Re: tesla coil current

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

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

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 <fireba8104-at-yahoo-dot-com>
>Hi Paul,
>So a good guess for my ,almost, 3 inch coil would be at least 500uA
>Cheers,
>John
>Tesla list <tesla-at-pupman-dot-com> 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 as 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 approximation on how
>much current a 3" medium 2kw TC can deliver.
>Cheers,
>John
>
>.