Re: Double Throw Spark Gap (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 14 Oct 2007 11:46:07 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Double Throw Spark Gap (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sat, 13 Oct 2007 23:46:38 -0500
> From: Crispy <crispy@xxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Double Throw Spark Gap (fwd)
> 
> Thanks a lot, Bert.  That pretty much sums up everything I've been
> saying.  Sometimes I'm not the best at getting across ideas, but that
> was great.
> 
> I'd just like to add one note - you talk about the charging arc drawing
> out due to the high charging current.  Although this is completely true,
> I don't think it's particularly relevant.  The time it will take to
> complete the charging cycle is, by my calculations, so short, that the
> arc really shouldn't have to stretch at all.

You may be right - I didn't do a thorough gap timing analysis before.
Let's plug in some actual numbers for your system. Your proposed tank 
cap (Cp) is 16.7 nF, the DC storage cap (Cs) is 400 nF, and the charging 
choke (Lc) is 25 mH. Let's assume that the DC resistance of the charging 
choke is relatively low - say 10 ohms or less, so we have a high Q under 
damped LC charging circuit. During the charging interval, Cs and Cp are 
connected in series with Lc. The combined series capacitance is about 16 
nF, and the resonant frequency (Fc) of the LC circuit will be about 7950 
Hz. Because of the dequeing diode, we know that charging will be 
complete in 1/2 cycle (at Fc), or about 63 usec. This is the electrical 
"dwell time" we need to insure that Cp completes a resonant charging 
cycle.

Referencing a picture of your gap:
http://tangent.cluenet.org/~chules/pictures/4/100_0628.JPG

The tungsten electrodes look to be 1/8" in diameter, and the radius of 
rotation appears to be about 4". If your gap rotates at "X" RPM, each 
revolution will take 60/X seconds, and each degree of rotation will take 
1/360th of this, or 1/X*6 seconds. The "mechanical" (edge-edge) overlap 
of the 1/8" diameter electrodes will subtend a total arc length of twice 
the electrode diameter (or 0.25 inch). This is equivalent to about 3.6 
degrees at a 4 inch rotational radius. The simple "mechanical" overlap 
time, expressed as a function of RPM, will be:

Mechanical Dwell time = 3.6/(6*X) seconds

Creating a table of mechanical dwell versus RPM:

RPM	Dwell (usec)
===	===========
1000	597
2000	298
3000	199
4000	149
5000	119
6000	99.5
7000	85.3
8000	74.6
9000	66.3
10000	59.7
11000	54.3

So, it looks like you are indeed correct - there should be more than 
enough time to resonantly recharge the tank cap when operating at 
"reasonable" RPM's. BTW, I would NOT recommend running the lashed up 
configuration at anywhere near the maximum speeds available from your 
angle grinder.

  >
> Also, I have another question somewhat unrelated to this thread, but
> goes along with the University limits.  I obviously need an RF ground,
> but I'm not allowed to pound anything into the ground.  So, I need a
> counterpoise.  I have virtually no idea on what is acceptable here.  Is
> a 1ft x 1ft square covered in the middle with many layers of chicken
> wire acceptable?

If possible, use a counterpoise whose diameter is least 2X the combined 
height of your secondary and toroid. Chicken wire, lots of radial wires, 
and even aluminum foil/sheet, will all work. Even connecting to the 
building's steel framework might work as a counterpoise (if you can 
access it). In any event, the counterpoise should also be connected to 
mains/building ground. Remember that anything in the vicinity of the 
coil will "see" the E-field from your coil - ungrounded metal objects, 
electrical equipment, speaker wires, etc. may develop voltages 
sufficient to spark over/damage sensitive electronics.

> 
> Thanks,
> Chris B aka. Crispy
> 
> 
Good luck - please keep us posted on your results.

Bert
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