Re: Tesla coil grounding and other questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: JOSEPH CACCIATORE <jocatch-at-us.ibm-dot-com> 


Thanks, Doc!

After reading all these responses and seeing pictures on the net of coils 
producing 3 or 4' sparks I decided to keep the old coil in retirement and 
build a new layout from scratch using the ideas you, Brett and others have 
given me.

I found one or two site that give basic guidelines for designing a TC so I 
think I will pick a secondary size, like 4" x 20" PVC pipe and figure out 
the primary turns from there. Sounds like what ever else, the capacitor 
will always be .0063 uf with the transformer I have.

Of course I am not sure how good the NST is. I used it years ago to fire 
the original TC and it makes a nice Jacob's ladder but I am not should if 
it is damaged and running at reduced voltage. I say that because I measured 
the DC resistance of the secondary with respect to the center tap (NST 
case) and on one side I measured 30Kohms and the other side 20kohms. 
Doesn't sound right to me!

I assume I can connect the bottom of the secondary to the NST case only and 
not use any other ground.

Regarding the toroid on top, I saw a picture where someone use 3" stove 
exhaust pipe. They connected 4 right angle pcs to form a rectangle of 
sorts. It is solid and holds up well but I would guess not a good idea 
because of the sharp edges where the parts slide into each other. For any 
toroid using aluminum hose, what do people use for the center section?

JC





Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>


The original Big TC was a very poor design, but it did work.

Best to use a flat Archemedian (expanding) spiral.  The original design
produced too tight coupling and made no provisions for pri tuning except
caps.  Use 25-30 turns of scrap wire held in place with cardboard with slits
cut in it for initial test.  Find best pri point to match your two parallel
caps.  Later replace with 1/4 inch dia copper tubing (refrigeration tubing).

If you have access to a good multimeter you can measure your cap, pri
inductance, and secondary inductance and then use John Couture's excellent
Tesla coil computer design program to get all your parameters close.

A large topload like a 14 inch sphere or a 14 x5 inch toroid will give you
much better sec spark length.

Do not connect the base of sec to the HV side of the NST.  You can use the
standard house ground on small coils like this, but also add a 20 ft long
piece of #10 AWG direct from bottom of sec to a reference ground like a
conduit box or waterpipe.  For small coils this will work well.

Use good line filters on the pri side of the NST and a "Terry filter" on the
HV side.

Sparkgap --- don't use sharp needle points like the article shows.  Carriage
bolt heads work ok (you need a large surface area to dissipate the high
heat) --- you should use two sparkgaps in series --- keep total gap under
0.240 inches --- tighter for initial tuning.  Also keep variac down to 30%
until you find initial tune point.

With the above listed modifications and a good topload you should see 12-16
inch long sparks with your NST.

Sparkgap parallel to HV input and put your two paralleled caps in series
with the pri --- don't connect the caps across the xmfr as shown --- xmfr
will blow.  Use a good filter to prevent transients from damaging your xmfr.

K soft copper tubing comes in a roll and you can carefully form it in a
circle as you work around using both hands to expand to tighten the spiral
as required.  Cardboard or wood with slots will hold the tubing.  Be sure to
use some 12 AWG for a scrap wire pri first to find tune point.

Dr. Resonance

Resonance Research Corporation
E11870 Shadylane Rd.
Baraboo   WI   53913
 >
 > Hello. I am trying to resurrect my old tesla coil I made in high-school 30
 > years ago with plans that came from the July 1964 Popular Electronics
 > magazine. I still have the secondary coil and 12kv, 30ma NST which I want
 > to use. The secondary was built pretty close to the article, 4.75"
 > diameter, 34.625" long using #26 (or was it #28) wire. The NST is center
 > tap to the case.
 >
 > First question is grounding. The 120 vac for the NST of course is not
 > grounded (only 2 leads to the outlet). The design I have shows the bottom
 > of the secondary connected to one side of the primary tesla coil in an
 > auto-transformer arrangement. That is how the original design worked and I
 > was able to get 2 or 3" sparks from it.
 >
 > But reading on the net I see some places which say ground the bottom of
the
 > secondary to the case of the NST only and some show the bottom going to
 > earth ground only. Which is correct? If I want to run it in a house or
 > display like at school, there won't be an earth ground to begin with.
 >
 > Also, if you have seen the PE issue, they had the spark gap in series with
 > the primary but all circuits I see on the net today shows the capacitor in
 > series. Is that better?
 >
 > For the capacitors, I am using thick window glass and aluminum foil, each
 > glass is about 18"x18". I am using 2 of them.
 >
 > Lastly, since I have about 75' of 15kv wire I want to make the primary
 > using the wire in a helical coil arrangement (I don't have copper tubing
 > nor anyway to bent it nor a mount for it). I assume I will need all the
 > wire since my basic calculations show I would need 52 turns for the
 > primary. Since 52 would require more wire than I got, could I go with a
 > harmonic and use 26 turns for the primary?
 >
 > Thanks for any input you guys may have. I got this beautiful secondary
coil
 > I made and NST and I really would like to make it work.
 >
 > JC
 >
 >
 >