Re: The Geek Group High Voltage Capacitors, making a HV switch, (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Wed, 8 Nov 2006 21:44:26 +0800
From: Peter Terren <pterren@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: The Geek Group High Voltage Capacitors, making a HV  switch,
      (fwd)

There are interesting design considerations with high current pulses.
I have two caps joined with 1 1/2 inch busbar, yet they were making big 
sparks between the cases when they were fired at 5kJ. A few calculations 
show why.

Consider the inductance of 20 cm of busbar compared with 60 cm wire in 3 
turns of a can crushing coil (?300 nH).  Without calculating it I would 
guess at the busbar being 10 % of the inductance of the coil.  So at 6 kV 
across the coil (busbar) would be 600 V at perhaps 80 kA peak current. Short 
that out and you will have plenty of sparks.

Good luck trying to get 20 caps to discharge in 10us. I have wire lengths of 
21, 25 and 30 inches for the full short circuit discharge path.  The 
resonant frequency of my 3 caps with a short circuit is 25kHz ie period of 
45uS. The first rise of the half cycle is 12us but that energy gets stored 
in the wire inductance and so current will lag. With 20 caps your wiring has 
to be longer.
http://tesladownunder.com/CanCrushing.htm#Can%20crusher%204

Peter


> Date: Tue, 7 Nov 2006 16:44:54 EST
> From: Mddeming@xxxxxxx
> To: hvlist@xxxxxxxxxx
> Subject: Re: The Geek Group High Voltage Capacitors, making a HV  switch,
>
> We have been arguing about whether total energy is  a meaningful measure 
> in
> safety considerations for protective  enclosures, since the chemical 
> reaction
> of a typical explosion takes 5-10  ms to go to completion, but the time
> constant for these caps could be a lot  shorter.
> For example: A 200 KJ discharge in one second is 200 kW peak power,  but 
> the
> same discharge in 10 msec is 20 MW and if the discharge were  in 10 usec 
> it
> would be 20 GW peak !!
> There are two sources of destructive capability in a cap bank. One is
> equipment fragmentation, the other is the shock wave from the 
> plasmalyzation of the
> air. It makes the designing for safety without  over-designing 
> interesting.
> In coin shrinkers, of course, the hope is that  most of the energy goes 
> into
> the EMP.
>
> Matt D.