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Universal fluorescent ballast

Discussion in 'Lighting' started by Mike Jackson, Oct 26, 2004.

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  1. Mike Jackson

    Mike Jackson Guest

    I understand that you have to hold the rapid start lamps at a constant
    voltage, and the ballast dims them by adjusting the frequoency. Correct? I'm
    trying to understand how a universal ballast would work for fluorescent
    lamps. Does the voltage always stay the same but the lamp draws more current
    or something because the resistance of the lamp is lower? I'm not quite
    sure...? For example, how does a ballast run both a 55W and a 80W
    fluorescent lamp? Can it? Thanks in advance, any online references explaning
    this would be appreciated.
  2. No. Lamps are operated from current sources and the lamp determines it
    operating voltage, not the ballast. The ballast must be able to
    accommodate the lamp voltage, but it does not set it.
    There is no true "universal" yet. As stated above, lamps are run from
    current sources and set their own voltage. All else being equal, a
    longer lamp will have higher voltage, Therefore, you can design a
    family of lamps that all have the same diameter, and are filled with
    the same gas pressure and all operate at the same current. Longer
    versions will have higher operating voltage and therefore will draw
    more power when operated at the same current. The new "universal"
    ballasts are designed to run a variety of lamps that all have the same
    or very similar rated operating currents but a wide range of operating
    voltages. As long as the ballast can accommodate the different lamp
    voltages, it can operate lamps of different powers.

    I said there is no true universal ballast because no way has yet been
    developed for the ballast to figure out what the rated lamp current
    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
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  3. Mike Jackson

    Mike Jackson Guest

    My other question; is there a maximum frequency rating for fluorescent
    lights, and they are dimmed (the rapid start) by changing the frequency
    correct? Do you have any documents on line that are good references to this
  4. Sorry, I forgot to answer your question about frequency. There is no
    known maximum frequency for operating fluorescent lamps. Fluorescent
    lamp products are operated routinely between 50 Hz and 2.5 MHz. Other
    prototype lamps operate at 13.56MHz and many people operate
    fluorescent lamps at microwave frequencies for a variety of purposes.

    The efficacy of fluorescent lamps increases as the frequency is raised
    from 50 or 60 Hz to about 10 kHz. Once the frequency is above 10 kHz
    the efficacy is just about independent of frequency.

    So, how does changing the frequency dim a fluorescent lamp? The
    changing frequency does not change the operation of the lamp. However,
    all lamps need a current limiting device called a ballast. (Yes, in
    the US we also call the entire system that operates the lamp a
    "ballast", but technically this term should refer only to the current
    limiting element.) If the ballast is an inductor, then increasing the
    frequency will increase the impedance of the inductor and therefore
    lower the current in the lamp. If the ballast is a capacitor then
    reducing the frequency will increase the impedance of the capacitor
    and reduce the lamp current.

    You can find a discussion of fluorescent lamp operation at

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.
  5. Mike Jackson

    Mike Jackson Guest

    I'm an american, so, my "ballast" just has to output X frequency and vary
    some amperes to dim the fluorescent light? For example, if I were to use
    some FETs to switch voltage -/+ and output a square wave AC signal, and I
    just changed the rails to the switching devices, that would provide a
    dimming function? So basically in european terms a ballast is some form of
    impedence circuit to control the current to the lamp via frequency from a
    power supply? Isn't it a little inefficient to change the current using an
    impedence circuit? I mean shouldn't you just have switching transistors
    switching the voltage from a switch mode power supply? Then you can just use
    PWM to control the switch mode power supply to control the dimming. In the
    end you would have a PWM switch mode power supply and transistors switching
    its voltage (mins and plus so it is an AC square wave....). Then just
    changing the voltage the transistors were switching would dim the lamp? This
    is also how a rapid start lamp is suppose to be operated correct?
  6. Yes, assuming it controls that current in some way and can supply the
    current at the voltage needed by the lamp.
    No! You have described a voltage source not a current source. If a
    fluorescent or other discharge lamp is connected to a voltage source
    it will draw as large a current as the source allows and either burn
    itself out or burn out the voltage source.
    Not sure what you mean by "European terms". Everyone uses a device to
    operate discharge lamps that provides starting voltage and operating
    voltage and acts like a current source to control the operating
    current. On this side of the pond we call this whole device a
    "ballast", even though the term ballast was originally limited to the
    current limiting element. The Europeans use the term "control gear"
    for the same box that we call a ballast, and therefore avoid the
    confusion created by the US terminology.
    No. Perfect inductors have no power loss. Even imperfect ones can be
    rather efficient if designed properly.
    You could use PWM to create a current-controlled source, and this has
    been done in certain ballasts. However, the lamp current can increase
    to destructive levels in far less than 1 msec, even 1 usec depending
    upon how much voltage is applied. If current pulses this short, and
    with fast rise and fall times to get high efficiency in the switching
    circuit, are applied to the lamp, it will generate a high amount of
    EMI and be in violation of FCC regulations. You therefore need some
    sort of filter on the output of the switching circuit to smooth out
    the pulses.
    Not unless you have a current limiting device. See above.

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.
  7. Mike Jackson

    Mike Jackson Guest

    Ok I'm starting to understand it. I still have to buy a book on switch mode
    power supplies I am saving up money for a really god one I have found. Is
    there a site that has the operating specs for most fluorescent lamps that
    you could refer me to?
  8. Operating specifications on fluorescent lamps are generally hard to
    find without paying $$.

    Complete data is published by the IEC (International Electrotechnical

    IEC 60081 (2002-05) double-capped fluorescent lamps (what we call
    linear fluorescent lamps). This can be purchased in PDF format for 296
    CHF or about $247. There also two amendments which are slightly less
    expensive. There should be an IEC spec on single-capped fluorescent
    lamps (CFLs), but I can't find it right now.

    Since I don't think you want to spend all this money, you could also
    download a copy of ANSI C78.81-2003 Double-Capped Fluorescent Lamps,
    Dimensional and Electrical Characteristics for FREE from,
    and ANSI C78.901-2001, Single-Base Fluorescent Lamps, Dimensional and
    Electrical Characteristics, also free for download at

    These may not contain operating data on the newest lamp types. You can
    sometimes find electrical data on the Philips Lighting or Osram Web
    sites. Look on the European sites first instead of the US sites. For
    some reason the lamp manufacturers think that European customers need
    this data more than US customers. GE has also started publishing
    electrical data for some of their lamps, but it is rather hard to

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.
  9. Bare in mind that the power ratings for many fluorescent lamps
    are slightly different in the US. Particularly, T8's are mostly
    lower powered in the US, and things like the 25W 4' T12 don't
    exist outside the US.
  10. Mike Jackson

    Mike Jackson Guest

    Well do most fluorescent lamps like use the same voltage but different
    currents are drawn? As far as limiting current, I read that the lamp has a
    negative resistance, so wouldn't an isolation transformer take care of that?
    I don't understand why you would use a coil. Plus a coil will act as a
    voltage divider if it is in series with the lamp, so it would change both
    the current and voltage. Of course if the isolation transformer was being
    used with the wrong frequency that would affect the current to. Any ideas?
  11. No. Lamp voltage and current varies quite a bit. However, it is
    probably more common for a "family" of lamps to operate at the same
    current, but to have different operating voltages and therefore powers
    because some members of the family are longer than the others. This
    allows one electrode design to be used for the whole family since they
    all operate at the same current.
    Yes, that is the term used, but the absolute resistance is actually
    positive, as it must be to absorb instead of generate power) while the
    incremental resistance is negative. That is, as the current increases,
    lamp resistance decreases, and so does the lamp voltage.
    No. I don't understand how an isolation transformer would solve
    operating problems associated with negative incremental resistance.
    The coil provides a current limiting function by "absorbing" "excess"
    power supply voltage as the lamp current increases. This is best
    explained graphically. I will have to post some graphs on my Web site
    and then post the link back here.
    Yes, that is inherent in the current limiting function of the ballast.
    An inductor is not a perfect current source, so it does allow the
    current to change as the lamp voltage changes. However, if the
    inductor is chosen to have a high enough impedance the lamp will
    operate in a stable mode in spite of its negative incremental
    Ideal transformers are frequency independent.

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.
  12. On Sat, 30 Oct 2004 09:50:08 -0400, Victor Roberts

    I have put a PDF file that explains negative incremental impedance on
    my Web site at:

    This was extracted from a larger presentation and is not fully
    self-explanatory, but should help you get started. I will respond to
    any reasonable questions posted here.

    The file is for personal, educational use only. it is not to be
    reproduced or used for any other purpose.

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.
  13. Mike Jackson

    Mike Jackson Guest

    Well let me just first make it clear to myself. We are talking about rapid
    start lamps right? I have been doing a lot of research on this fluorescent
    stuff. I was understanding that only rapid-start ballasts and lamps should
    dim because it will eventually damage other lamps? For example, you
    shouldn't interchange most fluorescent lamp technologies. Thanks for the
    reading it helped!

  14. No. The fundaments, such as need for current limiting, effects of
    frequency, etc. apply to all types of fluorescent lamps, and to most
    other discharge lamps.
    Yes, you are correct that dimming an instant start lamp beyond a
    certain point will lead to short electrode life. But then again, even
    dimming rapid start lamps on a rapid start ballast that is not
    designed to heat the electrodes properly while the lamp is dimmed will
    also lead to short electrode life. You need a properly designed rapid
    start dimming ballast.
    Correct, and thanks.

    Vic Roberts
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.
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