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filter question for arc lamp ignition

Discussion in 'Electronic Design' started by [email protected], Mar 9, 2007.

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  1. Guest

    Dear ng,

    I want to ignite a short arc lamp (HBO100) which basically is a sparc
    gap in special athmosphere. For ignition high voltage (~2kV) and low
    current (few mA) is needed, and for operation low voltage (20V) and
    high current (5A).
    Now I have some hv sources (simplest is a piezo from an old lighter)
    that make the lamp spark when no psu is connected to the lamp. If
    additionally a PSU (current regulated lab supply or battery with
    resistor in series) is connected to the lamp, no spark occurs in the
    Obviously the spark ends up in the psu/battery. Now I assume a low
    pass filter should do the trick, specifically of LC type, since I
    expect in an RC low pass filter the spark would just be dissipated in
    the filter resistor (or not?).
    The rise/fall time of my ignition spark presumably is extremely short
    and the voltage is rather high (about 8 kV) so that the dV/dt is
    extremely large and I'd expect the coil in the LC filter to just
    `block`the voltage pulse in this simple setup:
    | | |
    (C) arclamp hv source
    | | |

    (I hope the ascii art is readable)
    However, it doen`t work for any coil I used (ferrite core with up to
    200 windings).
    Probably I miss something here. Can the coil be too large (given low
    R) or has it got to be much bigger (sorry, I don't know the exact
    inductance)? How important is the dimension for the capacitor?
    Any suggestions?

    Thanks a lot,
  2. Guest

    Save yourself a lot of trouble and make a series injection ignitor.
    take a large torroid, 50 mm or so outer diameter and wind a 50-60 turn
    secondary on it.
    The secondary must be a guage of wire suitable to carry the entire arc
    Make sure NO secondary turns cross over the primary for HV safety
    Wind a 1 to 2 turn primary on the core and dump the energy of a 1-2
    microfarad cap
    charged to ~200 V through a fast scr. Place a 1 microfarad 600 V
    metal can fast capacitor
    or a similar wima high speed plastic cap across your DC power supply
    as the high voltage return.

    The lamp may need a "simmer" supply to help it start, in your case
    this would be 200-400 VDC across the lamp
    before starting, with the low voltage , high amperage 12V coupled to
    the lamp via a diode. The simmer supply is typically a cap, say 10 uf
    charged up and dumped in the lamp via a resistor of 10-100 ohms.
    Some lamps like to be illuminated with a small about of UV or visible
    light to liberate photoelectrons to aid in starting.

    if you want to look at similar circuits take a look at the "air cooled
    ion laser" power supply section of Sam's laser faq.

    fooling with a arc lamp is like playing with a small grenade, be
    careful and shield the lamp, they can easily explode.
    Gas pressure is typically on the order of several atmospheres cold,
    and much more hot.

    Steve Roberts
  3. Guest

    The resistor would either melt or evaporate when the lamp was running.
    The problem is probably the capacitance beween the turns of the
    inductor - every wound inductor has a self-resonant frequency, above
    which it looks like a capacitor.

    Your inductor has to be wound as a single layer winding on a toroid or
    something that looks very like it - I used a pair of biggish ferrite U-
    cores, and fitted some twenty turns of rubber-insulated lamp lead onto
    the available circumference, leaving a decent-sized gap between the
    final turns at the lamp end.

    My system started a lot more reliably when I gapped the core a little
    - I didn't have any trouble getting a spark across the lamp electrodes
    with the ungapped core, but you need to keep the inductor-capacitor
    circuit ringing for a few microseconds to convert the initial glow-
    discharge betwen the electrodes (which drops a few hundred volts) to
    an arc discharge (that only drops 20V). This involves heating a very
    thin layer of tungsten on the ends of the electrodes up to its melting
    point - the surface needs to become hot enough to deform into the
    sharp points that provide the thermally assisted field-emission of
    electros that sustains the arc - and that takes a few microseconds of
    glow discharge.

    The gapped core was still well and truly saturated under running
    conditions, and probably for most of the time the starting circuit was
    ringing down to drive the glow discharge, but gapping the core did
    seem to help transfer energy from the primary (starter side) to the
    seconday (lamp side).
  4. Satoru Uzawa

    Satoru Uzawa Guest

    That mercury arc lamp is widely used in many fluorescence microscopes. Why don't
    you just buy a used lamp power supply unit? Those units usually comes with hour
    meter for you to monitor the time the lamp was on (guranteed life time is 200
    hr, I'd seen a couple of bulbs explode before that). Having your room filled with
    mercury vapour is something you certainly want to avoid. Oh, by the way, you know
    that the lamp won't ignite if it's not mounted vertically, right?
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