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Audio modulate fluoro inverter

Discussion in 'Electronic Design' started by Barry Hughs, Jun 13, 2007.

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  1. Barry Hughs

    Barry Hughs Guest

    What is the best way to modulate an off-the-shelf 12V fluorescent lamp
    inverter with a squarewave in the audio range?

    Kind regards,

    Barry
     
  2. default

    default Guest

    A square wave, by definition, has two equal excursions - both time and
    deviation are equal, so there is no "modulation."

    What you want to try is a pulse width modulation scheme where the
    positive transition is modulated (made longer as sound, modulating
    signal, gets louder).

    PWM is fairly easy to implement - you feed a triangle wave (or the
    charge/discharge on the cap of a 555 astable) that continually moves
    between two voltages into one input of a comparator. You feed your
    audio signal into the other (properly massaged for voltage levels) and
    what comes out is a rectangular wave with varying on versus off ratio.

    An "off the shelf" lamp will operate at some frequency if it is very
    high with respect to the modulating signal no problems. If it is low
    enough you will experience aliasing - the clock of the PWM is in and
    out of sync with the natural frequency of the lamp oscillator and that
    is likely to be visible.

    You are dealing with an unknown lamp - they don't all behave the same.
    Some come on at a reduced level for a time before coming on fully. It
    should be a 100% cold cathode lamp with a fairly high driving voltage
    if you want modulation to work well.
     
  3. Barry Hughs

    Barry Hughs Guest

    Thank you for your reply. PWM is more like FM. There are existing
    "dimmer" circuits to do this with a TL494.

    Sorry if I was unclear, but what I am looking for is amplitude
    modulation. For example, most commercial 12V fluorescent inverters run
    at around 20KHz. How can I best adapt such a circuit to modulate, or
    gate, the 20KHz with a lower audio frequency squarewave?

    Barry
     
  4. default

    default Guest

    The TL494 will work for PWM, but I don't think you want the feedback,
    boost inductor, etc they show in the application notes - it is
    designed or optimized to provide a constant vantage in a boost or buck
    regulated power supply. Fixed output voltage - not what you are doing
    - but it will work,or can be adapted for PWM "modulation."
    Amplitude modulation is probably not the best idea. That would be,
    for instance, connecting the speaker output of an amp into a full wave
    bridge rectifier, then feeding that to your lamp. I don't think you'd
    like it.

    But you did mention PWM and that is the way to modulate a fluorescent
    light. So I think we have a communication problem.

    With pure amplitude modulation you don't have the "strike" voltage for
    the fluorescent tube. The tube really wants to see several hundred
    volts to start and warm filaments. Once it is ionized it may only
    require 50 volts with a current limiter.

    Straight amplitude modulation will be well below the strike voltage
    half of the time or more.

    Enter PWM - the audio signal is converted to PWM and gives the tube
    the full 12 volts (several hundreds after the inverter) and turns it
    on and off fast enough that you won't see it with your eyes. The
    important thing is that even when the "on" pulse is very short in
    duration, it still supplies the full 12 volts so may work for
    modulating a fluorescent tube inverter.

    Now, you say the tube is working at 20 KHZ - your modulation can't
    also go up to 20 KHZ (the oscillator in the pwm) it has to be slower
    to look good. The audio modulation source should probably be below
    5KHZ - voice only.

    You have three things that can "alias" with each other. The tube
    should be ~200 KHZ, the oscillator in the PWM at 20 KHZ, and the audio
    no higher than 3KHZ for things to be happy.

    High frequency is more effective at ionizing lamps than low frequency
    at the same voltage (or at some given voltage). So, Ideally you'd
    want the lamp to be running at a much higher frequency than 20 KHZ for
    audio - and not waiting for a heater to help the gas ionize.

    What you want to do, can probably be done. Lamps are moving to higher
    and higher frequencies as the switching parts get better and cheaper -
    they don't have to spend as much on the inductors and efficiency goes
    up.

    BUT you may have problems just taking some off the shelf 12v lamp and
    just adding pwm to it and expecting it to track an audio signal.
     
  5. Phil Allison

    Phil Allison Guest

    "Barry Hughs"

    ** What the hell for ?

    The light output will persist even when the AC drive is gated off for a
    short time - the phosphors have persistence.

    The tube may not ionise and re-light immediately if the drive is removed for
    a short time.

    If you gate the 12 volt DC power, the inverter will have a time delay before
    it kick starts.

    You have not thought this through much - have you ?



    ........ Phil
     
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