The Primary coil is the transmitter of energy to the secondary.
The primary coil emits a powerful
electro-magnetic field as current from the tank circuit oscillates
through
it. You
must tune a primary so the resonant frequency of the tank circuit is
the same as the secondary resonant circuit. Think of it like
pushing a kid on a swing, if you push him just right he will go higher
and higher, but if you push him at the wrong time nothing
happens. When a coil is properly tuned, the field transfers
energy to the secondary, and if the enough is transferred, arcs
will leap out of the top of the secondary. There are a few types
of primaries used, the helical, conical, and flat spiral. The
helical is used mostly on small coils and solid state stuff and I do
not really like them, it
is basically a conductor (copper tubing or thick wire) wrapped around a
tube sitting around the outside of the secondary. They generally
have
a really high coupling, which leads to problems with racing arcs,
primary strikes, and corona losses (plus when you
look at the magnetic field lines generated you will see they
tend to concentrate at certain parts of the secondary, unevenly, which
causes the problems).
The conical is much better, It is a spiral from the ground starting out
narrow at the base, and getting wider towards the top (between 10 and
30 degrees are good values). The flat spiral is what I use, it is
a flat spiral (as the name suggests) with the secondary in the
center (make sure you don't have any primary turns underneath of the
secondary, leave a hole slightly bigger than the secondary
diameter). The primary coil is tuned by lengthening and
shortening
it, which usually doesn't involve cutting, but simply sliding the outer
contact (like a heavy duty alligator clip). Generally copper
tubing is used
in these coils because it has a high surface area for low resistance,
and no sharp corners to suppress corona losses. I use 1/4"
tubing.
One more thing, the primary coil should generally be at least an inch
away from
the bottom of the secondary (contrary to what my left picture shows),
and
never have turns underneath of it (I mention this because my first
design had part
of it located directly
below, so part of the magnetic field interfered with the other
resulting in terrible performance). The reason it should be
farther away
is so it doesn't arc from the primary inner turn, to the bottom of the
secondary, which they have a
tendency to do as the secondary is grounded. The advantage to
having it
closer is that it is relatively
easy to get a tighter coupling that way. "Coupling" is the
physical
orientation of the primary to secondary, to adjust the coupling on a
flat spiral, you would simply raise and lower the secondary within the
primary until the best coupling coefficient is achieved. The
coupling coefficient determines the rate at which energy is transferred
to the secondary, sometimes you want it faster or slower than
others. Generally as fast as possible without having racing arc
problems is best.
