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Running two fluorescent tubes from one ballast - NOT! (in UK)

Discussion in 'Electronic Basics' started by Rick, Nov 1, 2003.

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  1. Rick

    Rick Guest

    Hi all,

    I've just built a UV exposure box using 12 inch tubes, tying two tubes
    to each ballast. The ballast (RS part 562-110) is rated for this
    according to its spec sheet and I've wired them up correctly with the
    tubes in series and a normal switch starter across each tube. The spec
    sheet showed the wiring diagram below:

    Note: Mains at 240V or whatever it is today in the UK.
    =======
    L o--------^^^^^^^-------+ +-----+
    Ballast | | |
    (Inductor) +|-|+ |
    | - | |
    | | +-+
    Tube 1 | | |S| Glow Starter
    | | +-+
    | - | |
    +|-|+ |
    | | |
    | +-----+
    |
    | +-----+
    | | |
    +|-|+ |
    | - | |
    | | +-+
    Tube 2 | | |S| Glow Starter
    | | +-+
    | - | |
    +|-|+ |
    | | |
    N o----------------------+ +-----+

    It does not work! The tubes do not even flicker. If I short across one
    tube, the other will start and when I remove the short, the shorted
    tube starts.

    Is it just that the ballast spec sheet is wrong about being able to
    support this?

    I have put a capacitor (X2, 0.68uF) across one of the tubes in place
    of its starter and the tubes fire up OK like this. Is this:
    a) safe?
    b) likely to reduce tube or ballast life?

    Thanks,
    Rick.
     
  2. Your approach is generally safe, but it could lower the lifespan of the tube
    with the cap.

    The problem seems to consist of wrong starters. You will need some that can
    act reliably at 110V, not the ones for 220/240V. The starters often have a
    small cap inside (some nF only), so the 2 caps form a 1/1 voltage divider
    and each starter gets half the mains voltage when the circuit is switched
    on. The starters must be able to handle this voltage, else the tubes won't
    power on.

    The correct solution would be 2 new starters, but you may want to try one of
    the following 2 options. None of them is really optimal, but they may be an
    alternative.

    1.) See Schematic (safe, but could take longer to start):

    =======
    L o--------^^^^^^^-------+ +-----+
    Ballast | | |
    (Inductor) +|-|+ |
    | - | |
    | | +-+ Only One
    Tube 1 | | |S| Glow Starter
    | | +-+
    | - | |
    +|-|+ |
    | | |
    | | |
    +|-|+ |
    | - | |
    | | |
    Tube 2 | | |
    | | |
    | - | |
    +|-|+ |
    | | |
    N o----------------------+ +-----+

    2.) Try using your approach with both starters, but attach a much smaller
    cap (0.1 - 0.4? uF) in parallel to one of the starters.

    Note that both options are non-standard. In the first case some tubes may
    not start while some will. If you get them started, use this solution, else
    try the second one. In this case, whether the tubes start will depend on the
    value of the capacitor. Use the smallest possible, just enough to make them
    start reliably.

    Dimitrij
     
  3. Rick

    Rick Guest


    Thanks for that, nothing I had read had mentioned different starters,
    and it didn't even occur to me. It makes perfect sense though.

    I'll try your option (2), I'll check my junk pile for some caps. I'll
    probably end up getting some new starters though, next time I place an
    order with Farnell or RS.

    Thanks again,
    Rick.
     
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