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Re: [TCML] Project Hathor 2000 Volt Test Fire

Assuming that the equivalent resistance of your capacitor string is relatively low, your capacitor bank and HV cap will ring like an LC circuit (at least for the 1st quarter-cycle). If you can estimate the inductance of your work coil, you can do a first-cut estimate of the peak coil current by assuming that almost all of the initial energy (0.5*CV^2) was transferred to your work coil 0.5*LI^2) 1/4 cycle later.

You can estimate your work coil inductance by using the inductance formula for an Archimedes spiral:

 R1 = innermost turn radius
 R2 = Outermost turn radius
Rav = R1 +(R2-R1)/2 = Average Radius
 N  = Number of Turns
 W  = R2-R1 = Coil Width

 L = (Rav^2)*N^2/(8*Rav+11*W)

"Guesstimating" the dimensions of your work coil, the inductance appears to be around 3 microhenries.

Assuming no resistive losses, the estimated peak current would be:

I = V*Sqrt(C/L)  amperes

With a 1560 uF cap bank charged to 2000 volts, the estimated peak current would be about 45 kA(!)

In reality it will be substantially less (perhaps 1/2 to 1/3 of this) due to the equivalent losses from series resistance of your electrolytic caps, wiring, switch, and work coil resistance AND some of the energy from the ringing LC system is converted to kinetic energy in the aluminum disk. The energy transferred to the disk can be estimated by measuring the peak height of the ring and the mass of the ring.

The ringing frequency can be estimated (C in farads and L in henries) :

F = 1/(2*Pi*Sqrt(LC)) in Hz

So, for your system, F is about 2.2 kHz

The period will be 1/F seconds, and the first current peak will occur at 1/4*T seconds, so your current peak occurs about 107 usec after your HV switch closes.

You'd need to use a current transformer and oscilloscope to accurately measure the current.

BTW, at the first current peak, all of the system energy resides in the magnetic field around the work coil. As the coil current then starts decreasing, the capacitor current reverses and the coil attempts to reverse-charge your capacitor bank. This is not a problem with film or paper capacitors, but your electrolytic capacitors are polarized, so they may not always handle being abruptly reversed-charged in a graceful manner. To prevent undesirable "surprises" and to extend capacitor life, many folks install high-current diodes across the capacitors to prevent them from being forced to conduct high reverse currents.

Good luck and play safely,


krux@xxxxxxxxxx wrote:
So brief little update on Hathor, the ring launcher I built.  We did a full
power test fire last night.  And apart from breaking most of the wire ties
holding the coil in place, everything worked nicely.


Main project page is here:


Now of course, that I have this fine scientific insturment, I'm curous of a
good method of meauring how many amps this thing produces when it fires, what
the disharge curve looks like, etc..

So 2000 volts, at 1560 uF, or 3120 Joules.

I'm guessing I can take the energy in Joules, some how measure how long it
takes to fire, which will give me how many watts.  Then it's a simple matter
of Ohms law to figure out my amperage, or at least amperage averaged over the
time it took to fire.

Any ideas, or other data points that would be useful to have here?

perl -e 's==UBER?=+y[:-o]}(;->\n{q-yp-y+k}?print:??;-p#)'

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