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Re: Please Help me design primary



Tesla List wrote:
> 
> Original Poster: Brent L Caldwell <stretchmonster-at-juno-dot-com>
> 
> All coilers:
> 
>         My unfinished TC needs a new primary.  The old one was an
inverted cone
> of copper tubing.  But I think I need more turns and the cone is almost
> half way up the secondary already.
> 
>         Here are my specs, first:
> 
> Secondary:
>         4" diameter pvc pipe.  From 1st wrap to final wrap is about 24".
Used
> .01" diameter magnet wire (whatever gauge that is, I forgot).
> 
> And here's my plan for new primary:
>         A 6.5" diameter drum will have about 6 or 7 wraps with 22 gauge
> insulated stranded wire.  Then, the rest will be a flat spiral of copper
> tubing, with 7 more turns.

The 6.5" is probably too small a diameter if it goes some distance up
the secondary coil.  If it is inverted (turns on the unwound part of the
secondary) that may work.  22 AWG stranded is too small for the primary
coil (tinned wire is another strike against you, as is the high loss
insulation).  You ought to be able to scrounge some 12 AWG house wire
from some construction site and strip the insulation off . . .

If you do go with something like that . . . use the tubing for the major
part of the inductor and use the 22 AWG for the movable tap part.  (I'd
do almost anything to avoid the 22 gauge altogether)
> 
> Will the 6.5" diameter be too tight?  I think so, but I'm not sure.  I
> need it because I don't have that much copper tubing and I need some way
> to take up the extra length of primary.  Also, I don't have much money to
> spend on this.  And how much spacing between turns on both parts?
> 
> Here's the rest of my coil:
> 
> X-former:  15kV 60 mA.
> Capacitor:  .009 uF Cap.
> Spark gap:  two nails 1/4" apart.

Seriously reconsider the nails.  (unless they are huge copper ones)  You
want almost flat surfaces, not points, and with a broad surface area. 
The gap has to be adjustable (but not necessarily while the coil is
running).  Iron and steel spark gaps don't work. Use nonferrous metals,
copper, aluminum, brass, zinc, brass, bronze, tungsten, etc..  

There has to be somewhere for the heat to go, so you have to provide a
heatsink or cooling air.  

Check out copper plumbing supplies.  You want something that has rounded
corners.  The sweat-on copper caps for rigid copper tubing make nice
gaps.  Solder a short length of tubing into the cap to act as a heatsink
(3-4" is enough) - Low cost and very effective.  You can easily make a
stand-off insulator for the copper stub gap, by drilling a hole through
the side of a piece of PVC pipe to hold the copper stubs.

There are several good spark gap designs on the web using short lengths
of parallel copper cylinders to form a multiple series gap.

>                 ( I have a torn apart hairdryer, and I'm thinking about
making it into
>                   some kind of rotary gap, but I imagine there's got to
be something
>                    wrong with that. )

The hair dryer should be almost useless for a TC.  You could use it for
a ballast for a transformer but with an NST you don't need or want a
ballast . . .

The tiny blower and motor could cool a spark gap.  They usually tap part
of the heating coil to provide low voltage for the motor via a small
silicon diode.  The spark gap doesn't need heat, so you would have to
add a low voltage DC supply to work the blower.

Turning a hair dryer into a rotary is tantamount to making a silk purse
from a sow's ear; it could be done, but the challenge of doing it, would
have to be more important than a good tesla coil.

A fixed gap, or multiple fixed gaps, are the inexpensive choices.  

Regards,
bob