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Xenon arc lamp igniter problem

Discussion in 'Electronic Design' started by [email protected], Mar 7, 2005.

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  1. Guest

    I found the igniter circuit from a short-arc xenon lamp power supply is
    not reliable. most of the time, ionized plasma can be seen in lamps ,
    but the lamp can't establish stable conducting current. The open
    circuit voltage is ok (~120v), and current regulation is working fine
    too. So I think the problem is the igniter pulse.

    According to the article "Cermax lamp engineering guide" from
    perkinElmer.com, peak voltage, rise time, and pulse width are all
    related with triggerability. Since the power supply was working well
    before, the following are the possible reasons that I'm now
    considering:

    1. The capacitance of the discharging (through primary winding)
    capacitor decreased. (decreased trigger pulse width? or decreased peak
    voltage?)
    2. The dv/dt of the discharging capacitor deteriorated. (larger rise
    time?)
    3. the aging of the spark gap tube. the spark gap looks good, and I can
    see spark inside and the sound is also a normal ping. but I don't konw
    if an aging spark gap can decrease its sparkover voltage or increase
    its on resistance.

    any advice?

    Another thing that bothers me is the estimation of the peak voltage.
    The trigger transformer has 2 turns on primary and 25 turns on
    secondary, and wound on a rod core (L~1.4", D~0.4", material unknown).
    If I want 35KV peak voltage on secondary, how large the discharging
    capacitor on the primary and how large the sparkover voltage I shall
    choose? It seems to me that the larger the capacitance, the larger the
    peak voltage and the pulse width on the secondary, is this true? The
    power supply uses 2 6.8nF series connected ceramic disk(rated 12kV) as
    the discharging capaictors. how large the sparkover voltage of the
    spark gap I shall choose(assuming the capacitors can be charged up to
    12kV)? Is 7.5kV too large?

    Thanks!
     
  2. Dan Mills

    Dan Mills Guest

    How much 'boost' capacitance do you have (and what series resistor), normal
    practice (at least on the 1.6 Kw + stuff I am familiar with), you have a
    bank of 20,000uF or so in series with a few ohms across the power supply
    outputs just before the igniter. The idea is that this cap bank supplies
    the energy to heat the surface of the electrodes before the voltage
    collapses from 120V to the normal operating region of 20V or so.

    Without this getting the lamp to strike reliably is hard, as the plasma will
    form then immediately collapse due to the cold electrodes having too high a
    work function.

    If the silly thing is flashing over initially but the plasma is failing to
    stabilise, then I would suspect the boost supply rather then the igniter.
    That said short arc Xe igniters are IMHO black magic and I wouldn't rule
    anything out.
    Ahh, but the whole igniter circuit is designed to be resonant, this is not a
    classical transformer! Think something more like a tesla coil (at least in
    the ones I have seen).
    Basically it is a high Q resonant circuit until the lamp ionises at which
    point it is damped by the low impedance of the lamp plasma.
    I would think half that or less, but without actually seeing the circuit it
    is kind of difficult to know.

    Regards, Dan (Sometime Cinema tech).
     
  3. Guest

    The boost capacitor is 10000uF, and it is also the power supply's main
    filter capacitor. The power supply parallels a triple voltage
    multiplier (boost) with a normal bridge (main) rectifier. The circuit
    is current regulated by a pair of IRF250, so the initial series
    resistor the lamp sees is nearly nothing. The boost supply seems fine
    to me. BTW, the xenon lamp is merely 200W.
    I think it's more like a pulse transformer without load.
    It seems that the calculation/simulation of such circuits is very
    difficult for me. maybe the only way to repair this power supply is to
    do some experiment. I really don't want to buy lot's of high-priced HV
    capacitors and spark gaps.

    Steve.
     
  4. Guest

    I put together a trigger circuit for a xenon arc lamp back in 1972.

    I used a mains-driven voltage multiplier to charge a stack of 1uF film
    capacitors up to about 2kV, where they discharged through a spark gap
    into a single-turn primary on a 24:1 step-up transformer, in which the
    secondary winding was the power lead to the lamp (12 turns of each
    lead).

    I used a pair of big ferrite U-cores with a roughly 0.1mm air gap as my
    core.

    The first prototype used a home-made spark gap between two steel
    spheres (1cm balls from a big ball-bearing) centred in a chunk of
    silica-glass tube.

    I let the UV flash from this spark-gap illuminate the electrodes of the
    xenon arc lamp, and this arrangement triggered the arc lamp better than
    90% of the time.

    Later copies used a commercial spark-gap in a borosilicate glass
    envelope, and they triggered the arc closer to 10% of the time.

    My theory was that the initial 40kV voltage spike had to coincide with
    the presence of a free electron between the electrodes of the arc lamp
    if it was going to initiate a discharge. With the prototype, the hard
    UV from the triggering spark-gap produced a photo-electron at the right
    time, and there was enough energy stored in the transformer inductance
    to sustain the discharge through the glow-discharge period until the
    electrode surfaces in the arc lamp had heated up enough to sustain an
    arc discharge - I found figures suggesting that only took about a
    microsecond.

    Once the we had an arc, the regular power supply - a linear constant
    current source with a peak voltage output of about 40V DC - could
    sustain the discharge.

    In the later copies, the transformer-capacitor circuit had to ring
    repeatedly until a cosmic ray or the like delivered an electron between
    the arc lamp electrodes to start the discharge, and by then, enough of
    the stored energy had been dissipated in the ringing that there was
    enough left over to carry the discharge through into the arc regime.

    Sorting out what might be going on took quite a while - even with the
    0.1mm gap, the ferrite core in the pulse trnasformer spent most of its
    time saturated, so the energy transfer from the primary to the
    secondary side must have taken a number of cycles before there was
    enough energy stored in the secondary side to generate the necessary
    break-down voltage across the arc lamp electrodes.

    Once the gap had broken down, the core would not have been saturated
    and the primary and secondary sides would have been tightly coupled.

    Back then, nobody made UV leds.
     
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