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CAP DESIGN
* Original msg to: Kukkonen-at-snakemail.hut.fi
KU> btw: I just acquired a 220V 6A variac..
That should be enough (proposed 10kV 80mA or .8kVA power supply
matched to a 1.3kVA variac) if you use power factor correction
across the primary of the step up xfmr.
KU> On Sat, 24 Dec 1994, Richard Quick wrote:
Talking about a primary coil design:
> The plastic wedges I was describing earlier give the angle of
> the slope. The slope is linear from the inside turn to the
> outside turn. Each turn of the primary is raised by the same
> amount.
KU> Oh, it is a straight cone.. Yes, that clears things a great
KU> bit..
Yes, a straight cone. Inverse conical section, but I heard the
proper english name (derived from the original Archimedes) but
can't recall what is was.....
KU> The angle of 30 degrees is ok with a toroid of 50cm diameter
KU> with the assembly you have described?
Yes, but don't think you are limited in any way by this design.
It is quite flexible. Suppose you built the cone primary with a
30 degree slope as specified, but decided to use a smaller toroid
for the initial power up (a common practice). The coil would over
couple rather badly with a small discharger, but this can be
easily resolved by rasing the secondary coil up in relation to
the primary coil (for example placing some plastic blocks under
the secondary to lift it up, thereby loosening the coupling).
But if the primary coil were built to couple loosely (like a flat
"pancake" type coil) and you wished to add a larger toroid to
peak the coil system out, you would have to design and build an
entirely new primary coil to take advantage of the increased
throughput that the larger toroid allows. Better build it a
little snug, than a little loose. A word to the wise though,
don't get much over 30 degrees, stay between 20-30 degrees.
One of my best primaries for 4" and 6" coils was measured with a
slope of 27 degrees.
Talking about bypassing capacitors:
> Glass, because of it's high RF dissapation factor (high loss)
> is ideal for construction of bypassing capacitors to be placed
> across the HV feed line to the Tesla Tank circuit.
KU> You used four 3nF 30kV doorknobs in series - will about the
KU> same figures meet my requirements - aka 120kV 750pF ?
That would be about right. Remember that the rating is in DC
voltage, I came up with .0007 microFarads (uF) at 120kV.
KU> If I shall not use a center-tapped x-former - I'll just
KU> connect the bypass-cap in parallel with the secondary,
KU> right?
That is correct. And If you WERE using a center tapped x-former
then you would divide the capacitance into two parts (use less
that 50% of the total) and place an equal capacitance between
each HV line and the system (RF) ground. I have cautioned this
before, and I will caution it again: when using grounded center
tap secondaries do not allow the capacitance between any one
winding and the ground exceed .001 uf. (very upper limit!) Too
large a capacitance across the ground will allow a parasitic
oscillation of great power to set up between the winding of the
step up transformer and the ground. I learned this one the hard
way. The oscillating ground current melted large holes in 1"
(2.5 cm) heavy braided ground strap.
KU> And the same applys for the safety-spark-gap as well?
Good question! It will work either way... But, I always use a
grounded center post between the electrodes of the safety gap.
The grounded center post greatly reduces the potential of the HV
arc when the saftey gap is active; this in turn puts less strain
on the HV step-up xfmr.
KU> About the rf-chokes to be placed to the primary circuit to
KU> protect the x-former: I've located a source for big
KU> 9,3cm*10,3cm "flatten" toroid-ferrites that have cross
KU> sectional area of 3cm*2,85cm.. Will these do - they are
KU> assembled of two U-shaped pieces..
If I am converting these numbers correctly these toroid-ferrites
are fine. It is unfortunate that all of my coiling relationships
are calibrated in my head in USA measurements. In this respect I
really feel "handicapped"!
KU> Or could I just use a couple of ferrite-rods together? aka
KU> many rods together with the coil wound around the "pile"?
Yeah, you could do that. I don't have any hard data to support my
preference for the iron power toroid RF chokes for this work. I
can tell you that if the choking inductance is affecting your
coil you will notice it in the quality of the tune. If the coil
tunes nice and "sharp" then your chokes are fine (as long as they
are doing their job as well). If the coil tune seems "soft" or
"mushy" then the RF chokes must be rewound or replaced. Soft or
mushy tuning is usually a problem with incorrect RF choking or
poor RF grounding of the secondary base wire.
KU> I did locate a possible source for RFI-filters - they have
KU> two models - for 250V 3A and 6A - of which the 6A model
KU> would do if one just calculates the power needed for the
KU> x-former - but should I use more of them is series (and/or
KU> parallel) to get more marginal? (will they get fried?)
I allow for "surge" currents. The large inductance of the variac
can draw some additional amps, as well as the power needed to
energized the core of the step up. Once these inductive delays
get worked out, things tend to settle down. Run two of the 6A RFI
filters in parallel to allow for the surge demand on start-up.
Talking about my air-blast spark gaps.
> I did make a modification on this gap later that cut down on
> the air required yet maintained the un-surpassed performance.
> I drilled a hole through the face of both electrodes, and
> plumbed in the air from the back. This method injected two
> opposing air streams into each other right in the center of
> the gap. The quenching was still excellent, but it used only
> 25% of the air previously required.
KU> What were the dimensions of the drilled holes? I presume
KU> that the holes were parallel with the longitudinal axis of
KU> the electrodes and in the middle of the faces..
The holes were about 3mm each (1/8 inch). They were in the center
of the face on each electrode, parallel with the longitudinal
axis, and the holes were exactly facing each other.
KU> You mentioned 25 psi in your article - will this do or shall
KU> one need more pressure?
25 psi is fine for power levels of 1 - 2 kVA in this gap.
> Thank you, I hope I have been of help!
KU> Yes, your help is extremely useful -
Thank you again!
KU> pps. I just finished making a compilation of the articles
KU> from fidonet electronics that I had.. I appended all the
KU> files into one huge file and then removed the duplicates..
KU> Now all your articles are in one nice 600k file that is a
KU> lot more handy than the separate files..
KU> The file is at nic.funet.fi : /pub/sci/electrical/tesla
You did all of that! I am impressed! I hope you weren't bored
stiff with all of that editing (I must write too much!).
KU> If you wish to get a copy and can't use ftp - I'll mail you
KU> the file, just ask for it.
No, I cannot use ftp. But I would like it very much if you would
take time and mail me the file! Thank you for offering.
Richard Quick
... If all else fails... Throw another megavolt across it!
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