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Re: Core Material

Subject:      Re: Core Material
       Date:  Thu, 08 May 1997 06:54:07 -0700
       From:  Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: Stoneridge Engineering
         To:  Tesla List <tesla-at-pupman-dot-com>
 References: 
            1


Tesla List wrote:
> 
> Subject:  RE: Core Material
>   Date:  Wed, 7 May 97 21:27:46 UT
>   From:  "Robert Davis" <Davis_Lewis-at-msn-dot-com>
>     To:  "Tesla List" <tesla-at-pupman-dot-com>
> 
<SNIP>

> 
> I have 25mm (1") Acrylic Rod, 50mm(2") Acrylic Tube, 80mm(~3") Acrylic
> Tube
> and can get 50mm Nylon Rod.  Teslac gives the following using 30swg wire
> (Diameter & Length in inches, Fres in KHz, Inductance in mH) :
> 
> Diameter   Length      Inductance   Fres     Loaded Q
> 2          7           5.126        1186     34.6
> 3          9           14.55        596.3    36.1
> 
> Teslac won't do the very small 1", I have seen a very small coil, it
> looked
> about 1" X 8" but had a very small spark.
> 
> I will go for the 3" X 9" (the solid is a lot more expensive, I just
> thought
> it might be easier in construction).  It will be a low power coil, maybe
> 50-100 watts and I will probably go for a solid state drive on the
> primary
> (probably MOSFET but I haven't decided yet), comments?

The 3" coil will give you better performance and will allow you to go to
higher power levels later on (something all coilers do once they see the
first sparks off their coil! :^) ). 

> 
> One other thing, am I correct in thinking that magnifier coils are
> driven
> directly from the base connection of the coil?  If so does the power
> supply
> have to supply a large current or large voltage to the base?
> 
> I look forward to receiving your replies.
> 
> Rob Davis, UK

Rob, a solid state magnifier typically does not use a primary coil, but
instead uses an RF step-up transformer (ferrite or air-core), the output
of which drives the base of an air-core resonator. Both high voltage and
high current are needed (i.e., we need to provide high power). A
simplistic way to look at a base-driven resonator is it's much like a
transmission line that's "tuned" to 1/4 wavelength. Little top load
capacitance is used. Under these conditions, the voltage at the top of
the coil is proportional to the voltage applied at the base multiplied
by the Voltage Standing Wave Ratio (VSWR) of the resonator. The "source"
needs to look very much like a voltage source, capable of providing
whatever current is required to drive the resonator. 

Although the VSWR for a given resonator is fairly difficult to
calculate, it's directly related to the ratio of the inductance versus
the effective resistance of the wire - the larger this ratio, the better
the coil will perform. That's why solid state coils tend to have
fat-squat resonators. 
Although many people on this list have successfully built solid state
coils typically by using circutis similar to switching power supplies,
be aware that these are rather difficult to keep running reliably, since
they seem to have a nasty tendancy to blow driver transistors. You might
want to get Duane Bylund's "Modern Tesla Coil Theory" as a start or
refer to his article, "Solid State Tesla Coil", Radio Electronics; Sept 
1991; p. 33. Also, some of the coilers on this list who HAVE
successfully built these finicky beasts may also be able to help you
out. Alan? Harri?

Safe coilin' to you!

-- Bert H --