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"The Mighty Corona Cone" (fwd)



---------- Forwarded message ----------
Date: Sat, 14 Feb 1998 11:36:28 -0500 (EST)
From: SBJohnston-at-aol-dot-com
To: tesla-at-pupman-dot-com
Subject: "The Mighty Corona Cone"



Ed Sonderman wrote:

>Something is not right in a couple of areas here.  First, I think
>the resonant signal readings are suspicious.  I have a 6" dia.
>secondary that is resonant at 270 khz with no top load and at
>132 khz with the large 5" x 40" toroid.  If I put your toroid 
>(est 18 pf) on my coil it would still drop 100 khz down to
>about 170 khz.  Your coil is much larger than mine and adding 
>the toroid would not have as much affect as the self c of the 
>coil is much larger - still I would expect it to drop about 25 to 30 khz.

Based on my specs (secondary form 5 feet high x 16 in in diameter, 48 inches
of windings, space wound with #22 at about 13 turns per inch) others have
calculated a resonant freq without toploading of about 160 kHz.  A Tesla coil
design program I downloaded figures it to be 148 kHz.  I measured it to be
134 kHz bare.

I suppose it is possible that surrounding objects in the garage are
influencing the resonant frequency, such as refridgerators, freezers, metal
racks of parts, 1951 military M38 Jeep, heating ducts, you know...  That
would drop it down to some degree.  I'll have to roll the coil out into the
driveway and repeat the measurement.

>I don't understand how the bottom of the toroid can be within
> a few inches of the top windings of the secondary when it is
> on a six inch ceramic insulator. Still, I have mine mounted 
>10.25" above the top winding and have no problem with
> corona.  

Well, the dryer duct is almost 5" in diameter so it hangs down two and a half
inches below the top of the six inch insulator.   This puts it about 3 and
half inches away from the top of secondary.  Other replies to my message have
suggested putting it closer to the top of my coil rather than farther away...

>Is the coil extremely over coupled?  

The secondary is about two-thirds of the way into the solenoidal-wound
primary.    

>Are you sure it is tuned
>properly?  I like to tune at about 1/2 power for the longest, 
>hottest spark (to a ground rod from a bump on the toroid) 
>then move the tap out another quarter turn for full power 
>operation.  

With my .018 uF of primary capacitance I calculate I'd have to use the full
11 turns of primary.  And that's where I get the biggest sparks.

>What are you using for a power supply? 

Not much, by Tesla-list standards...  a 15 kV 30 ma neon xfmr with only half
of the winding in use.  The tank cap consists of 4 mica caps in parallel,
each about .004 uF at 12.5 kV.  The caps are interesting -- I listened to a
voice recording of the coil's builder, now dead, who said that he removed
them from a 1920 Navy spark gap transmitter.

>With a large toroid and a beefy power supply this coil should
>produce some serious sparks.  I would think 9 to 10 feet with
>a pole pig and rotary gap.

Mmmmm... sounds delicious!  But I think I'd do well to find a way to clamp
down on this corona from the top windings or perhaps more input power will
begin to damage the secondary.

Steve Johnston

sbjohnston-at-aol-dot-com