This was my first main spark gap, using 2" copper sweat couplings and
connectors made from 1/2" copper bar stock. It looks nice, but the gaps
were rather difficult to set and I have gone to a different design.
This is a schematic top view of my new spark gap. It is formed from
a stack of eleven 3" X 6" X 1/4" and two 3" X 6" X 5/8" aluminum plates.
Each plate has four 1/4" holes to accept the four 1/4" threaded nylon rods
which hold the unit together. Each plate also carries a copper electrode,
made from 2" copper pipe which has been flame annealed (to remove the hard
temper), beaten and stretched to fit. The two end plates are drilled with
1/4" holes to accept 4 gauge battery cable, 5/32" holes to take banana
plugs (power from neon sign transformer) and 1/4"-20 tapped holes (on the
bottom) for mounting feet. The electrodes are spaced with 3/16" thick soft
silicone rubber washers between the plates. Total gap can be adjusted by
removing plates and fine tuned with tension on the nylon rods.
A side schematic view of the new gap.
Making the electrodes for the new spark gap. Far left, a 1 1/16" ring
cut from a piece of grade M copper water pipe with a pipe cutter. Next,
is a piece with all of the burrs and edges left by the pipe cutter filed
off with a mill-bastard file (just above the rings). The next ring has
been polished with a 220 grit sanding pad (top left). The last ring has
been annealed to remove the hard temper of the pipe by using a propane
torch to heat it to a dull red, it has a coating of dark brown copper oxide.
To the right is a ring that has been flattened in the workmate vise and
partially stretched using the steel bar jig. The electrode is then annealed
again (stretching and pounding causes the copper to regain a hard temper)
and stretched all the way out to 3" on the jig. Last, is a finished electrode
mounted on an aluminum heat sink.
This picture shows an eight gap assembly with silicone insulators and
nylon tie rods in place. It's a little hard to tell from the meters, but
the gap is handling 1600 watts of power (a few sparks are just visible
between the copper electrodes). It takes several minutes of operation before
the unit is warm to the touch.
This view shows the nylon rods which act as insulating feet. I have
more rod and some nylon wing nuts on order. I will make two or three sets
of main tie rods, to make adjusting the gaps easier.
Here are all twelve gaps and wing nuts (sorry it's so fuzzy).
This picture shows, from right to left, a clean copper electrode, the
same electrode after use, the same electrode with a 1/32" graphite facing
bonded on with conductive adhesive and the graphite electrode after use.
The bare copper works OK, but as the non-conductive oxide layer builds
up they require a higher input voltage before the gap will fire. Eventually,
they stop working and need to be cleaned. I thought that if I could coat
them with a material that was more temperature resistant, but still very
electrically and thermally conductive, I might increase the run time between
cleanings. The two materials that came to mind were tungsten and graphite.
I have not yet found a good source for tungsten foil, but sheet graphite
is readily available as gasket material. As you can see, it did not work
real well. After about a minutes operation in my Tesla coil, the graphite
surface was covered with tiny soot spots.
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