Tesla coils are notorious for destroying electrical equipment. The
extreme voltages generated can punch through insulators like they are not
there, shorting out capacitors and transformers. The high frequency output
can also induce currents in nearby devices and burn out sensitive electronics.
Protection is needed to keep this equipment functional. The first line
of defense is physical distance, don't put anything closer to the secondary
than it needs to be. Next is shielding, place a grounded conductor around
any sensitive equipment (this is known as a Faraday cage) or between the
equipment and the secondary (like the strike-ring above the primary).
Safety gaps are set to short out or ground voltages which are high enough
to be damaging. These are particularly important for capacitors and neon
sign transformers (even pole-pigs, which are designed to survive lightning
strikes, have been killed by Tesla coils when used without safety gaps).
Lastly, a band pass filter should be used between the transformer and
the Tesla coil primary. This circuit is meant to allow low frequency AC
from the power supply to pass with little resistance, but present a very
large resistance to the high frequency currents generated by the primary
or tank circuit. A schematic and construction details are shown below (the
transformer connects to the side with the safety gap).
Many coilers locate their filter on the base of the coil. While this
is convenient, it presents a problem in that the filter's inductors are
placed within the magnetic field of the primary and can thus become sources
of damaging currents. Since the current from the power supply to the primary
is low compared to the currents within the primary, heavy conductors are
not needed to connect the filter to the primary. Small diameter conductors
also present a higher resistance path to high frequency currents and help
to attenuate destructive kick-backs from the coil. I use 2 ten foot long
pieces of 30,000 VDC insulated, 20 gauge wire to connect my filter to the
spark gap of the primary. Although it looks wimpy, at 15,000 VAC (7,500
VAC each, since the neon sign transformer is center tapped) it can easily
handle 30 kW. I make connections between the transformer, filter and main
spark gap with banana plugs, which, surprisingly, are rated for 10 amps.
The finished high voltage radio frequency interference filter with
safety gaps to protect the transformer from Tesla coil kickback. The components
are all mounted in a polycarbonate NEMA 4X enclosure ~14" X 10" X 5". The
two large ceramic capacitors and the air core chokes are mounted on polycarbonate
stand-offs. The small yellow bits at each end are the banana jacks for
cables connecting to the transformer and primary. The white wire is 14
gauge for connection to the grounded transformer center tap.
A close-up of the safety gap, made from a chunk of UHMW polyethylene
with three 1/4-28 threaded holes, chromed steel ball knobs, 1/4-28 stainless
steel threaded rods and a couple of plastic knobs. The center rod is bolted
directly to the galvanized steel back plate in the mounting box.
The two air core chokes in the RFI filter each contain three 10" long
windings of 20 gauge magnet wire on a 12" length of 1" diameter polycarbonate
tubing. Each layer is separated from the others by a piece of PVC shrink
tubing (you can see an uninstalled one hanging from the winder's shaft,
just to the right of the coil).
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