TESLA COIL

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

1. BlackbeardGuest

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
suggest.

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
(how?)
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
helpful.

thanks
mike

2. John PopelishGuest

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
contacts.
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 WolfeGuest

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
2mm.

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
with.
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
coil.
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
waterpipes).

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
Yahoogroups.

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

Cheers.
Mark H.

4. Mark FergersonGuest

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:

http://www.pupman.com/

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

http://www.pupman.com/safety.htm

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. Larry BrasfieldGuest

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. Mark FergersonGuest

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 KelpGuest

Does a taser stun gun work the same way with smaller components?

8. Don BruderGuest

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. BlackbeardGuest

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
secondary.

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

How would one go about building a beer bottle cap?

10. Don BruderGuest

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
recommended).

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. Mark FergersonGuest

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:

http://users.tm.net/lapointe/SparkGap.htm

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:

http://users2.ev1.net/~nmyreality/tesla/sparkgap/rqgap.html

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 WolfeGuest

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"
streamers.
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.
Cheers.
Mark H.

13. Don BruderGuest

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.

15. Mark FergersonGuest

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 WolfeGuest

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.
Cheers.
Mark H.

17. Bill BowdenGuest

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?

-Bill

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
50/60Hz
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....
Cheers
Rob

19. BlackbeardGuest

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

20. BlackbeardGuest

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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