Re: tesla coil current

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: John <fireba8104-at-yahoo-dot-com> 

Hi Bert,
It makes sense. I believe it was Tesla that first noticed the high current 
produced by some of his coils. Plus taking into account resonant voltage 
rise with a ,somewhat, stable resistance the current has to increase, well 
in most cases.The only thing I don't get is why current is left out the TC 
explanations, It should be included since it does affect performance as 
with resistance. After reading Bert's post does anyone want to try out the 
skin affect? I'm not.
Cheers,
John
P.S I saw the article "Making Small Change Smaller" in pop sci.Cool.


Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: Bert Hickman

John,

The situation is actually quite a bit more complex. Even though the
"average" secondary current may be fairly low, the peak current that's
available in a moderately sized disruptive system can be in the range of an
ampere to tens of amperes depending on the circumstances.

First, there are the current pulses which transfer charge from the toroid
into the leader channel and into growing streamers. These pulses are an
essential consequence of streamer growth, and although they are of fairly
short duration (10's of nanoseconds), they have peak currents of 3-10
amperes in a medium sized coil with a moderately large toroid. And, once a
conductive leader has formed, there is a reactive displacement current
which flows from the toroid into (and out of) the leader's self-capacitance
at the operating frequency of the coil. This displacement current may be an
ampere or two for a disruptive coil operating with a peak output voltage of
250 - 400 kV.

However, the current peaks that occur during a power arc to ground can be
much higher. You've undoubtedly noticed that power arcs which connect
between the topload and a solid ground are considerably brighter and
noisier than free air streamers. Once the secondary rings up to peak
voltage, most of the energy that originated in the primary tank cap (bang
size = 0.5*Cp*(V^2)) is transferred to the topload and secondary
self-capacitance. In an efficiently designed system, the peak secondary
energy can be more than 85% of the initial primary bang size. If the
topload is then suddenly discharged to ground, virtually all of the
secondary's stored energy may be dissipated within a high current spark in
only a few hundred nanoseconds.

For example, let's take a coil that delivers a peak output voltage of 350
kV into an "effective" secondary/toroid capacitance of 40 pF. I've measured
actual discharge times of power arcs to ground of ~500 nSec or less for a
10" coil - let's use a 500 nSec discharge time for this example. At peak
voltage, the charge in the secondary/toroid system will be Q = CV or 14
microcoulombs, and the overall secondary electrostatic energy will be ~2.5
joules. When this charge is removed by a power arc to ground that lasts
~500 nSec, the resulting "average peak" current will be dQ/dt or about 28
amperes. Since the waveform is actually a high frequency oscillation with a
damped exponential envelope, the peak current is considerably higher.

Although YMMV, you can see how the peak currents available from even a
moderately sized system can be much greater than 10's of milliamperes. You
can also begin to see why you don't want to be in the path ! of a power
arc... and why there are significant risks associated with doing "sparks to
the fingers" tricks. :^)

Best regards,

-- Bert --
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Tesla list wrote:

 >Original poster: John
 >Hi Paul,
 >So a good guess for my ,almost, 3 inch coil would be at least 500uA
 >Cheers,
 >John
 >Tesla list wrote:
 >Original poster: "Paul Marshall"
 >This is going to sound very ambiguous but TESLA coils are totally
 >different animals. They seem to have a minimum required current and a max
 >limit as far a! s what a particular diameter coil will process. For instance
 >my 4" coil would take a maximum of 6 kW. Anything above that and I arced
 >out or flashed over. Below 800 w it was a poor performer. Coupling also
 >seems to be a big part of the equation. I have found also that a coil with
 >an aspect ratio of 3.5 to 1 seems to be the most effiiciant. My 4" coil
 >would step up the primary voltage about 22.5 times (Determined through
 >experimentation). I figure I was getting about 450,000v off the secondary
 >at 250kHz. I got 60" streamers at 6kW. If my coupling was tight I would
 >guess with the losses I probably was getting close to 10 mA.
 >
 >Paul S. Marshall
 >
 >
 > >From: "Tesla list"
 > >To: tesla-at-pupman-dot-com
 >Subject: tesla coil current
 >Date: Tue, 04 Nov 2003 08:07:33 -0700
 >Original poster: John
 >
 >Hello all,
 >I was wondering if any one could give a reasonable appro! ximation on how
 >much current a 3" medium 2kw TC can deliver.
 >Cheers,
 >John
 >
 >.