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Spark gaps -- making and triggering

Discussion in 'Electronic Design' started by Ignoramus29428, Apr 4, 2006.

  1. I finally picked up the caps from Fermilab. Each of them weighs about
    40 lbs and they look nice, very cute color also.

    So... I would like to make some sort of a pulse discharge thingy from
    readily available materials. Can crushing would be my first choice.

    My plan is as follows, to use copper pipe as conductor. I bought some
    HV diode stack (4 7.5 kV diodes) and a HV probe.

    The first thing to make would be a reliable system for discharging the
    caps safely, and test it at low voltages. I would probably want to use
    two 12 MEG Victoreen resistors in series a permanently attached

    With four 1 uF caps, RC would be 2*12,000,000 * 4/1000000 = 96, which
    is hopefully acceptable, and power dissipation in the resistors will
    be about 3W per resistor.

    My main questions concern the spark gap. How to practically make a
    decent spark gap, and also how to safely trigger it.

    For instance, would 3/4" brass balls such as McMaster item 9617K47, be
    adequate? If not, I have 3/16" tungsten electrodes. Would they make an
    adequate spark gap?

    If not, can I use sections of, say, 1" copper pipe, placed at some
    distance with their axes perpendicular?

    Also, what is the best way ot safely triggering a spark gap. My
    understanding is that there is a rule of one inch per 15 kV. Would
    making the gap 1.5" and injecting some argon into it be a safe
    and effective triggering method?

    I am going to do a lot more reading and I will post my "design" here
    and get it to pass some consensus before implementing it.

  2. Dry argon is not conductive at STP. Consider making a "trap" above the
    spark gap that drops a short section of, say, 36awg wire across the gap to
    initiate the arc.

  3. Dave Hinz

    Dave Hinz Guest

    I saw a coin-crusher guy's website a while ago that had a sliding
    arrangement that put two large brass balls, or maybe a ball and a disk, into
    contact with each other.
    Probably bigger.
    I think with HV you want to stick with a sphere.
    Slide 'em towards each other until they touch. Do this not with your
    body parts.
    If you can't find that coin shrinking website let me know and I'll dig
    around until I recognize it.
  4. surftom

    surftom Guest

    Don't have any real info for you, but you may want to contact the guy
    who runs this site:
    He seems to have some really fun HV stuff

  5. steamer

    steamer Guest

    --Time to cruise over to sci.physics.electromag which is where the
    folks you need to gab with hang out. Good idea using pipe for conductors;
    when I was messing with mass drivers I discovered that floppy, unsecured
    wires will thrash about when pushed to their limits; it can be a little
    disconcerting, heh.
  6. Thanks, Dave, Surftom, steamer, and Lloyd. I am going to subscribe to
    that newsgroup now. I have Bert's website in my bookmarks now.

  7. OK... I realized that I have two steel bearing balls stolen two
    decades ago from a Soviet factory. About 2" in diameter. Their
    downside is that they are very heavy and thus would require some
    support, with the attendant issues pertaining to HV. McMaster has some
    balls, but relatively expensive.

  8. Fred Bartoli

    Fred Bartoli Guest

    Maybe have 3 spheres. 2 to make your spark gap, the 3rd slowly rolling on
    nylon rails right between the two others...
  9. I like this idea. How would one go about calculating the sphere size
    given the energy stored and perhaps voltage?

  10. Guest

    96 seconds is a pretty long time for the RC discharge time but much
    better than nothing.
    Remember that 3 rc times will be about 4.5 minutes and the caps will
    still have about a thousand volts on them. Be careful out there.

  11. Boris Mohar

    Boris Mohar Guest

    You can experiment triggering a gap by firing a photo flash at it.


    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things)

    void _-void-_ in the obvious place
  12. Ignoramus29428 wrote:

    I once saw some guys using car spark plug for
    triggering the arc between bigger copper spheres.
    This means at least one of the spheres was at
    GND potential. The spark plug was mounted on the
    inside of one sphere and made almost even, located
    close to where both spheres are closest.

  13. kell

    kell Guest

    Check out
  14. True, but at a thousand volts, they would hold about 1/2 joule of
    energy, which could be discharged with just a crowbar.

  15. Doug White

    Doug White Guest


    The proper gadget for triggering a discharge is a special vacuum tube
    called an "ignitron". The ones I used to play with were about the size
    of a beer can. I have no idea where you could get one surplus. They
    basically have a pool of mercury in them, with a small electrode just
    over the pool, with one terminal connected to the pool, and one terminal
    on top. You discharge a relatively small HV capacitor between the
    electrode and the pool, which creates mercury vapor. The main high
    voltage then discharges through the vapor. It produces a very low loss
    HV switch. They used to show up occasionally at the surpls yard in Los
    Alamos, where I grew up, along with all the other HV toys you could
    imagine for 35 cents a pound. Those were the days...

    Doug White
  16. OK, what about using "hitch balls" from walmart for spark gaps? They
    are cheap and I could buy them locally.

  17. Bert Hickman

    Bert Hickman Guest

    For your application, keep it simple. You can use heavy flexible cabling
    (such as welding cable) to make the interconnections. A little bit of
    cable jumping from magnetic forces won't cause you any problems.

    For the spark gap, there's no need for pressurized gases, inert gases,
    etc. - air at STP works just fine. Copper or brass balls will work great
    (from McMaster-Carr), and copper end caps on copper pipe should work as
    long as you're not going to do thousands of shots. Tungsten or
    copper-tungsten would also work, but because of the high peak currents,
    I'd recommend using electrodes that were at least 1/2" in diameter. I'd
    avoid using steel electrodes since steel tends to generate a messy spray
    of pyrotechnic sparks. For similar reasons, avoid aluminum electrodes.
    Aluminum would erode rapidly, and the spray of sparks and flash from
    oxidizing aluminum vapor might make quite an impression. Do think about
    enclosing the spark gap to help deaden the sharp "BANG!" when it fires.

    I previously used a triggered gap (a home-built Trigatron) that used a
    modified spark plug and 40+kv pulse to fire the gap. However, I've
    recently converted to a simple solenoid triggered gap with brass
    electrodes, since it provides much wider operating voltage range and I
    don't require precise switch timing. Solid brass electrodes show
    relatively little wear even after thousands of 50-100kA shots. The only
    downside is a bit of zinc oxide film on nearby objects, apparently from
    some zinc leaching out and oxidizing during the discharge. After
    thousands of shots, the surface of the brass looks similar to a dry
    river bed, with little islands of copper surrounded by small fissures.

    A picture of my solenoid gap can be seen here. The Lexan case helps
    dampen the noise somewhat. It's STILL loud, but not as sharp. You could
    rig something similar using a (long!) string to pull the electrodes
    together, or you could swing a third electrode between the main pair to
    fire it.

    As someone suggested, you could also use a pulse-rated ignitron. These
    use a molybdenum anode (instead of a graphite one) to handle current
    reversals and 100 kA peaks on an infrequent (1/minute) basis. However,
    although an ignitron is quite a bit quieter when firing than a spark
    gap, there's no real performance advantage. In fact, an ignitron may
    actually be significantly lossier than a well designed spark gap for
    your application. Unfortunately, pulse-rated ignitrons tend to be scarce
    and costly even on the surplus market... and you never know how much
    life remains on a used pull. However, if you do want to pursue this
    route, some pulse-rated 20 kV ignitrons include the 7703 series,
    NL1037H, and NL1039.

    BTW, if you are near Fermilab, you are also near me. Contact me off
    Usenet if you'd like to see the coin shrinker in action or discuss some
    of this in greater detail.

    Good luck, and play safely!

    We specialize in UNIQUE items! Coins shrunk by huge
    magnetic fields, our "Captured Lightning" Lichtenberg
    Figure sculptures, and Out-of-Print technical Books.
    Visit Stoneridge Engineering:
  18. Bert, thanks. I am going to make a big Gnumeric worksheet with all
    values, etc, to calculate stuff. Prior to that though, I would like to
    get a "feel" of things. When you talk about heavy cables, what size
    cables would apply to 4 uF charged at 11-13 kV, discharging through a

    I think that for paralleling the caps, I will simply use aluminum or
    steel angle, which I already have.

    For connecting paralleled caps to the actual can crushing setup, I
    would indeed use some cabling. I have some six gauge wire, for
    What size balls? They have 3/4" diameter balls pretty cheaply.
    Definitely not thousands, so, it would be great to not need anything.

    If I can buy copper tubing (what diameter?) and some end caps, and be
    in business, that would be awesome.

    Just what forces act upon the coil? Do I need to wrap it in fiberglass
    cloth with epoxy to make it safe from exploding?
    All I have is 3/16" electrodes.
    I like the idea of copper pipe with end caps.
    Yes, I saw your solenoid triggered gap, and wondered to myself,

    1) does high intensity discharge impact the colenoid
    2) how do you switch the solenoid without exposing yourself to
    possible shock

    Otherwise I like the solenoid idea a lot, a push solenoid would not be
    that expensive. I will start with something simplier though.
    That's what I will do, indeed!
    the second ipcture does not work.

    If you want a free hostname, like, let me
    That's a little above my head at the moment.
    Will do! Thanks a lot!

  19. How about McMaster item 94050A475 (1/2" silicon bronze carriage bolt)
    for spark gaps? The nice thing about them is that they can be
    adjusted, unless they would weld themselves to threads after a few

  20. RoyJ

    RoyJ Guest

    I haven't played with these but I think you can just stop the contacts a
    few thousanths short of contact. At that voltage, it just doesn't make
    any difference. Anything between .010" and .060" should work. using some
    1" copper pipe with smooth end caps would be fine, they have flat heads
    and rounded corners.
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