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Re: Polyethylene Capacitors



Tesla List wrote:
> 
> >From jim.fosse-at-bdt-dot-comWed Sep 25 20:56:02 1996
> Date: Wed, 25 Sep 1996 06:11:11 GMT
> From: Jim Fosse <jim.fosse-at-bdt-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Polyethylene Capacitors
> 
> >From: Skip Greiner <sgreiner-at-mail.wwnet-dot-com>
> >To: tesla-at-pupman-dot-com
> >Subject: Re: Polyethylene Capacitors
> >
> >
> >Richard and all
> >
> >I have a very basic question about placing all of these rolled caps in
> >series. These caps very definitely have inductance. Inductance ADDS when
> >placed in series. Inductance also retards the rate of discharge of the
> >total cap. It seems to me that we are defeating our purposes when we put
> >a bunch of caps in series which have inductance. Wouldn't it make more
> >sense to use flat plate caps with end foil terminations in all cases????
> >
> Skip,
>         Yes, inductances in series add. Now what are you discharging
> this capacitor, with several hundred nanohenries of series inductance
> (I'm guessing here, I've not taken the time to run the numbers for an
> 8' by 1' sheet conductor's inductance; I hope to be guessing high),
> into? A 100uH+- inductor. The primary inductance is the current
> limiting factor here. Even given a shorted secondary, the coupling
> factor (K=~0.2) only decreases this inductance by 20%; say to 80uH.
> 
> You could also view it as 0.1uh/100uh = 0.001% of off axis inductance
> by treating you capacitor as a perfect cap and a 100nH series inductor
> driving a 100uH coil. To a first order approximation: this can be
> viewed/treated as a capacitor driving a 100.1uH inductor.
> 
>         cheers,
> 
>         jim


Jim, Skip

Jim has put it in the proper prospective.  The actual inductance of the 
wires going to the primary from any capacitors exceed, by a full order of 
magnitude, any type of even amateur construction internal capacitor 
inductance!  I have measured no more than about .5uh (500 naonhenries) of 
inductance in any of our crudest rolled caps. three in series would be 
1.5uh. (this assumes really shoddy construction of the caps).  The tank 
circuit wiring is usually on the order of 8-16uh (due to the size of a 
large closed turn).  That's another reason I opt for lotsa' primary 
turns.  Sloppy construction will not impact the coupling all that much or 
waste power off-axis.

Richard Hull, TCBOR