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Re: [TCML] Telsa coils + capacitor bank = ?
I 99% sure Bill Wysock and/or Dr. R. did a similar experiment years ago. Perhaps one of those folks will chime in?
Cheers,
Aaron, N7OE
--- On Thu, 9/4/08, Bert Hickman <bert.hickman@xxxxxxxxxx> wrote:
> From: Bert Hickman <bert.hickman@xxxxxxxxxx>
> Subject: Re: [TCML] Telsa coils + capacitor bank = ?
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Date: Thursday, September 4, 2008, 12:39 PM
> Hi Josh,
>
> I'll give it a try... :^)
>
> Josh Bailey wrote:
> > Hi Bert;
> >
> > I recently made friends with Greg Leyh and attended
> the recent Norcal
> > Teslathon - 'twas grand (I made my first tiny coil
> back in July so have
> > had a... rapid introduction to the field).
>
> It looked like quite a fun event! And Greg's lab makes
> me drool... :^)
>
> >
> > While there we worked on using Greg's largest coil
> to strike a graphite
> > rod, connected to ~100uF of capacitors charged to
> ~20kV.
> >
> > I had some naive questions about what we observed, and
> Greg suggested you
> > might have some interpretations? I'm a network
> engineer by trade so please
> > forgive being a newbie - just want to understand what
> is happening.
> >
> > Here are two series' of photos (frames from an
> MJPEG capture running at
> > about 30 fps):
> >
> > http://vandervecken.com/images/capbanktest1/
> > http://vandervecken.com/images/capbanktest2/
> >
> > Q1. What conditions must exist for the capacitor bank
> to discharge? We
> > noticed that sometimes the graphite was struck
> multiple times before it
> > discharged. We also noticed that sometimes the bank
> was not fully
> > discharged (still had 3-4kV on it).
>
> In general, the brightness of a spark is a proportional to
> the amount of
> current passing through it. The base of air discharges from
> a Tesla Coil
> (TC) is the brightest and hottest part since the sum of all
> the
> branching discharge currents pass through the main channel.
> As you move
> away from the terminal, the discharges begin branching,
> with each branch
> carrying a portion of the total current. As the current
> flowing through
> more distant branches becomes progressively lower, the
> channel diameters
> become smaller, and the discharges become more purplish and
> dimmer. The
> tips of the discharges transition to a bluish glow (the
> streamer-corona
> region) where the gas temperature is barely above room
> temperature.
>
> Unconstrained sparks and arcs in air have a negative
> resistance
> characteristic: the more current you force through them,
> the hotter
> their cores become, and the lower their electrical
> resistance. Because
> the more distant channels are cooler and smaller in
> diameter, these
> remote channels are considerably poorer electrical
> conductors.
>
> When a Tesla Coil is operating near the limit of its spark
> length, you
> may see sparks sporadically/weakly connect the TC terminal
> to ground
> without the characteristic brightening that we usually
> associate with TC
> ground strikes. In this case, although the spark has indeed
> bridged the
> gap, the combination of high resistance from the discharge
> path, and the
> fact that we are close to being max'ed out on TC
> terminal voltage,
> prevents any major increases in channel current - even
> though we've
> actually bridged the gap. This will occur for regular coils
> and those
> using a high energy capacitor bank for secondary boost. It
> takes a
> significant increase in channel current to start the
> runaway process
> that culminates in the hot arc that's necessary to
> discharges the
> capacitor bank: the increased current heats the channel,
> reducing the
> channel resistance, which further increases channel
> current, further
> heating the channel, etc. etc.
>
> While a barely connecting spark may not provide a
> sufficient current
> "bump" during the final jump to initiate the
> runaway process, a slightly
> "stronger" spark will. Once a bridging arc has
> been initiated, it will
> then persist, discharging the capacitor bank pretty much
> independently
> of further Tesla Coil operation.
>
> >
> > Q2. What is the sequence of events at discharge?
>
> Assuming that we've bridged the gap, initiated the
> runaway process, and
> formed a bridging arc, the energy in the capacitor bank
> begins flowing
> through the secondary, terminal, and arc, completing a
> series LC circuit
> that oscillates at a low audio frequency (that you can
> easily hear when
> the capacitor discharges). The combination of secondary
> winding
> resistance and arc resistance eventually dissipates most of
> the stored
> energy from the bank.
>
> During each oscillation, the arc current passes through
> zero,
> temporarily extinguishing the arc. Usually it is quickly
> reignited by
> the high voltage from the capacitor bank. As the energy in
> the system
> decreases, so does the arc current, causing the arc
> resistance and
> overall arc voltage drop to rise. The voltage drop across
> the arc is
> also proportional to its length. As the bank energy
> decreases, a point
> is eventually reached where the arc can no longer be
> reignited, and the
> arc goes out for good.
>
> Once the arc finally extinguishes, residual energy will
> still be left in
> the capacitor bank. This is typical behavior for an LC and
> spark gap
> system. Because of the relatively long arc involved in
> Greg's system,
> the stranded bank voltage is proportionally higher. You may
> also see
> significant variation in residual bank voltage if the Tesla
> coil
> continued to operate during the bank discharging process,
> since it would
> assist in reigniting the arc channel.
>
> >
> > Q3. The arc changes colour several times (before and
> after the strike).
> > What's going on there?
>
> There may be several things occurring here. The arc itself
> changes color
> depending on how much current is flowing through it. Low
> current arcs
> tend to be purplish, transitioning to a whiter color and
> then a
> brilliant blue white at increasing current levels. In
> addition, the
> carbon rod that was used as a target may contain metallic
> salts in the
> core and a copper jacket - these may add other colors to
> the arc,
> particularly at the end of the arc nearest the carbon rod.
>
> >
> > Q4. I was thinking about trying to get a better
> photographic record of
> > what's happening - other than longer lenses/faster
> cameras etc - please
> > might you have any suggestions on how to better
> capture what's going on?
>
> Part of the problem is the huge optical dynamic range - you
> may need to
> use 2-3 cameras simultaneously, each with different neutral
> density
> filters in order to capture the entire discharge sequence.
> The main arc
> channel is primarily a black body radiator. However, you
> may be able to
> detect various spectral lines emitted by vaporized
> electrode materials
> and excited gases near the arc core using a slit and a
> prism or
> diffraction grating. Good luck!
>
> >
> > Thanks,
> >
>
> Bert
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