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Re: [TCML] first coil help
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In a message dated 3/20/08 12:13:17 P.M. Eastern Standard Time,
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
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
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!
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