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Tesla Coils and more (fwd)





---------- Forwarded message ----------
Date: Sun, 2 Aug 1998 18:05:37 EDT
From: RWB355-at-aol-dot-com
To: Tesla List <tesla-at-pupman-dot-com>, alfred.skrocki.sr-at-juno-dot-com
Subject: Tesla Coils and more


Hi Alfred and all,

Thanks for correcting me about the bug zappers. I never really took one apart,
 so my reply to Dave was just a train of thought.

 I have been fooling around (without being foolish) with high voltage / high
 current for a pretty long time now. I love man-made lightning. I guess itīs
 because of my dad (God bless his soul). He was a prof (physics & astronomy) at
 U.C. Long Beach. He built his own unique plasma lab, where students could
 really work with plasma not just bla bla about it. Everything was self built,
 from the plasma chamber to the vacuum pump, filaments and high voltage supply.
 The filaments were heated by using two "Minute-Man" starter units rated at 28
 volts and 800 Amps for a total of 1600A. The high voltage supply had a rating
 of 25kv-at- 100 (one hundred!!) Amps. In fact the faculty had to wire up his lab
 seperatly with 3 Phase 440 Volt mains wiring (1MW power capability). Now
 thatīs what I call p o w e r!! I have built all types of welders for my own
 needs including MIG/MAG, stick & TIG welders with all options included. They
 are simply to expensive (esp. TIG units) to buy and as I am unemployed at the
 moment this holds true even more so. Besides itīs fun when a home-made device
 works and is as good or better than a bought unit (for much less $$$$$).

I have built quite a few small Tesla coils with less than 1.5kW input rating.
 However, I plan to build a bigger one next. I want to get 55" to 60" sparks
 from this baby and keep the frequency high enough to stay out of danger. I
 have a few questions to you coilers out there.

1.) The power supply will look something like this: I will be using 10 units
 of furnace burner xformers 15kV-at-20mA (voltage & phase matched) wired in
 parallel to get 15kV-at-200mA. I will pull these babies out of their shell and
 emerse the whole set in a plexiglas container filled with castor oil. Does
 this sound okay? Over here (in Germany) you canīt get hold of neon sign
 xformers. The people who sell them are rare and they want big bucks, even for
 used ones. The same holds true of pole pigs. You will get strange looks from
 power companies when you tell them what you want and what it will be used for.
 I though of using MOTīs but I canīt get a hold of enough of the same sort to
 make my needed 15kv and I donīt need 700mA of power. The 2.3Kv-at- 700mA seems to
 be a common type in german microwave ovens. They pull power like crazy. My
 2.3kV-at-700mA unit pulls almost 15A-at-230v under full load (Okay really its
 overloaded because it doesnīt limit the current. It does have a thermal fuse,
 however). This means if I wire up seven of these xformers to get my 15kV I
 would need a 100A slow blow fuse in my primary side. Wow man, I donīt have
 6mm2 wiring in my house (other than the wire that comes from the electric
 company, itīs 10mm2) Of course I could use a current limiter in the primary,
 but who wants unnecessary heating and why use more parts than necessary. The
 primary side will be controlled by some sort of non holding (meaning you have
 to press & hold the button) fused remote controller. What about x-ray xformers
 from x-ray machines or x-ray viewers ? I know x-ray machines are pulsed, so I
 would suspect that they probably will burn out after a time if they are
 operated continuesly. However, I donīt know how x-ray viewers are constructed.
 They probably donīt have a high amperage output, but they should provide
 35-40kV at least. Or is this too much, so that you have corona problems on the
 primary side of the TC ?

Circuit description of my planned Tesla coil:

2a.) The mains enter the circuit via a switch, fuses and a power "on" light
 and have a hefty varisistor connected across them. The next step is the remote
 control box. Across the mains and in series there will be a voltmeter and
 ampmeter (respectivly) A simple non holding relay with big amp capacity from a
 high frequency generator for a DC TIG tourch. (300A capable) will switch the
 current to the HV xformer. The coil is 24 volt AC and will be activated by a
 separate small xformer.

2b.) Next the mains will pass through an over dimensional RF filter consisting
 of two caps connected in series across the mains and their center-tap
 grounded. The center tap (the iron core) of the HV xformers will also be
 grounded. The next step are two chokes in each leg of the mains wound on
 ferrit toroids. Behind the chokes there is a second set of caps connected in
 the same way the first are. Now we enter the set of HV xformers.

2c.) On the high voltage side I want to set up a safety spark gap using 3 ball
 bearings (dia. 3/4") with the middle one grounded to an RF (real earth stuff)
 ground. Is this okay ? I would rather use this setup than a set of RF-
 supressing "safety caps" like they are on the primary side of the HV xformer
 in my circuit. Who knows if the caps will stand up to a real backward blow
 from the tank circuit!!

2d.) Moving on to the primary coil of the Tesla circuit: The next step will be
 two chokes (short, thick wire on ferrit toroids). These will be connected to
 the spark gap which lies parallel to the hv xformer. Please read note 5. 

2e.) My caps goes in series (see note 5) with secondary of the HV xformer and
 the primary of the Tesla coil. The caps have a safety spark connected parallel
 to them. Or does this pose a danger in case of a kickback ? In effect the
 capacitor is no longer present in the circuit, should the safety gap fire (for
 the time of the arc at least).

3.) Can I use a plexiglas tube as a capacitor carrier ? The capacitor will be
 self made and the plexiglas tube will contain caster oil, too. This way I can
 keep up on the insides and also be able to show the capacitor. Both caps of
 the tube will contain small holes plugged with a rubber "skin" as a safety
 valve, that can rupture if necessary. Somewhat like a "real" capacitor safety
 valve.

4.) What kind of spark gap should I use. Rotary or stationary? I have never
 used (needed) a rotary spark, but I do have a lathe and it should not be a
 problem to design one. If I used a stationary spark gap it would be a multi-
 gap (4 gaps in series) to distribute the load and get richer harmonics. As
 electrodes I would use 3/16" thorium free tungsten TIG electrodes. The rotary
 gap would contain a synchron motor and tungsten electrodes. It would also
 contain a fan to de-ionize the air and carry off the heat.

5.) Corresponding to 2d & 2e I have a question about the tank circuit. I have
 seen numerous different configurations. I even saw an oddball with a center-
 tapped primary (tc) that is grounded. I couldnīt figure out what this is
 supposed to improve. Does anyone know ? They other configurations can be
 divided into the following two basic catagories:

        a.) Looking from the HV side of the HV xformer: The spark gap is
 connected
            parallel to the HV xformer and the "tank-caps" are in series with
 the xformer 
            and the primary of the Tesla coil.

        b.) Again looking from the HV side of the HV xformer: The capacitor is
 now 
            connected parallel to the HV xformer and the spark gap is in
 series with the
            xformer and the primary of the Tesla coil.

Does it make any difference (I have always used the "A" version) ? If so, why?
 and what would be better for my Tesla coil.

6.) What kind of  primary should I use on my Tesla this time. Up to now I have
 always used a (single) helical coil. I am thinking of adding a strike rail
 (that is RF grounded) between the primary & secondary in order to prevent the
 sparks (from the secondary of the TC) from hitting the primary coil. Is this
 idea any good ? Does it make a difference where (along the length of the
 secondary) the primary is placed. Of course I mean somewhere along the bottom
 of the secondary coil.

7.) My RF ground and my ground on the primary side of the HV xformer are not
 connected. The secondary has itīs "cold" end connected to the RF ground.

8) What is better on top of a Tesla coil: a sphere or a toroid. I have never
 used either because I could not get hold of something good looking AND cheap.
 Of course the sparks which my Tesla were making were not the maximum possible
 length because  the charge "evaporated" before it could reach itīs maximum. So
 this time I will use an "extra capacitor" (sphere or toroid). But which is
 best? From the physics point of view I canīt imagine how a toroid made from a
 dry hose (as a lot of people shown on the internet have) could work all too
 well because the surface is not as smooth (because the hose is flexible) as
 possible which in turn lets some of the charge leak off.

One last question: 

Does anyone have a cool "Danger High Voltage" sign for me (or at least a scan
 of it) ? The "Hochspannung !! Lebensgefahr !!" sign from Germany is pretty
 boring.

If so and if anyone has any ideas, corrections or improvements for my Tesla
 coil please send them to me at:  

<A HREF="">RWB355-at-AOL-dot-com</A>

Thanks and keep coiling,
Reinhard