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Fluorescent lighting

Discussion in 'Electronic Basics' started by Dan Beck, Jan 26, 2004.

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  1. Dan Beck

    Dan Beck Guest

    In the typical set-up, what is the purpose of the "ballast"? Is the
    "starter" simply a capacitor? Does the "ballast" ever wear out? If the
    "starter" is a capacitor, I would imagine that can wear out?

    I tried Google to research this first, and it told me this newsgroup no
    longer archives :-(

    Thank you for reading,
  2. Joel Kolstad

    Joel Kolstad Guest

    To limit the current through the tube so that it doesn't self-destruct as
    well as helping to start the tube. Traditional ballasts are just big
    inductors, so there's a series impedance of j*omega*L with the tube.
    No, it's a little bi-metallic strip that slowly heats up and, after a few
    seconds, opens up. This interrupts the current flow through the ends of the
    tube (each of the two pins on either side of the tube are a filament...
    running current through it heats up the filament so that many electrons are
    boiled off). SInce the ballast is a big inductor, the interruption also
    generates a high voltage pulse that creates the arc in the tube. Once the
    arc is struct (the gasses in the tube are ionized), the voltage required to
    maintain the arc is relatively low compared to the starting voltage. Still,
    your available voltage limit how long the tube can be, generally speaking --
    in the U.S., with only 120V, I don't believe one could do better than 4'
    tubes. Commercial installations have higher voltages available, though.
    Starters wear out pretty commonly, being a quasi-mechanical device.
    Ballasts are much sturdier -- many seem to go for years without problems. I
    don't know their typical breakdown mechanism either.
    Keep in mind that, these days, many so-called 'ballasts' and 'starters' are
    all-electronic affairs. These are effectively switching power supplies that
    output a regulated voltage and can provide benefits including instant
    starting, higher reliability, more efficient operation, and dimming.

    ---Joel Kolstad
  3. Steve Dunbar

    Steve Dunbar Guest

  4. You read it wrong. Google no longer archives
    It still archives
  5. w_tom

    w_tom Guest

    Lamp has two modes. First a high voltage (little current
    required) must convert gas to plasma. Second circuit must
    change from a voltage source to current source - to limit the
    amount of current through a near short circuit created by that
    plasma. As long as sufficient current is maintained, then
    plasma will remain as plasma, conduct the current, and emit
  6. JeffM

    JeffM Guest

    what is the purpose of the "ballast"?
    Good answers, all, from Joel, Steve, and Michael.
    Here's a simple drawing to boot:

    Typically the thermostatic switch is inside an envelope (a bulb)
    which contains a noble gas (argon or neon).
    When power is applied, the neon glows and creates heat.
    The thermostat gets warm and closes, shorting out the gas-filled gap.
    With the neon no longer lit, it cools and the thermostat opens.
    The shunt path offered by the now-ionized mercury vapor
    inside the fluorescent tube doesn't allow the neon to strike again.
  7. Dan Beck

    Dan Beck Guest

    Thank you all, it was very informative!

    Now, to satisfy the chemistry geek in me, does anyone have the physical
    chemistry/engineering background to explain the relationship between Ar
    atoms, (or Ne) with Hg atoms, electrical energy, and the resulting UV
    photons that light the phosphors? I probably should post this to a
    chemistry group, I realize; I just thought I would try here.

    Thank you again.
  8. w_tom

    w_tom Guest

    For more details of the concepts, seeks books on plasma
    physics. Typically requires an engineering library with a
    good supply of books published on or before the 1950s. Also
    IES handbook (industry bible) may provide good background.
    Again, only a better library would have this.

    Conversion of inert gases to plasma is a function of
    voltage, pressure, the gas (or composition), and other lesser
    factors such as temperature and material of electrodes.
    Generally the tube is a low pressure inert gas or combination
    of inert gases - depending on light frequencies desired. These
    are UV frequencies which is why a moderator inside the tube
    converts UV light to visible light.

    Trace amount of mercury is added to assist startup. Other
    techniques include keeping the glow filaments slightly warm to
    last longer. These are details that refine a design. Hg is
    not essential to basic operation. Gas composition and its
    pressure more determine tube operating characteristics.
    Unfortunately, trace amounts of Hg improved operation.

    Related example is a vacuum tube called 0B2 which provided a
    regulated 105 volts. Again, plasma physics.
  9. Doug

    Doug Guest

    Also look at posts in there are some very knowlegable
    people there.
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