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Re: Super simple beginner coils
At 11:25 PM 10/29/98 EST, you wrote:
>SNIP...........
>"Neat idea with the beginners TC. They wouldn´t be real TC´s ..."
>
>As such they would teach NOTHING of the principles of operation of a real
>Tesla coil!
>All automobile ignition coils (as far as I know) have a ratio of
>transformation directly related to the ratio of turns, whereas a Tesla
>coil's ratio of transformation is dependant on the ratio of primary to
>secondary inductance or likewise the ratio of primary to secondary
>capacitance. Your suggested circuit would make a very nice driving supply
>for an actual small Tesla coil though. It really wont at any significant
>cost to the project to make a small primary perhaps out of #10 or #12
>gauge house wire and one could strip the
>wire out of an old audio transformer to make a little secondary, say
>around 1 to 2 inches in diameter and 4 to 8 inches long and of course
>adding a top load like a 2 to 4 inch sphere or equivalent sized toroid.
>Now that would demonstrate the true principles of a Tesla coil and still
>be very inexpensive. As far as "the problems building the primary and
>secondary to specs", well that's all part of learning what a Tesla coil
>is all about! Besides if one is going to draw up complete plains to post
>on the Net, the builder won't have to do any of the calculations or get
>involved with any other of the multiple design compromises. I would
>suggest including the why and wherefore of the specs of the design to aid
>the student in understanding the coil they would be building, this is not
>much different than the approach used by Heathkit and Eico in the kits
>they used to sell.
>
> Alfred A. Skrocki
snip
Hi Alfred and All,
The difference between an ignition coil (or any iron transformer) and a
Tesla coil really isn't that great. The first problem is the coupling in
the GM ignition coil is too high. A hacksaw can easily cut the core off
one of these in a few minutes (once cut, the cores in these fine American
auto parts fall apart just like the rest of the car :-)). The reason Tesla
coils transform voltages the way they do is due to the matched resonant
circuits on the primary and secondary. All a transformer really needs are
the appropriate capacitors added to each side. Then they need good high
voltage design. These coils typically operate in environments that would
disintegrate our coils. This is probably easier service than they see
under the hood. Hopefully, they could make a very robust coil that could
survive many odd experiments. The specs for a GM coil with the steal core
removed are as follows:
Primary resistance 0.047
Secondary resistance 8488
Primary inductance 148uH
Secondary inductance 6.12H
Coupling 0.0576
The secondary has considerable turn to turn capacitance. It is around 40pF
giving an Fo frequency of around 10kHz. Not real bad. A matched primary
cap is about 1.7uF. Kind of odd by our standards. However, there is a
wonderful property at work to help use. The voltage "gain" is about 200!
The huge primary capacitance and secondary inductance really help the
voltage fly up! Modeling shows that a 500 volt primary cap charge should
push about 50000 volts with all the losses (including the arc) considered.
A 500V 1.7uF cap can be put together with commercial parts for about $30.
A simple rectifier circuit can easily charge it from any moderately high
voltage source. I have a $20 (mini-pig) circuit that could charge it very
easily (as could a microwave transformer) but the current is well into the
very lethal range so maybe something like the camera flash idea would be
safer at first. The coupling really needs to be 0.21 instead of 0.056 but
one can loop some steal wire through the core to raise it to the proper
value. Since the Fo frequency is so low this should work well. Quenching
and a low voltage spark gap should be interesting....
So perhaps you can build a very conventional Tesla coil out of a GM
(General Motors) coil. It actually is fairly complex theoretically in that
it has very unconventional circuit values compared to what we are used to
(modeling is a big help here :-)). It is easy to build although the
primary cap could cost a bit of money to build from many high voltage poly
commercial caps (same problems our caps have :-)) Of course, the cap could
be made from poly and foil too (it needs to be adjustable unless another
trick is employed for primary adjustment). If a beginner could build this
coil, it would have all the "fun" properties of any Tesla coil and the
knowledge gained would be directly applicable.
Sooooooo will it work???? I'll let you know :-)) So far it looks fast to
build not counting the design and measurement stuff the beginner would be
spared. They have enough mechanical stuff to worry about without worrying
about tuning and other stuff where fancy equipment or expert knowledge is
needed. The theory will come much faster when they have their working coil
in front of them to fiddle with. It is also cheap (free) but I think I'll
buy the caps to get it designed right the first time. So far, it looks
simple, fast, and cheap. The real test will be if it makes cool streamers
or not. Only the finished coil can determine that. A number of people are
getting me other ideas too so something should come out of all this.....
Terry
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