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Toroids, Strike Shields
For those of you who out there who are, or will be, building
high inductance, low aspect ratio, secondary coils coupled
to large primaries; I thought I would cover an area of con-
struction technique that will find a real use in your coil
system: proper toriod positioning and the use of strike
shields.
One of the difficulties of operating a highly efficient Tesla
Coil, where discharge lengths are exceeding the length of the
secondary winding, is the tendency of the spark discharge to
couple into the primary/secondary field flux. This problem shows
up in the form of frequent strikes to the primary conductor.
Heavy striking to the primary coil and tank circuit can result
in a condition that I have dubbed as a "runaway" oscillator. What
happens is this: The secondary discharge strikes the primary. The
high frequency high-voltage secondary discharges form ionized
"leaders" that bridge the spaces between the turns of the primary
conductor. These ionized pathways or "leaders" allow the high
current that oscillates in the tank circuit to jump the turns in
the primary coil. With the primary coil effectively short
circuited, the tank circuit shifts to a higher frequency.
This shift in the tank circuit frequency due to arc shorting
on the primary coil does not produce a single monochromatic
oscillation frequency. The short circuit pathways vary in length
and duration; some current still travels the length of the
primary coil, but some current is diverted into the developing
short and crosses turns.
As the primary frequency rises and drifts away from the resonate
frequency of the secondary, the secondary coil is no longer able
to efficiently absorb energy. Without a source of tuned input
energy, the secondary stops resonating, and any energy stored up
in the secondary coil or primary/secondary field flux is dumped
back into the tank circuit. If the secondary was sparking well,
the sparks look as if they were sucked back into discharge
terminal with a vacuum. The arcs jumping the primary turns
brighten noticeably and primary "goes to flames".
If the coil has not already been powered down by this point the
entire system energy shifts into the shorted primary. The tank
circuit frequency continues to rise as the shortest pathways
become firmly established; the blue arcs shorting the primary go
to bright white, sparks and corona begin to spray from the tank
circuit wiring and connectors. The safety gap fires continously
and the capacitor terminals can flashover...
If the system is not powered down then critical components such
as the step up transformer, capacitors, or low voltage power
supply circuits will fail and circuit will power down for you.
As you may understand from reading this, this describes a very
powerful and unstable condition. I have had energy from a runaway
oscillator find it's way back to the supply circuits where it
ruptured a filter capacitor, punctured the PVC insulation on a
low voltage buss wire, and arc scored three inches of heavy poly-
urethane coated plywood before reaching a ground strap. All I saw
was a bright white flash, an explosion like a high-powered gun-
shot, and the thick smell of ozone and electrical damage. After
replacing the blown filter cap and a neon with a short to the
core everything worked fine. But I credit the heavy ground strap
in my power cabinet with saving me from what would have been, at
the very least, a nasty shock.
So what do you do?
The first line of defense is the proper use of toriods. A large
toroid, properly positioned, will allow the spark to uncouple
from the primary\secondary field flux. The height of the toriod
from the top of the secondary coil will need to be raised with
the input power levels. I generally do not mount a permanent
insulator on the top of the secondary coil, but will use some
scrap sections of PVC pipe set on the top of the secondary. I
simply set the toroid on top of this. After a coil is all tuned
in and you decide you want to make a permanent mount, go ahead
and score the top plate with a scribe or other tool and epoxy
a permanent standoff insulator to the end cap. In addition to the
correct height from the top of the secondary, the toriod needs to
have the proper diameter, and I like them large. Lower input
powers requires a toriod that is at least an inch or two larger
in diameter than the secondary coil form, and this is only
sufficient for low power testing. For any practical operation the
minimum toroid diameter is twice that of the secondary coil form,
and as the coil efficiency and input powers increase, the toroid
can get to four or more times the diameter of the coil.
BTW, one of the first benefits you will see from using the proper
size and position of toriod is a longer lateral discharge that is
less likely to strike down on the primary.
The next little trick for protecting the primary was taught me
by Richard Hull of TCBOR. It consists of the use a strike shield
to protect the primary coil. A strike shield or "strike rail" is
simply an incomplete turn of primary conductor that is mounted on
standoff insulators and positioned above the last turn of the
primary. It is important that the strike rail not make a complete
or "shorted" turn, and the strike rail must be well connected
somewhere to system ground. With a strike rail in place; if the
coil by chance does make an occasional strike downward, the
discharge will stike on the grounded rail where it will not
short out the primary or damage any system components.
Along with a strike rail, small sections of hardware cloth,
aluminum flashing, or other conductor, may be positioned to
protect tank circuit wiring, spark gap motors, and other con-
nections (and components) where needed. Proper use of grounded
shielding to prevent essential oscillator components from getting
struck will increase the life of the system. It may also prevent
some gray hairs on behalf of the coiler!
Richard Quick
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12