasts on 208 volt building systems.
    Another explanation could be that Mr. Fluorescent had a brother-in-law that
    made ballast boxes and wanted to help him get rich.
     

  4. What led you to the conclusion that fluorescent lamps don't need ballasts?
    I can assure you they do need them whenever they are going to be operated
    from stiff voltage sources (which the AC mains is an excellent example of).
    If the voltage source isn't stiff then you can sometimes get away without a
    separate ballast (with very poor regulation though) since the voltage source
    itself will perform the ballasting function.

    If you have had lamps that didn't have ballasts, and they worked properly,
    then you simply didn't see the ballast but it was there. Sometimes they are
    made small and integrated into things so they are harder to find/see. Many
    of the new compact fluorescent lamps (with basic medium screw bases on them)
    are good examples of that. The ballast is small and electronic and is
    integrated into the base of the lamp.

    The only electric arc based lighting that doesn't need a separate ballast
    that I am aware of are the high pressure mercury vapor "self ballasted"
    lamps. They use a resistive tungsten filament to perform the ballasting
    function and are built into the lamp itself.
     
  5. JeffM

    JeffM Guest

    /
    / START
    +---------------o ---------------------+
    | |
    | ================================ |
    +--I -+ +--I----+
    I \ / I
    I / \ I
    ---o --------I -+ +- I----+
    \ ================================ |
    \ OFF |
    Vin |
    _________________________ __________________|
    UUUUUUUU
    ========

    HOW A FLUORESCENT LIGHT BALLAST WORKS

    When the starter switch is closed, the filaments at each end of the bulb
    start to heat up and the ballast inductor develops a magnetic field.
    The glowing filaments ionize the gas inside the tube.
    When the Start switch is released,
    the magnetic field in the inductor collapses
    and the resulting high voltage helps to further ionize the gas,
    making a better electrical path through the tube.
    Once the gas is ionized, it conducts with just the normal line voltage
    and the ballast limits the current.
     
  6. Fred Abse

    Fred Abse Guest

    'Cos they ain't transformers, they're series chokes used to drop the
    supply voltage without dissipating (much) power.

    I've seen some old fittings with a tungsten lamp as ballast. Ugh! how
    wasteful!

    BTW, the correct spelling is "fluorescent"
     
  7. Good explanation, Jeff. Actually the only reasonable one.

    You only forgot to mention the fact that when the starter closes the
    ballast is virtually the only series resistance in the circuit as the
    filaments' internal resistance is pretty much negligible. Without the
    ballast the lamp (and the starter) will most likely explode when powered
    on even before the lamp actually starts.

    Mr. 'BananaPannaPoe', you see, that sort of thing won't work without
    current-limiting on a fixed-voltage source. P.S. What about a real name?

    Dimitrij
     
  8. SHAUN

    SHAUN Guest

    Can this explain when you like hit the light switch and the light will not
    come on, but if you keep turning it off and on it then finally come on?

    Shaun
     
  9. JeffM

    JeffM Guest

    It's harder to ionize the gas in an old bulb
    but more tries may give a greater chance of success.
     
  10. SHAUN

    SHAUN Guest

    So in other words try replacing the bulbs first, then if this does not work
    what should I try next? Thanks for the previous reply Jeff!


    Shaun
     
  11. Wade Hassler

    Wade Hassler Guest

    Replace lamps first, then curse because you didn't notice the oil
    oozing from the ballast, THEN replace the ballast.
     
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