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Re: Peak Primary Current

Original poster: Mddeming@xxxxxxx

In a message dated 4/15/05 6:45:21 P.M. Eastern Daylight Time, tesla@xxxxxxxxxx writes:
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?).

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.

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.

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

Straighten me out, please.


Hi Mark,

I think the problem is that di/dt is only approximately equal to delta_i/delta_t, for delta_t very small. since you are comparing values with two very different delta_t values, it introduces an error in the approximation. Remember di/dt is not linear, so you have to compare over the same time base and one is four times larger than the other one.(5 vs 1.3)

Hope this helps,

Matt D.