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Spark gap as a snubber (TIG inverter)

Discussion in 'Electronic Design' started by Ignoramus6607, Oct 28, 2005.

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  1. I am curious...

    For high current snubbing, would it be possible to make a spark gap
    from 4.8mm tungsten rods, with a certain gap between them, so that it
    would spark at, say, 600 volts.

    If so, then would this simple thing be a better snubber for complete
    bridge turnoff at full power, than huge diodes, caps etc. Note that
    intermittent, short duration turnoff during switching can be taken
    care of with a small capacitor. The above applies to one of a kind
    conditions that cause complete turnoff of the bridge under load, an
    unusual situation.

    Using a spark gap strikes me as a very foolproof approach.

    Any thoughts?

    I have 10 4.8 mm tungsten electrodes (cost $19) and could use one for
    this purpose.

    i
     
  2. You would get a lot more predictable results with a gas tube arrestor.
    At least, the weather (especially humidity) wouldn't alter its response.

    Here are some examples from the Digikey catalog:
    (bottom of page)
    http://dkc3.digikey.com/PDF/T053/1226.pdf
    http://rocky.digikey.com/WebLib/Bourns/Web Data/2036.pdf

    Some are even in stock. ;-)
     
  3. Thanks. It looks just like what I need.

    Do you think that it is, in fact, a solid idea and the right way to
    go, as opposed to having a monster capacitor, big ass wires, big
    diodes etc.

    I really like this concept, to be honest. Seems like there is very
    little that can go wrong with a spark gap in inert gas.

    i
     
  4. Don Foreman

    Don Foreman Guest

    You would have this on the output of an --uh -- Tungsten Inertgas
    welder designed to sustain an arc (once struck) across a gap between
    tungsten and metal in inert gas?

    Brilliant! Well, it will be for a very short while anyway.....
     
  5. Don, I am confused. I would think, naively, that these arrestors are
    designed to extinguish the arc once voltage drops back to operating
    range. If they start an arc and do not interrupt it, what is their
    value? A clarification would be appreciated.

    For example, think of a tig torch and HF arc starter. I can hold my
    torch so far from work, that arc starter creates sparks between
    electrode and work, and yet the main arc does not start.

    That's why I am curious as to what these surge arrestors do once they
    spark from a surge.


    i
     
  6. Guest

    They are designed for surges that are self limiting in duration, ie the
    pulse of energy induced in a phone line from a near by lightning
    strike, in your case, they probably would form a arc and melt, as your
    welder, by definition, is a spark producer.

    Gas gaps that are designed to quench CW arcs use high magnetic fields
    , moving eletrodes that open out the gap distance or puffs of air, to
    blow out the arc.

    600 V in air is not conducive to a small gap breakdown.

    steve Roberts
     
  7. Don Foreman

    Don Foreman Guest

    They extinguish when there is insufficient power to keep them going.
    This works for brief transients; we used them in traffic controllers
    as partial protection against nearby lightning strikes that induced
    spikes in powerlines and other wiring. If there is enough power to
    sustain the plasma, a spark becomes an arc --as in a Jacob's ladder.
    The voltage necessary to sustain it is much lower than the voltage it
    took to initiate it.

    See
    http://tinyurl.com/awpvf

    Note how the voltage drops on the gas tube (green plot) microseconds
    after initiation.

    A better bet for you would be a large MOV -- and you need to commutate
    your IGBT's quickly. Use the IGBT's to chop, but don't try to use PWM
    (off time) to regulate the current. Use your welder's controls to
    control welding current. You can certainly vary the duty cycle of
    the opposite polarities, you just need to minimize "off" time.
    Use the welder's controls to regulate welding current.

    If you used a 200 volt MOV, at 200 amps it would dissipate 40KW when
    snubbing -- but if your commutation time is only a microsecond then
    the energy per commutation is only 40 millijoules. For 300
    commutations per second that's about 12 watts. A couple of large
    MOV's should be easily able to handle that.
     
  8. Guest

    Unfortunately spark gaps aren't "designed to extinguish the arc" - they
    simply exploit the physics of a gas discharge through a gas.

    The process of breaking down a spark gap starts with the appearance of
    a large potential difference between the electrodes. This persists for
    an indeterminate time (a microsecond?)until a free electron shows in
    the space between the electrodes.

    If the electric field is high enough, the electron will be attracted to
    the positive electrode, and accelerate to a speed where it can undergo
    an in-elastic collision with a gas molecule, knocking off a second
    electron and creating a positive ion, which goes the other way. This
    produces an avalanche of electrons going one way, and positive ions
    going the other.

    When the positive ions get to the negative electrode, they hit it hard
    enough to knock off secondary electrons, which proceed to generate more
    positive ions ....

    This is a "glow" discharge. The voltage drops at the electrode surfaces
    are of the order of a few hundred volts.

    http://science-education.pppl.gov/SummerInst/SGershman/Structure_of_Glow_Discharge.pdf

    If the current density at the cathode is high enough, the surface will
    get hot, get soft, and distort under the electric field to emit
    electrons by a process which is a mixture of thermionic emission and
    field emission.

    This is an "arc" discharge. The voltage drops at the electrode surfaces
    are an order of magnitude lower than in an arc discharge.

    The glow-to-arc transition can be fast - sub-microsecond - if the
    current is high enough.

    The arc-to-glow transition occurs if and when the current density at
    the surface drops to the point where the surface can cool off to a
    temperature where it is solid, and the termionic electron emision
    stops.

    The glow discharge stops at the point where there are so few electrons
    in the discharge that the statistical nature of the avalanche process
    allows it to self-extinguish - the was only one electron in the gap,
    and it didn't gnerate any descendants.

    This is a rather low current density.
     
  9. I see now. Thank you.
    I agree with you regarding the use of a varistor.

    My plan is to have a RCD circuit that would handle regular commutation
    events. I calculated that a 4 uF capacitor should handle these events
    just fine and limit the circuit voltage to 200 volts.

    Then the varistor would only conduct during much higher spikes other
    than those that occur during regular commutation.

    Does that make sense to you?

    If so, what kind of capacitor is appropriate. I have looked at some
    C-D capacitors, but want to hear your opinion.
    Don, do you have a suggestion regarding a particular varistor?

    I have some 14N391K varistors. Are they usable for this application,
    perhaps paralleled?

    As always, I appreciate your input.

    i
     
  10. Don Foreman

    Don Foreman Guest

    I'll defer to others on RC snubbers for IGBT's.
    Not off hand. I'd have to look at specsheets for rated power.
    Can't say without a datasheet.
     
  11. Don Foreman

    Don Foreman Guest

    It kinda does, and 4 uF in series with maybe 0.75 ohm sounds like a
    reasonable value though I still defer to others with more IGBT
    experience. What does RCD mean, BTW?
    Again I defer to others on that, but I've had best luck with
    polypropylene caps. My experience may be dated -- last time I did
    any of this sort of work was nearly 10 years ago already. Time
    flies when you're having fun -- I retired in 1999 (grin!) Now I get
    to play in my own lab/shop every day (when I'm not gone fishing) and
    do what *I* wanna do! Tonight I was sticking surfacemount parts
    on a board on the bikelight project for my daughter, tomorrow I
    plan to start learning how to weld 4130 moly with TIG and O/A for
    building bike frames.

    I was actually sticking an SOT-23 transistor and a 1206 resistor
    where there are no board traces -- this is a patch. It really is
    possible to "glop wire" with SMT parts, #30 wire, some wee dabs of
    epoxy and a stereo zoom microscope. It ain't pretty, but it works.
    I can only say "probably" based on their physical size. If they
    don't get hot, they should work OK.
     
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