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Re: Double Throw Spark Gap (fwd)
---------- Forwarded message ----------
Date: Sun, 14 Oct 2007 17:28:25 -0500
From: Crispy <crispy@xxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Double Throw Spark Gap (fwd)
Thanks for the advice on the counterpoise - I'll try that. I'm
wondering why the problems of feeding RF into the building's ground
disappear when a counterpoise is used ... or is this true at all? I may
also try to completely bury a real ground and have some mechanism where
can attach and detach the grounding wire to this underground
construction even where the construction itself isn't visible.
By the way, I have been testing the gap at maximum speeds (11000 rpm)
and it seems to work fine. The configuration may appear to be lashed
up, but it's actually relatively stable. The angle grinder rotor itself
is very secure with a tight fit to the internal gears. I'm using the
angle grinder's original method to attach the G10 bars onto it, and that
is very stable as well. The rotary tungsten rods are a tight fit and
are secured with shaft collars - even so, the direction of force is not
such that it should put stress on the shaft collars. The angle grinder
is attached onto the base by bars screwed into its handle holes which
are screwed into the base. There are also some other HDPE sections used
to make the angle grinder level with the base. Hot glue was used to
provide additional support where the angle grinder contacts these
supports, and large amounts of duct tape was used to provide some
resistance from vibration. The stationary contacts are made out of G10,
which is very stiff. All of the parts are screwed together well. The
space between the rotary electrodes and stationary electrodes is
sufficient to prevent damage from vibrations. I've run this up to its
full speed on several occasions, even without ramping it up, and it
appears to be fine. Even if something did break, it is enclosed on all
sides.
On Sun, 2007-10-14 at 10:40 -0600, Tesla list wrote:
> ---------- 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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