Re: Spark Gap Sustaining Current (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 02 Oct 2007 23:23:51 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Spark Gap Sustaining Current (fwd)

Hi Christopher,

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sat, 29 Sep 2007 16:28:45 -0500
> From: Crispy <crispy@xxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Spark Gap Sustaining Current (fwd)
> 
> Thanks a lot, that helps me greatly.  Perhaps I should elaborate
> somewhat on my end goal with all of this.
> I'm trying to design and make a type of disruptive discharge Tesla coil
> that should be able to create very long sparks with minimal power input.
> Classic TCs have a single point of disruptive discharge with the primary
> spark gap being the switching device.  The large peak power in this
> discharge is what causes the large sparks as compared to a CW coil.
> However, with the same basic primary circuit, spark length can be
> further increased by increasing the bang rate, such as with an ARSG.  My
> understanding of this is that the ionized trails off the topload have
> less time to dissipate.  The problem is that to have many disruptive
> discharges in rapid succession, the overall power draw increases
> dramatically.

And, for a given bang size, overall spark length no longer increases 
after you reach several hundred breaks per second... the discharges may 
get hotter and more "frantic", but they get no longer.

> When I was considering a way to fix this, I first started thinking about
> DC Tesla coils.  The first idea I had (which has apparently been tried
> once before) was to use a variation of an ARSG instead of the normal
> ARSG with a large charging inductor.  This ARSG would essentially act
> like a SPDT relay (instead of the SPST relay that normal spark gaps
> operate with) with an oscillator connected to the coil.  It can be
> connected in such a way that the charging supply is physically
> disconnected from the primary circuit when the primary circuit starts
> resonating.  Because of this, such a large charging inductor (with a
> value in the Henries) is unnecessary, because the charging circuit will
> never be shorted out.  A charging inductor of a much lower value can
> still be used to limit charging current and to double the charging
> voltage.

This should work. However, you'll want to use a suitably robust HV 
"dequeing" diode in series with the charging inductor. Also, the ARSG 
(during the "charging" sequence) should have sufficiently long "dwell 
time" so as to charge the tank cap to full voltage. This should be a 
MINIMUM of 1/2 cycle of the effective resonant frequency of the charging 
circuit when the gap is operated at maximum break rate. The charging 
circuit resonant frequency will be the series combination of the tank 
cap, DC storage cap, and charging inductor. This mode of operation 
allows the tank cap to sinusoidally swing up to full voltage (approx. 2X 
Vsupply). Once the tank cap is fully charged, the dequeing diode 
prevents any further current flow. After this point, the tank capacitor 
charging current is zero, and you really shouldn't have to break any 
"sustaining" current. If your gap has insufficient dwell time, you'll 
waste some energy arcing in the charging gap - the relatively large 
charging inductor will make trying to prematurely open the charging 
circuit an exercise in futility.

> After I planned out a coil of this type, I started working on the power
> supply for the coil (which is now finished).  It consists of a 12/30
> NST, relevant filter circuit, a hv bridge rectifier made out of a total
> of 120 1N4007 diodes, and a smoothing capacitor made with 50 450V 22uF
> electrolytic capacitors in series.  

Sounds OK. The higher equivalent series resistance (ESR) of the 
electrolytics may make the overall circuit a bit lossier during the 
charging cycle. Also, watch that you aren't significantly exceeding the 
ripple current rating on the electrolytics. It's possible to have many 
amperes of charging current in a resonant charging system when using a 
lower value of charging inductance. Your dequeing diode must also be 
capable of handling the charging current and standoff voltage.

> Relevant equalizing resistors and
> protection diodes were added across the components.  To test this setup
> for durability and functionality, I connected it up with a simple spark
> gap in parallel with the smoothing capacitor.  As expected, the gap
> periodically fired as the capacitor charged to the gap's breakdown
> voltage.  Interestingly, the capacitors all appear to be completely
> unharmed after a number of repetitions.  

The acid test will be how well the electrolytics handle the ripple 
current at the desired break rate...

> While watching these
> discharges, I came up with an idea - what if this same "smoothing"
> capacitor was discharged in impulses into the whole primary circuit?

Directly firing the electrolytic bank into the primary would not be a 
good idea. Electrolytics do NOT like the voltage reversals that occur 
when "ringing" with a primary.

There was a very clever circuit discussed a few years ago. A 
comparatively large DC storage cap was used with spark gap version of an 
H-bridge circuit. The tank cap and primary winding were connected in 
series. Pairs of spark gaps then connected the tank cap (and primary) in 
one direction across the DC storage cap, and then with the opposite 
polarity during the next gap firing... etc.

Each time the a pair of gaps "fired" (pair A on one cycle, pair B the 
next), it would disruptively reverse the charge on the tank cap, through 
the primary. As with a DC resonant charging system, the tank cap would 
"see" a voltage swing of 2 times Vsupply, and you get a bang each time 
the gaps fired instead of the DC resonant circuit above which will 
charge in one position and discharge in the other. Use a 
non-proportional font (such as Courier) for the following ASCII-art 
schematic:


    + --------------o------o------------------------
                    |      |                       |
                    |    A X     Cp     Lp       B X
      HV            |      |    | |                |
    DC In         -----    o----| |--OOOOOOOO------o
                  -----    |    | |                |
                    |      |                       |
                    |    B X                     A X
                    |      |                       |
    - --------------o------o------------------------


> This way, the ARSG could be run at very high speeds to take the most
> advantage of remaining ionized trails in the air, and the bang size
> would be relatively constant over the short period of time that the
> primary spark gap was firing.  The repetition rate for sparks visible to
> the human eye would be slower than the normal ARSG speed or the 60Hz
> main line feed in classic TCs, but the spark length should be
> substantially longer.  In my setup (about half done so far), the
> repetition rate should be very roughly around 5Hz by my calculations.

I'm not sure I understand what you are proposing. However, you'll get 
longest streamers when using break rates of 100 Hz or more. At lower 
break rates, previous spark channels cool down too much to permit 
bang-to-bang streamer growth. This causes each bang to become pretty 
much an isolated event, accompanied by individual, but short, sparks.

> What I am currently considering is what method I should use for
> discharging the "smoothing" capacitor (I put it in quotes because it no
> longer really serves that purpose).  I've built a high voltage relay
> based on 2 solenoids and tungsten contacts, but I was wondering if a
> simple non-quenched static gap would function just as well.  The key
> would be to keep the arc sustained between ARSG charging cycles -
> otherwise the entire purpose is lost, and that is what my original
> question was about.

Hmm... I'm having difficulty understanding exactly what you are 
proposing. Can you generate a schematic?

> I'm working on implementing this design right now, but it's going a bit
> slowly since this is my first year at college and couldn't bring many of
> my materials and testing systems with me.  Also, an aggravating aspect
> is that the rules are so strict here that I can't even use a hacksaw
> without jumping through a number of hoops.  I will of course let
> everyone know how it goes.
> Are there any thoughts on this design?
> 
> Christopher Breneman
> 

Bert
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