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TC as pulse transformer
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From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Friday, June 05, 1998 2:34 PM
To: Tesla List
Subject: Re: TC as pulse transformer
> > At some point, particularly as the Ctop gets big, can't the TC be
> > considered as a air core pulse transformer.
> >
>
> A closer model seems to be a dual-resonant air-core transformer because
> of the way energy actually transfers from the primary to the secondary
> over a number of half-cycles of Fo.
Only if k is low. Why not make k=1?
>
> > You charge up Cpri, then discharge it into the primary of a transformer
> > (which happens to have inductance Lpri). The flux of the primary is
> > linked to that of the secondary, so the current in Lpri induces a
> > current in Lsec. That current charges Csec (=Cself + Ctop), which then
> > breaks down the air dissipating the energy.
>
> Yes, but primary-to-secondary energy transfer is not nearly complete in
> one quarter of a cycle due to the relatively low coupling coefficient in
> a 2-coil system. Even if we stopped the proces at 1/2 cycle (at the
> first point of zero primary current), only a relatively small portion of
> initially available primary energy will have been transferred to the
> secondary.
But, you don't need to have a low k. That is just an artifact of how
TC's have been built in the past, probably due to quenching
requirements, etc.
>
> >
> > Assume for a moment that your spark gap can act as an ideal switch. You
> > close the switch, and at the perfect moment (i.e. when the capacitor
> > voltage is zero, and the inductor current is maximized) you open the
> > switch. All that energy has to go somewhere, and the somewhere is the
> > secondary. Isec = Ipri * sqrt(Lpri/Lsec). (It is, of course, quite a
> > trick to open a sparkgap switch when the current is at a maximum).
>
> It's not clear that this would be the perfect moment, since only a small
> portion of the primary circuit's energy will have been transferred to
> the secondary at this instant. If we were somehow able to open the
> switch, we'd develop a huge primary voltage spike from the Ldi/dt from
> the rapidly collapsing primary field.
But the di/dt is exactly what you want, that is what produces the high
voltage on the secondary. You want that energy to couple into the
secondary.
Unfortunately only a relatively
> small (10-25%) portion of these flux lines engage the secondary - the
> remaining flux lines collapse _only_ back onto the primary, resulting in
> most of the original energy being dissipated as heat, light, EMC, and
> "stranded" capacitor charge in the primary circuit.
If only 10-25% of the lines engage the secondary, then the k is low (.10
to .25??). Why not build a coil with a k of 1, which is an impulse
transformer.
>Practically speaking, [barring a single-shot explosive-type opening switch] it's not
> possible to open the primary circuit at a current maximum with any
> switching devices typically available to coilers, including
> semiconductor or gas tube devices.
Well, of course, that IS the problem....
>
>
> >
> > Returning to making big sparks. We know from laboratory research that
> > making a big spark requires a slow rise time voltage pulse (many, many
> > microseconds, if not milliseconds). We also know that we want to get
> > energy from our storage reservoir (the primary cap) into the
> > transformer, and then open the switch. This is starting to look like the
> > desired switch is something that is unidirectional, reasonably fast and
> > low loss. Perhaps a thyratron or an SCR?
> >
> > We want a slow rise time (necessary for developing a big spark), which
> > means low resonant frequency in the secondary, and high inductances for
> > both the primary and secondary.
> >
> > This is what Terry and Dave have arrived at, although by another route.
> > They advocate high inductances, small C, and high primary voltages (to
> > get the energy up).
>
> I disagree with the part about a unidirectional switch. With a loosely
> couple system, the main switch needs to be _bidirectional_ so that
> energy can transfer to completion over several half-cycles, and the
> switch must then be capable of being turned off once all the energy has
> been transferred to the secondary system. This will "force" a perfect
> quench, independent of variations in spark-loading. Heavy toploading, a
> lower Fo, and relatively high (>=400 BPS) rep-rates will make big
> sparks.
However, this would seem to be more in the fashion of generating
> repetitively arrested streamers, each one following part of a weakenned
> trail blazed by its predessor, with far ends "blindly" searching for
> ground.
It is unlikely that a streamer can repeat the path of a previous one.
The ion recombination time in air is very fast (on the order of
microseconds). Also, the spark leaders in a typical tesla coil are small
enough in diameter that they cool almost instantaneously after the
current is removed. More likely, the apparent behavior is due to the
sparks tending to follow Efield irregularities, and in a statistical
sense, they tend to always go in the same place.
In the case of lightning, you do see current flowing through the channel
in between strokes,and the strokes making up a flash are essentially in
the same place. However, lightning dissipates something in the area of
100 kJ/meter, which makes a fairly large column of heated air, which
tends to cool more slowly than the very thin sparks of a tesla coil. It
is the classic surface area/volume thing.
In some exploding wire experiments, I (and others) have created multiple
sparks in the same ionized channel, but my energies were in the 10's of
kJ/meter range, and my "strokes" (to use lightning terminology) were
separated by a millisecond or so.