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Discussion in 'Electronic Basics' started by Blackbeard, Jan 29, 2005.

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  1. Blackbeard

    Blackbeard Guest

    My kid is building a tesla coil for his science fair project. I'm a
    plumber, not an electronics guy. Right now, he has put together a
    primary coil (made from 1/4" OD copper tubine) and he has a secondary
    coil made from (approx. 1500 winds of 24-gauge wire around a piece of
    4" PVC pipe). He made his toroid out of semi-rigid as the plans

    Now he's getting held up on the capacitors and spark gap. His plans
    call for 1-1/2 copper to make the spark gap. I can't see any point in
    using 1-1/2" copper (mostly because I have 3/4" immediately
    available). Will it make any difference if we use 3/4" copper for the
    spark gap instead of 1-1/2" copper. The plans call for 7 pieces of
    1-1/2 copper (2" in length). You put them in series and the spark arcs
    from one piece of copper to the next. Seems to me it would do the
    exact same thing with 3/4" copper. But 3/4" copper has less copper
    (thus (I assume) less resistance). Will that make a difference or can
    we go with the 3/4"?

    Also, the plans call for a home-made capacitor made by rolling a piece
    of aluminum flashing and piece of plastic or rubber. They say to roll
    it tight. But they give no directions on how to hook this thing up. I
    have TWO posts on the high voltage transformer (15,000 volts).

    I assume the sequence I need to follow is
    1. Connect both posts from the transformer to the home-made capacitor
    2. Run a wire from the capacitor to the spark gap (those pieces of
    copper pipe..again, how?)
    3. Run a wire from the spark gap to the primary coil

    4. How does the secondary coil get power? One person told us that the
    secondary coil should just be grounded and that the 1500 turns would
    generate a magnetic field and generate energy (ie, the lightening that
    hops of the toroid)

    Our plans are pretty vague. Any experienced advice would be very

  2. I am pretty sure the pipe diameter will make a slight difference in
    the result.
    With wire that is spaced inches from any other conductor, because the
    voltage on them will be about half of the transformer voltage, since
    the transformer has the midpoint between the two terminals grounded.
    One terminal goes positive and when the other goes negative. Like an
    electrical see saw. The average current is very small (see the
    current rating on the transformer).
    This wire carries the big capacitor discharge current ot the coil. It
    should be as big as the coil wire (pipe). The connections to the
    aluminum flashing also have to be high current connections, not tiny
    I think the spark gap is in series with one of the wires that connect
    one side of the capacitor to the primary coil. When the spark
    ignites, it forms a conductive path that connects the cap to the coil
    and dumps a large current through the coil. As long as the spark is
    lit, it allows the cap and coil to exchange energy and this ringing
    creates a radio frequency magnetic field inside the coil.
    I don't understand what "the toroid" refers to.

    The bottom of the secondary coil needs something that can absorb a bit
    of charge each half cycle that matches the charge being sprayed of the
    top end. This can just be a fan of wires splayed out in a circle
    beneath the secondary. The primary coil should be several inches
    above this fan, to prevent arching to it.

    The magnetic field produced by the primary generates a similar (though
    smaller, because the secondary captures only a part of the magnetic
    field of the primary coil) voltage per turn in the secondary as the
    cap applies per turn to the primary. Since there are more turns in
    the secondary, it makes more voltage.
    Do you have a web link we can look at that shows similar plans?
  3. Vidor Wolfe

    Vidor Wolfe Guest

    3 quarter inch pipe will be fine.
    I've used everything from copper wire, bolts, copper pipe of all dimentions
    and brass stock.
    It'll be fine, just get the spacing between the pipes nice and even at about
    2mm each.
    This will give a max of 12mm of gap which is therefore adjustable down to

    I'd steer clear of homemade rolled caps unless you want a lot of work, mess
    and an unreliable cap.
    Just use a series/parallel combination of smaller pulse caps of
    polypropelene construction.
    Caps with a value of 0.68uF or 0.47uF, or 0.1uF at 1500v are good to start
    REMEMBER... 10 in series will give the required voltage value (15000V for
    your transformer.) Then you use parallels of these for the final value.
    What value cap are you using? If you tell me, I can give you a good idea of
    the caps you need to buy.

    The coupling between the primary and secondary is by magnetic induction.
    However, a Tesla coil is not a transformer that operates on a pure
    transformer action such as the 240V/120V to 12V stepdown units we have in
    the home, or the 240V/120V to 15000V you're using here.
    The primary coil and capacitor makes up a high frequency reasonant cct.
    The secondary coil has both inductance and capacitance. This should match
    the primary cct and you'll get a reasonant rise in voltage as you go up the
    If everything is in perfect tune, maximum voltage will occur at the top of
    the secondary, resulting in 10s of thousands, if not 100s of thousands of
    volts of high frequency electricity.
    This needs to go somewhere and manifests itself as "streamers" of lightning.

    The toroid is important as it forms a capacitance to earth and also shapes
    the electric field.
    Some people use a ball such as in a Van Der Graaf, but a toroid is better.
    Old literature shows a knob such as a doorknob. Not really a good idea. The
    capacitance is way too small.

    The bottom of the secondary should be connected to a good RF earth such as a
    minimu of 4 feet of 3/4" copper pipe driven into damp earth.

    Do not connect it to any other earth on the system.
    Do not connect it to the mains earth, gas pipes or any home pipe (inc

    There are two ways to connect the transformer, gap and cap.
    Try putting the cap in parallell (across) the transformer and the gap in
    series with the primary.
    The gap is just a switch.
    When the cap charges, it gets to a value where the gap breaks down, there's
    a spark, electricity flows back and forth through the primary until there's
    no more energy then stops and the cap charges again.
    This happens hundreds, if not thousands of times per second.

    Hope that helps.

    I've built quite a few Tesla coils in the past.

    For more info, you could try TCBOUK (Tesla Coil Builders Of UK) on

    If you're in the UK, we have a Teslathon in Nottingham on 26th February.

    Mark H.
  4. The curvature of the pipe's surface partly determines when the gaps
    "fire" (what voltage they can get up to before they arc over). The
    greater the curvature (smaller diameter), the smaller the voltage. This
    is ordinarily not an issue, but if your primary circuit isn't running at
    a high enough voltage it won't arc at all.

    But mainly, the pipes are going to get hot just from carrying all
    that current. The more thermal mass (more copper), the better.

    Duplicating somebody else's plans allows you to duplicate their
    results. Change something and you get different results, and you might
    not be able to tell what's going on (or what will happen); change too
    many things and you're almost guaranteed to let the magic smoke out of
    something with no clue what went wrong, leading to tossing the thing
    aside in favor of video games (assuming you don't burn the house down). ;>)
    Oh, dear. Rolled caps are multiple disasters waiting to happen. "Beer
    bottle" caps are simpler and more tolerant of, um, errors in execution.

    Hooking the thing up should be fairly obvious; do the plans you're
    working from not include a wiring diagram?
    Where'd he find the plans you're using? I don't expect you to draw a
    diagram, but if you could refer us to a webpage...
    "One person" doesn't quite know what he's talking about; all the
    energy comes out of the wall plug and is transferred (minus losses)
    through the system from gap, to primary circuit, to secondary circuit,
    to ground.

    Ordinary transformers consist of coils that transfer energy very
    efficiently through their magnetic fields, and are designed to work at
    line frequency (60 Hertz here). Tesla coils aren't quite like ordinary
    transformers; they're coupled resonators and you can design them to work
    pretty much at your frequency of choice, typically a few thousand Hertz.

    Put way too simply, when the gap fires the primary tank circuit
    (inductor and capacitor) "rings" at its L-C frequency like a struck bell
    except it generates an oscillating magnetic field instead of radiating
    sound, and the field induces oscillating current in the secondary (it
    "rings up"). The toroid forms one plate of a capacitor, and the ground
    the other (which is why you ground the bottom of the secondary; "one
    person" got that right). The secondary current charges up the toroid's
    capacitance. When the voltage on the toroid exceeds the breakdown value
    for air, the "lightning hops off the toroid".

    I said "way too simply" because there are subtleties like the fact
    that the secondary will not resonate at the exact same frequency as the
    primary. Being able to point out and explain those subtleties will get
    your son a better grade, and teach him a few things.

    Also, the voltage gain in Tesla coils is _not_ directly proportional
    to the secondary/primary turns ratio of the coils like in ordinary
    non-resonant transformers, but rather to the ratio of the primary
    circuit/secondary circuit capacitances. The secondary needs lots of
    turns to get enough inductance to resonate with the miniscule
    capacitance of the toroid. You want lots of voltage gain, so you want a
    large primary cap, which means the primary coil needs few turns to
    resonate at (nearly) the same frequency as the secondary.

    If the toroid doesn't "break out" into feathery streamers or an
    outright blazing arc, all the energy stored in the secondary capacitance
    sloshes back into the primary circuit (because the secondary current
    generates its own magnetic field, inducing current back into the
    primary), which can do Very Bad Things to the primary circuit
    components, and maybe feed back into the house wiring (depending on your
    actual circuit). It's a good idea to deliberately put a "breakout point"
    on the toroid to prevent this. Just solder a thumbtack to the toroid
    point-out, and don't allow anything you value (like your son) to get
    between it and the ground.
    Go to:

    and look through the FAQs on the left. Definitely read the Safety Sheet:

    to keep the fun from overriding prudence. We can't learn from our
    mistakes if we don't survive them.

    If your son seems to be "bitten by the Tesla bug", he might want to
    subscribe to the pupman mailing list. Lots of advice available from
    helpful, friendly coilers; some with experience, some with degrees, and
    many with both. If that would be too time-consuming for this project,
    check out the Tesla webring; many pre-tested circuits out there to use
    as guidelines.

    Mark L. Fergerson

  5. Nice article, Mark. I post this for interest rather than a nit-pick.

    An effect often overlooked in gas arcs is that they often have
    a negative resistance characteristic. This can cause resonant
    circuits to be excited to much higher levels than the "struck
    bell" model would ever predict. The negative resistance leads
    to a positive exponential growth of the oscillation. This effect
    is especially important for the Telsa coil.
  6. Yep, another of those subtleties I mentioned. AFAIK it doesn't
    usually become noticeable (in "magic smoke" terms) until you get to high
    power levels though, all other things (like coil impedance) being equal.

    If the OP's son is up to monitoring his project's voltage and current
    (Tesla didn't have oscilloscopes) he might get an A+. ;>)

    Mark L. Fergerson
  7. Rodney Kelp

    Rodney Kelp Guest

    Does a taser stun gun work the same way with smaller components?
  8. Don Bruder

    Don Bruder Guest

    No. A taser is little more than a (probably multi-stage) voltage doubler
    that boosts the 9V (or whatever source it uses) into the "high enough to
    arc through the air" voltage that gets applied.

    They're not all that much different from the circuit that fires the tube
    in one of those disposable flash cameras, except in the way the voltage
    is finally used - They build up a big voltage from a small source, to be
    dumped all at once.

    A tesla coil is a whole different concept. For one thing, a tesla coil's
    output is ALWAYS high frequency AC. ALWAYS. World without end, amen.

    A taser's output is, with a very few exceptions I believe are purely
    experimental at this point, pure "flatline" DC. (discounting incidental
    "AC-like" effects that may happen due to the final capacitor ringing
    during the discharge cycle)
  9. Blackbeard

    Blackbeard Guest

    Well you guys are talking electronics and

    Alright. You guys are talking over my head. Electronics is definitely
    NOT something I am familiar with. So I figured I could attach the
    pictures of what my son has completed so far.

    From what I gather, the only thing he has left to do is connect a
    transformer and a spark gap.

    When we tested his spark gap by connecting one wire to one of the
    copper pieces and the other wire from the transformer to the other
    side, the sparks to jumped across. However, he used hot glue to hold
    the copper pipe in place and the heat melted the hot glue. So we've
    gotta re-think that plan.

    If you can take a look at the pictures, maybe it will give you a
    better idea of where he is and what he's trying to accomplish.

    Sorry for my ignorance on all this stuff. If you have a plumbing
    question, I'm your guy. Electronics...nada from me.

    One the pictures, the primary is made using 50-feet of 1/4OD
    refrigerant tubing. The secondary is made by winding 24-gauge wire
    approx. 1500 turns onto a piece of 4-inch pvc. The toroid is made from
    a piece of semi-rigid duct attached to a 5-gallon bucket top. The
    5-gallon bucket top/toroid uses an upside-down toilet flange to easily
    attach to the extra pipe that runs through the center of his

    We are using a 15,000-volt neon transformer for power.

    How would one go about building a beer bottle cap?
  10. Don Bruder

    Don Bruder Guest

    How would one go about building a beer bottle cap?[/QUOTE]

    Buy a six-pack of longnecks of your favorite brand. Drink the contents.
    While drinking, remove the labels. Wait until sober.

    Once you sober up, carefully wash and rinse the bottles to remove any of
    the previous contents and any remnants of the label-attachment goo.

    Wrap each bottle in tinfoil, doing the wrapping as smoothly as humanly
    possible. Cover the sides and the bottom, but leave the necks "naked".

    Load up a good sized pan with water - 6 bottles times 12 ounces - That's
    what? A bit over half a gallon or so? Go for a gallon to be sure you end
    up with enough. Heat the water on the stove - No need to boil it, just
    get it good and hot. Now start stirring in salt - plain ol' table salt
    or rock salt - until no more will dissolve. Let cool, and strain through
    a cloth to remove any of the salt that crystalizes out of solution
    during cooling.

    Using the salt solution, fill each bottle to the top edge of the foil
    wrapping or a hair more - it's not critical, just make sure you've got
    at least enough saltwater in each bottle to match the point where the
    tinfoil stops. Top up with mineral oil from the drugstore (optional but

    Cork the bottles using rubber stoppers with a hole. Run a long bolt of
    suitable size through each hole - Long enough to immerse it in the
    saltwater inside.

    Place the corked bottles in a suitably sized wooden or plastic box lined
    with tinfoil - you want all of the foil "skins" of the bottles
    electrically connected to each other - by setting them on tinfoil, you
    get the same result as if you tried to wire them together. Run a
    good-sized wire (12-14 gauge should be more than sufficient unless
    you're going into the super-high-power range (Doesn't sound like it from
    what you've described so far of this proeject) to connect all of the
    bolts together, and leave a pigtail off of one of the bolts. That's one
    lead of your capacitor. Run another wire from the tinfoil lining of the
    box to give you your other terminal.

    The tinfoil wrapping of each bottle is one plate of the cap, the
    saltwater inside the bottles is the other plate. The glass of the
    bottles is, of course, the dielectric of the capacitor.

    Your capacitor is now ready to use.

    I used to know the math that would tell you what the value of your
    capacitor is, but it's been so long since I used it that you (or some
    other brave volunteer) is going to have to look it up. I'm too lazy
    right now.

    Unlike your rolled cap, which may well turn into a firebomb if
    overdriven too far, the worst likely scenario in case of failure is a
    broken bottle and some spilled saltwater/oil (which can be made into a
    complete non-issue if you use a sturdy watertight box with a lid - one
    of those "six-pack coolers" works quite well on both counts - Contains
    any flying glass (unlikely, but not impossible) that might happen from
    severe abuse, and contains the spill if/when a bottle breaks or pops a
    cork) More likely is that you'll just blow a cork. Big fat hairy deal...

    Need a cap with more "oomph"? Too easy: Add more bottles.
  11. Whoops! No binary attachments (pictures) allowed in this group.
    Repost to alt.binaries.schematics.electronic and we'll take a look.
    OK, but we'd still like to take a gander to help make sure the thing
    works, and he survives.
    Epoxy works too.

    You could get mechanical about it. Take a look at the top couple of
    pictures at this link:

    for ideas how to mount your pipe sections. Notice particularly that
    sharp edges on the little brackets, bolt heads, and like that are kept
    far apart (and the pipe is deburred) so that the sparks will "want" to
    jump only between the pipes themselves. Oh, yeah; the gaps between the
    sections ought to be a millimeter or so.

    You won't have to get quite so ornate; I picked this page pretty much
    at random from a Google search for "copper pipe" +"spark gap" to show
    some typical general construction methods.

    Here's another that uses a fan to blow out the arcs to prevent what's
    called "power arcing" and provide some cooling:

    Power arcing is when the gaps are still firing after the cap has
    discharged into the primary coil and the transformer is basically
    shorted across the gap, wasting its power instead of feeding more into
    the secondary. Getting the gaps to stop firing when the cap has
    discharged is called "quenching" them. The idea is to feed energy into
    the secondary in lumps (every time the line voltage peaks, 120 times a
    second), not to try to do so continuously regardless of the fact that it
    LOOKS like it's continuous. Power arcing just overheats the transformer,
    but if your son's coil isn't drawing lots of power, and/or he doesn't
    run it for more than a few minutes at a time, this may not be worth
    worrying about.
    Ignorance is not knowing how far what you already know can be applied.

    You know about water hammer, right? That's the exact same kind of
    resonance principle that your son's coil will use, except with
    electricity. When water hammer gets extreme pipes break. When his Tesla
    coil works right, the toroid similarly breaks out in sparks.
    That primary seems to have way too many turns. <mumble> gotta see
    that diagram...

    I say that because like most electrical gear, a TC ought to be
    designed and built from the business end backwards. You have your toroid
    and secondary, which pretty much determines the frequency they'll run
    at. You have a primary coil and its dimensions fix its inductance range,
    so what's left is to figure out the capacitance the beer bottle cap will
    need (how many bottles, how big a bucket) so the primary circuit can
    resonate with the secondary circuit.

    Then there's tuning the primary; this is analogous to adjusting the
    lengths of the parts of a plumbing system to get the loudest water
    hammer, but since the "customer" end is fixed you have to adjust the
    "supply" end.

    When everything's built and powered up, don't be surprised if he
    doesn't get lightning right off the bat. It's really difficult to adjust
    a beer bottle cap in small steps, so it's simpler to move the primary
    connection ("tap") along the copper tubing to find the "sweet spot"
    where the primary coil is in resonance with the secondary. This involves
    connecting the wire to the copper tubing with something he can loosen,
    slide along the tubing say an eighth of a turn at a time, then retighten
    (the part of the tubing "hanging loose" doesn't affect the primary's
    inductance). Some folks prefer modified fuse holders, other use a short
    piece of copper pipe strap. Whichever, only move it when the system is
    UNPLUGGED and he's shorted the primary capacitor to remove its charge.
    The charge stored on it can KILL!

    Your son will have to learn a little math and get familiar with some
    equations to guess how much of his primary will resonate (where to put
    the tap to get started) with the cap you end up using.
    Is it an older one, or a newer one? The difference is that the newer
    versions have "protection" devices built into them that make them almost
    useless for TC use without surgery. If so, there's always microwave oven
    transformers. Gutting a microwave isn't all that difficult, and they're
    always available at yard sales.

    For more details on that I really suggest the Tesla coil mailing list
    archives, and/or a tour of the webring.

    Don Bruder gave good advice. I'd add that the hardware that actually
    touches the salt water ought to be galvanized because salt water is
    corrosive. One other thing; this is for a school project and they might
    or might not get a little cranky if the bottles are identified by their
    previous contents. ;>)

    Mark L. Fergerson
  12. Vidor Wolfe

    Vidor Wolfe Guest

    Another excellent transformer is the OBIT (Oil Burner Ignition Transformer.)
    Generally 10KV at 20+mA and to my mind, "bullet proof!"
    I've used two 10KV at 23mA in parallel on a 4" coil and gotten 30"
    And watch out for Hydrogen generation in the caps...

    Further to my previous postings, MMC (Multi Mini Capacitor) are really the
    cheapest, most reliable way to go with an almost home made cap.
    Not that I'm trying to stop an experiment, but I've never had brilliant
    results with beer bottle or glass plate caps.
    Mark H.
  13. Don Bruder

    Don Bruder Guest

    True enough - Although the lack of air in the bottles (The main reason
    for the top-up with mineral oil) seems to negate the corrosion problem.
    Let me stress "seems to"... Using galvanized is by no means a bad idea,
    but may end up being wasted expense.
    That's true, too... Trouble is, beer bottles, and specifically
    long-necks, are the only commonly available bottles I can think of that
    have the requisite straight, essentially same-thickness-throughout sides.

    A variant that might work (If you're a serious tinkerer) would be
    sputtering aluminum or copper on the inside of glass gallon jugs -
    basically, make a leyden jar (the beer bottle cap is essentially nothing
    but a "wet" leyden jar...) I never got THAT far into the concept,
    though, so I can't speak with certainty whether it would work. It's also
    probably beyond the realm of "reasonable" for a one-off school project,
    since one would almost certainly have to either hire that part out to a
    commercial plater, or design/build a sputtering machine from scratch.
  14. YD

    YD Guest

  15. Yeah, I forgot about those. Bullet-proof indeed; I managed to short
    one with a really poorly-built Jacob's ladder, but it worked fine after
    I unscrewed the mechanical setup.
    Right. Bubbles are Bad Signs.
    Yeah, but bottle dielectric punctures are easier to fix. ;>)
    It's a school project. Explaining its failure modes is better than
    having it work perfectly and not being able to explain why.

    Mark L. Fergerson
  16. Vidor Wolfe

    Vidor Wolfe Guest

    Too true.
    Then again, if he made both, he could show a reliable working coil and with
    the bottles, show problems that Tesla himself had with his jar/bottle caps.
    I'd imagine though, if his caps are too far out, it'll be out of tune and
    not work.
    This brings the new problem of racing sparks on the secondary or sec to
    primary flashover. (Both symptoms of poor tune)
    This could destroy a lot of hard work, ref 1500 turns of wire.
    Just a thought.
    Mark H.
  17. Bill Bowden

    Bill Bowden Guest

    If you have a plumbing question, I'm your guy.
    I have a plumbing question. I get this loud humming
    noise from my water pipes when I shut off the water
    after taking a shower. I can't find any leaks and if
    I flush the toilet, the noise goes away and doesn't return
    until next time I run the hot water for some time.
    Any idea what is going on with the water pipes?

  18. Guest

    HI to all.
    Not to many posts have hit on the SAFETY aspect of what you are doing.
    Just remember that those 15000V neon sign transformers are running at
    and can be lethal.The ones I have can supply up to 30mA at 15000v.Not
    very pleasant if you get a shock :0(
    Remember to take care....
  19. Blackbeard

    Blackbeard Guest

    Thanks for giving me a heads up. I have posted out pics at
    alt.binaries.schematics.electronics as you suggested.
  20. Blackbeard

    Blackbeard Guest

    Sounds like you either need to install a hammer arrester or a pressure
    reducing valve. You can buy a hammer arrester that will screw right
    onto the threads of your stop vavle. Then you connect your water
    supply tube to the auxilliary threads on the the new hammer arrester.
    That's probably your cheapest solution. The threaded hammer arrester
    will cost you approx. $20. I would try the hammer arrester. If that
    works, don't worry about that PRV
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