[TCML] first coil help
Thomas.Ryckmans at pfizer.com
Thu Mar 20 08:43:04 MST 2008
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 attachement, 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 -
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):
Other references state that the RF ground should be distinct from the
mains ground, to avoid frying electronic equipment etc.
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?
I am using a 6kV 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.
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 pathetich 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.
Many thanks for your comments!
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