Re: Capacitor Purchase

> >From Esondrmn-at-aol-dot-comThu Jun 20 22:15:45 1996
> Date: Thu, 20 Jun 1996 12:23:55 -0400
> From: Esondrmn-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: Re: Capacitor Purchase
> In a message dated 96-06-20 00:33:03 EDT, you write:
> [snip]
> >Now, all I need to do is clean up the oil mess and hook up the new
> capacitor.
> > I think I will lower it down in the coil mounting cabinet.  The cap that
> >blew up was mounted only about 18 or 20" below the primary - maybe this had
> >something to do with its demise??  I had raised it in order to shorten all
> >the primary interconnects.
> I measured this again this morning.  The distance from the lowest (center)
> primary winding to the center of the capacitor is 10.5".  The primary is
> wound at 30 degrees, so the remaining turns rapidly increase in distance from
> the horizontal plane.  I would like to mount the new capacitor in the same
> place (which is on top of the rotary gap enclosure) because it allows me to
> have the end of the cap where the tap lead attaches located directly under
> the center of the coil.  This makes the tap lead as short as possible and
> reaches all sides of the primary equally.  I really need some advice here.
>  Is this too close to the primary?  I have seen the field generated by the
> primary when I had the two coils too closley coupled.  It is shaped like a
> Christmas tree, fully engulfing the secondary.  Is there much field generated
> below the primary?  At 10.5", would the capacitor be in this field?  Would it
> damage the cap?
> >
> >Has anyone else ever ran their coil with the cap mounted this close or
> closer
> >to the under side of the primary?  At 5 to 9 kva?
> >
> >Ed Sonderman


I think there are really a couple of different fields at work: 
electrostatic and electromagnetic. The electrostatic field is created by 
the high potential at the top of the coil relative to the primary and 
strike rail. A visible corona discharge can appear when the secondary is 
heavily driven and the toroid is slightly too high (i.e., not enough 
shielding), or when overcoupling or primary mistuning results in the 1/4 
wave voltage peak occuring at a point somewhat lower than the top of the 
secondary winding. The latter situation is very common on higher power 
systems with large toroids particularly when the coil just barely "breaks 
out".  Additional capacitance of the ion field surrounding the discharge 
terminal, and the additional capacitance of the secondary discharge 
combine to lower the ideal frequency by as much as 15 kHz. If this is not 
compensated for, the primary frequency will be too high and the 1/4 wave 
voltage peak will occur below the top of the secondary winding.  The 
resulting "Christmas Tree" shaped corona discharge is displaying a region 
of electrostatic overstress, and is a usually a precursor to flashovers 
between the strikerail and secondary coil. This problem is more prevalent 
when an inverse conical primary is employed in high power coils: a flat 
spiral primary should help for your next coil design. The GOOD news is 
that ANY electrostatic field from the secondary is effectively shielded 
from the tank capacitor by the primary coil above, and should not be of 

The electromagnetic field consists of expanding/contracting closed loops 
of magnetic flux surrounding the primary coil whenever the gap fires. The 
strength of this oscillating field is offset, to some extent, by an 
opposing field created by current flowing in the secondary coil (opposite 
in direction due to Lenz's Law). However, since a 2-coil system typically 
uses relatively low coupling coefficients (0.10 - 0.17), MOST of the 
primary flux lines do NOT intersect the secondary winding, and 
instead extend above, below, and around the primary coil for quite some 
distance (many feet). This field is not directly observable to the 
operator. However, rough magnitude and shape can be determined via the 
use of a pickup coil while exciting the primary (LOW POWER!) with a 
signal generator as suggested by R.W. Stephens in an earlier post. 

This strong "induction field" induces currents into any nearby conductive 
materials, especially if the conductor forms a closed loop oriented 
parallel to the plane of the primary coil.  A flat conductive sheet 
parallel to the primary plane will develop substantial eddy currents and 
resulting energy loss. Power lost to "induction" heating reduces 
the energy that would otherwise go to into producing longer secondary 
sparks. As long as your tank capacitor is oriented horizontally (its long 
axis parallel to the plane of the primary coil), the internal foil layers 
should be perpendicular to the plane, and should not suffer significantly 
from being immersed in the strong induction field immediately below the 
primary coil. However, it's probably NOT a good idea to mount the 
capacitor VERTICALLY directly beneath the primary coil even though it 
will reduce your primary circuit wiring length. This orientation may 
induce substantial eddy current heating in the internal capacitor buses 
and foil, and may induce premature failure.

I currently run a 6" coil run off banked 15KV neons, and have 
observed the "Christmas Tree" discharge even with a flat primary system. 
I try to keep all metal out of the primary induction field (including the 
tank capacitors) since I think the caps are already taking enough of a 
beating from high pulse currents/rep rates and resonate voltage rise 
without adding potential induction heating stresses. However, it's hard 
to argue with the success that other coilers are having with caps placed 
below the primary. In most of these configurations, however, the 
capacitors are horizontal.

Hope this helps... good luck on your new cap!!

-- Bert --