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From: keithd@cadovax.UUCP (Keith Doyle)
Newsgroups: net.physics
Subject: Re: Tesla
Message-ID: <443@cadovax.UUCP>
Date: Fri, 8-Mar-85 18:13:36 EST
Article-I.D.: cadovax.443
Posted: Fri Mar  8 18:13:36 1985
Date-Received: Sun, 10-Mar-85 07:48:31 EST
References: <1025@hou4a.UUCP>
Organization: Contel Cado, Torrance, CA
Lines: 194

[............]
>I have been reading the discussion about Tesla with great interest.
>Some of his experiments have always facinated me, in particular
>the Tesla Coil. I was wondering if anyone out there knows where
>I can find the plans to build one using modern components such
>as transistors, flyback transformers (from TV) etc.etc.
>I do not relish the task of winding my own coils.

>Any information would be appreciated (I would like BIG sparks)

>			Thanks, Werner B. AT&T-IS Holmdel, NJ

I'll give this one a try.  There are plans out there for coils, you
might check the readers guide at a local library.  I once found a
Popular Science/Mechanics/Electronics article that used a T.V. HV
regulator tube as driver for an oscillator.  Although you indicated
you're not crazy about coil winding, I found that it really wasn't
all that bad.  I used a bolt in one end of my coil form and put it
in the chuck of a variable speed hand drill, and anchored the other end
so it would turn.  With a buddy on the drill, I wound my coil in about
1/2 hour.  If anyone knows of the source of some less crude (than my
following instructions) yet accesible method of constructing coils,
I'd like to hear about it too.

My present coil is constructed using a 15000v neon sign transformer.
Check for local neon sign repair outlets in your area.  You can usually
obtain used sign transformers fairly cheap ($10-$20).  CAREFUL, 15000v
is a lot of juice, and mine will deliver 60ma (30 is also common).
These come in about a 3x4x10 size package that weighs 30lbs or so, with
large ceramic insulators on either end.  Just plug one into 115v, and
instant 15kv.  ALso watch that the center tap of the transformer may
be connected to the metal case of the transformer, which means that
either insulated end (or your resulting coil) are a good 7.5kv hot
with respect to this transformer case.

Next you need a H.V. capacitor.  I initially went to a glass shop, and
had them cut me about a dozen glass tiles 1 foot square, out of 1/4 inch
glass.  You can home make a H.V. cap by layering glass and aluminum
foil in a stack.  Every other aluminum layer should be connected together
on one side, and the leftover layers on the other.  Make sure that you
have plenty of margin between the end of a sheet and the side it's NOT
connected to, as 15kv will jump a couple of inches easy.

Illustration:

            -->    |--------------------|
plenty of margin   |      glass         |
   	    -->	   |    |----------|    |
		   |    |          |    |
15kv worth	   |    |          |    |
		   |    |  foil    |    |
		   |    |          |    |
		   |    |          |    |
		   |    |          |    |
		   |----|---|  |---|----|
			    |  |
                   ^    ^   |\----- connection here
		   |    |
		   7.5k worth of margin

basically, you can just contstruct one of these for each sheet
of glass you have, and then pile them up so that every other one
is rotated 180 degrees.  Connect all of the connection points
on either end together, and you have a H.V. capacitor.  Note that
you can make them bigger than 1 foot square, I found that I needed
so much margin that the foil area was fairly small, and didn't deliver
much capacitance.  Thinner glass might work too, but you're depending
on the thickness of this glass to insulate to the full 15k.  I feel the
more capacitance the better, as it keeps the frequency of the resultant
coil down to a signal that is more effective and carries more 'punch'.
After my first glass cap, I was later able to find a surplus oil-filled
H.V. cap which GREATLY improved the efficiency of my coil.  Unfortunately
these have been hard to find. (I'd grab another if I found one).
I used Scotch tape to anchor the foil to the glass.

Ok. Now on to the coil.  I used a 2-1/2 to 3 inch mailing tube.  Larger
tubes are certainly workable, but I didn't have a lot of space, and the
tube was handy.  Smaller ones probably work too, you can certainly get
more turns/ratio on a smaller core.  You probably want a tube that is
at least a couple of feet long.  The next step is to buy a load of
enameled magnet wire.  Basically, you just start winding from one end
to the other, or until you run out of wire (splicings ok, but be neat!).
The voltage step-up ration of your coil is theoretically the number of turns
in the secondary divided by the number of turns in the primary (haven't got
to the primary yet).  Realistically, I have no idea how to compute the
practical ratio as there are losses all over the place in a kludge like
this.  At any rate, use your own judgement, generally, the more turns the
better.  I would suggest that you wrap cleanly, and don't over-wrap previous
turns.  There are ways of over-wrapping such coils, but I don't know what
they are, and you don't want the coil to arc accross the windings.

When you're done winding, you might want to consider some kind of fixative
process.  Some people boil the whole thing in wax, I just anchored the ends,
and painted them with corona-dope (an insulative paint that you can get
at local T.V. repair oriented electronics shops).  It's not necessary
to do either, but it may cut down on inter-coil (?) emmisions.

Ok, now we need the primary.  I used very-few turns of a T.V. H.V.
wire (I think 6 turns), and just wrapped it on the base of my secondary
coil as if it was a continuation of it's windings.  The resonant
frequency of the coil is related to the number of turns in the primary
and the capacitance of the H.V. cap.  So few turns and so little 
capacitance tend to run at frequencys so high that the efficiency of
the coil is not that great.  You can improve efficiency by adding more
primary turns but this will drop the turns-ratio you worked so hard
to get for that H.V.  It would be better to use more capacitance
to bring the resonant frequency down to more efficient ranges.
You don't have to use H.V. wire for the primary,  just about anything
will work.  An earlier coil of mine just used some medium gauge solid
wire insulated to 600 volts.  The voltage difference from one winding
to the next is not enough to worry about.

Lastly, you need a spark gap.  I used a couple of peices of heavy gauge
wire mounted in a plastic box so that they are insulated from each other.
I've heard of people using lead pencils mounted with the points facing
each other, and using the lead as the conductor, but I've never tried that.
You want to position whatever you use as conductors so that they never
come closer than about 1 inch at any point (for 15kv anyway, lower voltages
you may have to position closer).  I mounted mine in an enclosed plastic
box, because when the sparks fly, it makes ALL KINDS of NOISE, and emits
ALL KINDS of OZONE (operate in well ventilated area).

Ok, now lets see if I can draw a schematic:

			   (---->    output
			   (
			   (
			   (
      secondary windings   ( 
			   (
			   (
			   (
			   (
			   (---------------|
    primary windings	   (               |
			   (----|        =====   capacitor
				|          |
				|          |
	spark gap		+--->  <---+ 
				|          |
				|          |
				~~~~~~~~~~~~
	neon sign transformer	#          #============< from 115v A.C.
				~~~~~~~~~~~~

(whew!)  That's about it.  All sorts of substitutions can be made, you
might try using a car coil instead of a neon transformer as a driver,
(I've seen circuits that use an SCR and a little neon glow light that
smack a car coil with 115v for turning flouorescent tubes into strobe
lights, the coil can get a little hot, you might try a lower-voltage
oscillator circuit to drive the car coil).  You can play with all
the parameters, capacitance, size of coil, number of turns, etc. to
try to get a more efficent coil.  I noticed, for instance, that if
you use a fan to blow over the spark gap, it's extra agitation works
to improve efficiency, some schemes use a rotating disk with holes
in it to 'interrupt' the circuit more often which helps.  Basically,
the spark gap serves to generate high frequency 'glitches' that exite
the coil, as only the high frequency components of the signal into the
primary actually goes through the coil.  The capacitor takes these
'glitches' and by working as a tuned circuit with the primary windings,
serve to sustain oscillation at these high frequencys.

What can you do with it now that you've made one?  Well, besides
fouling up all the T.V.'s in the neighborhood, and amazing your
friends, the coil will exite the gas in any fluorescent tube held
within a couple of feet of the coil, and other than that, I can't
think of anything right now.  Some of my Tesla books indicate
experiments with light pieces of wire hung of the secondary that
emit sparks and spin around, etc.  Note that because of the
spark gap, the thing is VERY noisy, and the gap itself emits
more light than the secondary (another good reason why I hid
it in a plastic box).

Any more questions about this one, address them to me.  And if
anyone has further suggestions for improvements, I'd like to
hear them.  About once every 6 months I drag the thing out and
start it up, and think about what I might do to improve its
efficency, or quiet it's noise etc. (mine puts out about 10-inch sparks)

This was inspired by a coil that was in my High-School electronics class,
a slightly larger model that would produce sparks of 1-1/2 to 2 feet long,
with basically the same design but a larger coil and capacitor.  I don't
have much access to machine shop tools etc, and didn't have a lot of space
for storage, so I kept mine rather small.

Have fun, and don't electrocute yourself.  Keep in mind that the
15kv primary is a lot more dangerous than the secondary emmisions.
If you get zapped by the secondary, chances are it will do little more
than curl your hair or give you a tickling sensation, but 15kv could
knock you around the room!

Keith Doyle
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