Re: [TCML] Tim's Qs was: can you connect the secondary *directly* to the primary

[Mddeming@xxxxxxx]

Hi Tim,
 
    A complete answer to your questions will require  more information than 
you provided. See comments interspersed below:
    
In a message dated 11/16/07 1:56:03 A.M. Eastern Standard Time,  
btmeehan@xxxxxxxxx writes:

"This is my first tesla coil, I built it without consulting any  sensible
source of information on the web."
 
This is far more common than you'd think.
 
 
"All I had was a healthy respect (fear) of high voltage and a neon sign  
transformer."
 
A little fear and a lot of respect is healthy at this point. What  voltage 
NST? What current rating?
 
 
  "I made a spark gap out of two screws and a u-shaped block of  wood." 
 
NOT GOOD! At ~10 kV and above, wood, with any degree of humidity, is  
CONDUCTIVE!! Several heavy coats of varnish or polyurethane helps, but it's  still 
better to use HDPE or similar plastic. For your gap, use 2 brass bolts  head to 
head, or two round brass lamp finials, or two brass drawer pulls, or  two 
round brass door knobs, etc. You can also use short sections of copper  pipe in 
parallel but your sum of all gaps should not exceed  2kVrms/mm. Also use plenty 
of air flow, vacuum cleaner, fan, pressure hose,  etc. 
 
 
 "The choke coils are wound on a 3.5" long 3/8" diameter slugs of  ferrite 
and they had a 1millihenry reading on the LCR meter at the  office.  I put them 
there to protect the NST, but I
have since found  out from members of this list that there are much better 
ways to do  that."
 
Ditch the choke coils, use a terry filter if you can, but at least use a  
safety gap. See the archives for details.


"The secondary that I built had a 1.5" diameter, 10" tall and has  a first
resonance at about 500kHz.  I used the frequency generator to  drive the base
of the secondary and read the peak from an oscilloscope  probe used as an
antenna."
 
This is smaller diameter than most people build, but it's not bad for a  
first time. Without knowing what size wire and how many turns, it's not  possible 
to analyze.

"The primary is 15 turns in a flat coil with an  inside diameter that is 1"
away from the base of the secondary coil (3.5"  diameter).  The primary
circuit and the NST are not grounded at  all."
 
Again, without knowing the spacing between turns, wire or tube size, or  
outer diameter, there's not enough  to analyze. If this is fastened to a  wooden 
board and/or your using metallic fasteners, you probably have a high  
resistance short circuit or two.



"The capacitor is a 1 uF 20 kV rating that I borrowed from someone at the  
office.  I did let them know that there was a fairly good chance it might  not 
come back alive."  
 
What type - mylar, mica, foil, film, ceramic, etc?? NOTE: Your cap  voltage 
rating should be not less than twice the output of your transformer.  If you 
have a 12 or 15 kV  NST you may have already punctured your  cap.
 
 
"I have a crude
ASCII art drawing, (obviously this only works with  fixed width font):

+---CHOKE---+--||--+
|         |      |          o
|           v      |   |
NST             PRI       SEC
|       ^      |          |
|           |      |   |
+---CHOKE---+------+---------+"
 
Ditch the chokes and add a safety gap. The general layout is OK. You  should 
have a least a Variac with a switch and a fuse for safety and control  as a 
minimum. You can add metering,  safety lockouts, contactors,  etc as resources 
permit, and as your lust for power grows. (And it  WILL!)


"It never sparked, however I have more enthusiasm than smarts for  RF ...
I also had the spark gap about 1/4" apart, and when it ran there were  big
cloudlike angry blue sparks ..."
 
Depending on the size of your NST, that may or may not have been too  wide, 
as mentioned above.
You may indeed have blown the cap and are now feeding all your power into  
the gap.

"I am guessing that I am breaking about 20 good design rules  at this point,
so I sought help before I shocked myself or blew up my  secondary coil."
 
 
You got here, and enlightenment does not care which door you came through  or 
how long your journey was.


"I am open to all criticism of the design, or pointers - I have  found some of
the online resources for design such as JAVATC, etc.   and I'll probably
start working on version two using some of the designs  that I see on other
pages ..."
 
Copying designs, even good ones, without understanding what or why, could  be 
VERY hazardous to your health. Ask many more questions, read more on the  
principles and safety, and make haste very slowly.

Thanks  again,
Tim





Welcome,
 
Matt D.



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