Re: [TCML] Tesla Gun

[Michael Twieg <mdt24@xxxxxxxx>]

Hi Steve,
1A through a human body is a tremendous amount, even at "RF."  I know in
medical applications, 100mA is the highest you're ever allowed to go, even
for research purposes (for clinical applications it's lower).  I'm
surprised you haven't been suffering from peripheral nerve stimulation, or
experienced any significant heating in your arm.  I think the fact that you
haven't suggests you aren't seeing nearly that much current in your body,
thankfully.  You should still try and suppress E fields along your body
though.  If you already have screens on your shoes, directly connecting
those to the grip of the gun should help a lot.  Adding a small
counterpoise/faraday shield extending the grip around your forearm should
also help prevent E fields from terminating on you.

-Mike

On Mon, Jun 18, 2012 at 3:15 PM, Steve Ward <steve.ward@xxxxxxxxx> wrote:

> Hi Guys,
>
> Recently got a project running here thats been a lot of fun.  Well, as
> fun as fiddling with electronics stuff gets... I think this one
> certainly pushes the state of the art in terms of tesla coils driven
> by electronics!  I'll cut to the chase with a link to some pictures of
> the device:
>
> http://www.flickr.com/photos/kickermagnet/
>
> Where to begin...
>
> Essentially its a DRSSTC that runs extra long pulses to grow sparks at
> a low voltage of about 55kV pk.  I believe the reason the sparks stay
> pretty straight instead of splitting is that the low voltage allows
> growth only with negative coronas, which have been shown to make
> branchless sparks, whereas positive coronas tend to branch.  But this
> is merely my speculation!  I've not seen any research on spark
> development under these conditions (~400khz).
>
> In order to maximally control the straightness of the sparks, the
> electronics driving the primary circuit are fairly sophisticated.
> Essentially im using a "phase shifted  full bridge", where 1 leg of
> the AC output is perfectly in phase with the primary current (so it
> experiences wonderful zero current switching), but the other leg of
> the AC output is phase shifted, which essentially controls the drive
> voltage.  If the 2 AC legs are 0 degrees phase, then the effective
> output voltage (differential mode) is zero.  If the 2 AC legs are 180
> degrees, then the entire 395V is applied to the primary circuit.  So
> what happens is the phase is "ramped" over a period of about 17mS in
> order to gradually drive more and more power into the coil.  This
> allows for growing long straight sparks that do not branch.  If the
> voltage (or phase) is ramped too quickly, the sparks become very
> forked and jagged, forming multiple branches in 1 shot.   This
> parameter is of course tweakable during operation ;-).
>
> The bridges themselves utilize some cheap little IGBTs (FGH60N60SMD).
> Each bridge has been tested to 60A peak, hard switched (due to the
> phase shifted bridge control) for 17mS pulse durations at 400khz.  The
> calculated thermal rise for the die is 52*C during such a pulse.
> Luckily real operation is not this harsh.
>
> The output of 2 bridges (operating synchronously) is combined through
> a pair of transformers, with the secondary windings in series.  Each
> transformer is wound 6:3, so with 2 combined, its a 1:1 ratio.  The
> reason for doing this is 1) i can ground one end of the primary now,
> and 2) it absolutely forces equal current sharing between the 2
> H-bridges.
>
> Power is derived from a 22.2V 5000mAHr LiPo battery, the type used for
> RC helicopters.  It only lasts for about 8 minutes of continuous fun
> as the DC-DC converter uses about 700W.  The DC-DC conversion itself
> is a resonant type converter used very commonly for capacitor charging
> supplies, as its short-circuit tolerant.  The DC bus for the bridges
> is then 395VDC with 20,000uF of storage capacity.  One spark eats up
> 200-250 Joules of this capacitors energy, so even 20k uF is a bit
> insufficient, the DC voltage drops by about 30V at the end of the 17mS
> burst.
>
> Another interesting feature of this system is a secondary MMC.  That
> is, there is a string of 72 x 1.2nF 700VAC capacitors INSIDE of the
> secondary coil.  This capacitor adds about 17pF to the coil.  There
> are a few reasons for this.  1) due to the gigantic streamers this
> thing produces, detuning is a serious issue, so the coil needs to have
> a pretty decent amount of self capacitance.  2) more Csec drops the
> impedance of the coil, which seems to improve spark growth behavior.
> 3) adding more energy storage to the secondary coil means slightly
> less circulating current in the primary.  The way i think of it is
> that the system Q probably stays about the same, but since there is
> more energy in the secondary, there is less stored in the primary.
> This capacitor also provides convenient measurement of the toroid
> voltage by measuring the current through this capacitor only.  This is
> where i obtained the 55kV number, which is in good agreement with
> previous direct HV measurement of my last QCW system using a Jennings
> HV vacuum cap divider.  So indeed, the voltage is low.
>
> The primary coil is wound with home-made Litz wire.  4 bundles of 4
> strands of 24awg were twisted such that each strand sees the outside
> equally.  Its important to do this, otherwise strands that remain
> burried inside the bundle will not carry their share of current.  This
> is more surface area than 1/4" copper tubing at a fraction of the size
> and weight.  Only needed about 18 feet of the litz, so it wasnt a big
> deal to make it using a drill with a jig.
>
> Grounding for the system is questionable!  To be totally un-tethered,
> i have a pair of shoes with steel mesh on the bottom for contact with
> the floor (dont stand on anything flammable!).  My body is charged to
> a few kV with the 2.5A of secondary ground current.  Generally,
> though, i prefer to add an additional connection to mains ground.  The
> current into mains ground is about 1.5A, meaning my body capacitance
> is still seeing about 1A in this condition.  I'm uncertain of the
> effects of this current through my body (it must primarily go through
> my right arm, which is gripping the conductive gun grip which is the
> only connection between me and the electronics).  Anyone have any
> research on RF currents in the human body?  People expose themselves
> to pretty similar fields when playing with SSTCs and VTTCs, just being
> up close to the secondary coil must induce quite a bit of current in
> them (according to the mutual capacitance between the person and the
> TC).
>
> Well, im sure i forgot to describe certain aspects of the project, so
> ask questions if you like.
>
> Steve Ward
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