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Re: Peak Primary Current

Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote:
Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx> 

I have used many different wire combinations for my primary wiring
thinking that the primary wiring was a major resistance in the circuit.
Interestingly, no matter what I do with the primary wiring, I see no
change in the performance of my coil.  I have tried the following for
the primary wiring #18 ga, #14 ga, (2) #14 ga, #14 ga 20 KV, (2) #14 ga
20 KV, #12 ga, #8 ga, and 1/4" tubing(a real pain to set up).


I am always saying this. Another of the myths about Tesla coils is
that you need heavy wiring for the primary circuit. In most cases
several Ohms of resistance don´t make any significant effect.

I have run (2) coils with my primary and power suppy(Note I am using DC
resonant charging so I have one extra degree of freedom than you AC guys
and can pick my tank capacitor independent of the power supply).
My small coil resonates and 383 kHz and runs great with an 18 nF
capacitor(18 to 24" streamers).
My medium size coil resonates at 100 kHz and runs with a 34 nF
capacitor(48 to 60" streamers).
In both cases, I charge the capacitor to 20 kV.
It takes 1/2 cycle to empty the capacitor - 1.3 uSec for the small coil
and 5 uSec for the medium coil.
Using I = Cdv/dt, I get 277 amps for the small coil and 136 amps for the
large coil(Have I made a math error?).


If the voltage is v(t)=A cos wt, the maximum dv/dt is Aw. For the 383 KHz coil:
I = 18e-9*20e3*2*pi*383e3 = 866 A
For the 100 KHz coil:
I = 34e-9*20e3*2*pi*100e3 = 427 A

The actual values are a bit different because of the double resonance and losses, but very close to these.

These values seem absurd for my wire sizes.  How can I be moving this
level of current?  Also, note Peak current is dropping as the coil size
increases because the frequency dropped faster than the capacitor
increased in size.


The currents are high, but flow only for tens of microseconds. Not
anough time to even heat the wires significantly.

I think you're going to tell me that the spark gap creates such a large
loss that the capacitor is significantly discharged during the first
cycle and does not recover until the next re-charge period.


No, but the spark gap produces practically all the primary loss,
unless you use really thin wire.

I tried to measure primary current with an isolated CT and got 88 amps.
This is obviously not accurate because resonance only occurs for about
40 uSec(383kHz coil) until we reach the first notch and quench.  I'm at
460 BPS so charging occurs for 2200 uSec.  During charging available
current is absolute max about 200 to 250 mA(documented by
studying/measuring power supply capability and typically only ~75
mA(~1.5kW).

You need an oscilloscope to see the current waveform, or at least a
(fast) peak detector following the current transformer to measure the peak current.

Antonio Carlos M. de Queiroz