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Re: [TCML] first coil help

Welcome, Thomas,
See comments between:
In a message dated 3/20/08 12:13:17 P.M. Eastern Standard Time,  
Thomas.Ryckmans@xxxxxxxxxx writes:


I am building my first Tesla coil, but having issues -  sparks are
pathetic - about 1 cm from topload to a grounded object. Help is  much
I am adding the same document as a Word attachment,  in case the pics
don't make it. <<tesla.doc>> 
First of all  here is a picture of the thing on my kitchen table; scale
is provided by  the yellow yardstick sitting on the primary (21 cm). In
the background  there is a crate of the best beer in the world -

Unfortunately, TCML system deletes all pictures AND attachments before  
distributing as plain text.. 

My first problem is that I find books and Internet references on  "RF
ground" a bit confusing. Several books (e.g. electronic gadgets for  the
evil genius, gadgeteer's goldmine) show a RF ground connected to  the
mains ground as here (evil genius):

Many authors reprint verbatim things that they have no personal  experience 
Other references state that the RF ground should be distinct from  the
mains ground, to avoid frying electronic equipment etc. 
This is the preferred method for avoiding potential trouble for you and  your 

Unfortunately I live in an apartment, so I would find it difficult  to
drive an iron rod into the ground to create my own RF ground...
What  would you recommend?
Use a counterpoise under the coil. This can be a circle of aluminum foil  
with radius equal to the coil height (more eddy currents), wire screen (more  
efficient), or wires laid out "daisy-wheel" fashion (least eddy  currents)

I am using a 6 kV 50 mA NST to power

Here are some  values for the coil; the calculated values are provided  by

Primary is 
Salt water bottle caps,  24 bottles for a total of 10 nF (measured)
Copper tubing (8 mm) with inside  diameter 189 mm, spacing 25 mm
Measured values are 
4 turns 1uH
5  turns 5 uH
6 turns 9 uH
7 turns 14 uH

Secondary is using a 130 mm  (diameter) black tube, 320 mm high, with
(about) 14 turns/cm hence (about)  450 turns.
Most, but not all, black plastics use carbon powder in the pigment,  making 
it conductive. This has three effects:
1) Shorting from the secondary to the form.
2) Arcing down through the form to the ground.
3) The form looks like a large, closed loop absorbing and wasting  energy.

Measured value for the secondary coil is 3.53 mH

Calculated  value for the top load is 7.3 pF and self capacitance of the
coil (calc) 5  pF assuming spherical top load with R=65 mm. In reality
the top load is  cylindrical and I expect its capacitance to be a bit

This  gives me a calculated resonance frequency of 767 kHz.
Using a signal  generator with a LED indicating max current I get a
maximum current at 833  kHz - not too far away from calculated values.
Using a signal generator and  an oscilloscope I get a resonance value at
909 kHz.

With my  primary's capacitor at 10 nF I would need my primary inductance
around 3.5  uH to give a similar resonance frequency of about 850 kHz -
this turns out  to be just between 4 and 5 turns of the primary coil.

Using the signal  generator at 833 kHz and the oscilloscope, I get a
maximum voltage value  for the primary coil+cap in parallel at around 3.5
turns - again, not too  bad.

So far, so good. However when firing the coil, I get pathetic  sparks
(about 1 cm long) between the top load and a (insulated) screwdriver  I
hold above it. 
What am I doing wrong?
I am using this  configuration:

I tried the other configuration (cap and primary in  series, below) I get
the same kind of results... which configuration would  you recommend? The
series one (below) is said to offer more protection to  the NST
apparently - I will add a Terry filter soon anyway.

The gap across the NST has been shown, both theoretically and by body  count 
to increase the life expectancy of NSTs. Your gap spacing should be  limited 
to ~3 mm for a 6 kV supply.

Many thanks for your  comments!


Thomas  Ryckmans

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