I better make a few corrections to my previous post...
In the paragraph on the charging circuit I said that we need to be able to
charge the capacitor up in 1/4 of a "cycle". We really have most of a half
cycle if we are using a static gap. I was thinking quarter cycle because
that is where the peak voltage of the AC waveform occurs (along with 3/4
cycle). Since we don't have to charge to the peak voltage, only to the
spark gap firing voltage, we have more time.
I also mentioned charging the cap "a couple of times a second". I meant to
say "a couple of time a cycle".
Obviously, this isn't the best job of explaining TC cap charging so you guys
feel free to help out!
-----Ross' 1st reply to Bryan-----
Original Poster: "Ross Overstreet" <ross-o-at-mindspring-dot-com>
I realize that I am probably providing more questions than solutions in this
post, but here are a few things to keep in mind as you begin your journey
into the world of Tesla Coil design.
There are really only component groups in a tesla coil
1) Charging circuit
2) Primary circuit
3) Secondary circuit
The secondary is composed of pvc pipe (or whatever) covered with magnet
wire, the topload (usually a toroid), and an earth ground. Some general
rules of thumb are as follows. Use a height to diameter ratio of between 4
and 5. Professional Telsa Coil builder D.C. Cox (Dr Resonance) recommends
4.5 as optimal. Pick a wire size that gives you about 1000 turns at your
chosen winding length
The secondary has a resonant frequency. You should take the time to
calculate this and, if possible, experimentally verify it with a signal
generator and o-scope. Remember this resonant freq, it will be important in
the construction of the primary
The primary is composed of a few turns of heavy conductor, the primary
capacitor, and a spark gap. Experiments have shown that the easiest primary
to build for the average hobby coil is a flat spiral. The primary can be
cylindrical or even funnel shaped, but you are usually begging for over
coupling problems or difficulties in the setup and tuning with these
configurations. The primary capacitor needs to able to handle the high
voltages and currents that are encountered in a TC primary (more on this is
a minute). The spark gap acts as a switch that dumps the cap through the
primary coil. This can be as simple as 2 rusty nails hammered towards each
other or as fancy as a synchronous rotary gap with a G-10 disk and thoriated
The primary also has a resonant frequency (once the gap fires). The trick
to getting good performance out of a coil is to get the resonant freq of
primary to be very near that of the secondary. You can alter either the
inductance or capacitance of either the primary or secondary to do this, but
the easiest way is to simply adjust the inductance of the primary by moving
a "tap" in or out.
It is a balancing act - all we have to do is get the primary resonant freq
near the secondary resonant freq and then connect a charged capacitor to the
primary coil via the spark gap. This brings up the next point...
Now we need to be able to charge a capacitor. This is a bit tricky since we
want to get the cap to a high voltage (5-15KV for typical hobbyist coil) in
a limited amount of time. That period of time is about one-quarter of a
cycle. Do a search on "resonant charging" at the pupman archive. Typically a
hobbyist will find the best HV power supply that he get his hands on and
then pick a capacitor size based on the voltage & current rating of the
power supply (and the amount of time we mentioned earlier).
Now there are many caveats here, here are a couple that I can come up with
at the moment
- the quarter (or even half) cycle thing is a general guideline, usually
more "breaks" per second is fine. Most "static gap" coils have many
hundreds of breaks per second.
- remember that the capacitor is charged to the voltage where the spark gap
fires, not necessarily the max voltage of the capacitor (assuming static
When you manage to charge a capacitor to a reasonably high voltage at least
a couple of times a second, your spark gap fires, and the resonant
frequencies of your primary and secondary are close, you simply can't go
Now there is about a million little things that I didn't mention here. If
you are new to resonance, read up on 2nd order systems such as RLC circuits
or even spring-mass-damper mechanical systems - it will give you a lot of
Drop by my page and see the "Resources" section. Everything there is a
must-read for anyone new to the hobby. Take care, good luck, and let us
hear about your progress!
Coiling, outside, with a proud grin stretched across my arc-lit face,
-----Original Message from Bryan-----
Original Poster: Football <ftbll-at-MailAndNews-dot-com>
I am just starting to build my first coil. I am having a little
trouble finding some information about everything that has to be equal, such
as in the inductance, frequencies, etc.. Any resources or help would be
greatly appreciated. Thanks.