This was my first capacitor, 0.02 microFarads, with safety gap. The
mount was made similarly to the main base. In
operation, it lasted about 20 seconds at half input power. This was a commercial
unit from Plastic Capacitors Co., non-inductively wound and rated for 30,000
VDC. I had previously tested it at full voltage with my DC supply and had
set the gap to fire at full voltage (this produced a very impressive flash
and bang). Just before it blew, the safety gap began firing (a shot-gun
blast counterpoint to the chain-saw on steroids sound coming from the main
gap). This was followed by a white cloud of vaporized mineral oil drifting
out from under the coil.
If you ever wanted to see the guts of an end foil constructed capacitor,
here 'tis. The end cap was forced off by internal pressure. I had mopped
up most of the oil by the time I took this picture.
This is the capacitor I am currently using. It consists of eighty-one
2500 picoFarad (rated, ~1850 measured) 30,000 VDC ceramic "door knob" capacitors
arranged in three series of twenty-seven parallel for 17 nanoFarads at
90,000 VDC. Aluminum flashing and 1/4-20 hardware holds them all together.
Half inch square copper bars are used to make electrical connections and
hold the safety gap. My new static gap can also be seen behind the capacitor.
The heavy black cable which exits the bottom of the picture is the RF ground.
Update! 8/22/99 Ceramic capacitors can, and do, explode!
I had just switched from 8 to 10 gaps in my main static spark gap.
As I cranked the power up, there was a flash and bang from the edge of
the capacitor bank. After shutting down, I found a half dollar size chunk
of 1/4" thick insulation had been blown off of the capacitor and a fair
bit of the ceramic dielectric (barium titanate?) was turned into gravel.
Fortunately, I have a couple more of these caps, so fixing the bank was
just a matter of replacing the one bad cap. I also noticed that the cap
which blew was lighter in color than all of the other caps, perhaps it
was compromised to start with (wishful thinking? you bet!).
I am also working on a capacitor of my own design, which should be able
to handle 100,000 VDC. It will be a stacked plate capacitor, with forty-eight
6" by 14" plates made from aluminum roof flashing. The plates will be stacked
with 12 inches of overlap and separated by 15 sheets of 0.006" linear low
density polyethylene. The trick is to stack the plates and sheets in a
pan (also made from the flashing) and then run the whole lasagna through
a vacuum oven at 140 ºC. I built a small test capacitor using this
technique, just to make sure it would work, once I realized how much effort
it would require.
This was my first effort, with six 4" by 6" plates, 4" of overlap and
3 sheets of 0.006" LLDPE between each plate. I trimmed the corners of each
plate to reduce corona and of each plastic sheet to provide a path for
out-gassing. I also added filler strips of plastic sheet around the plates,
so there would be no spaces around the edges of the plates. After the assembly
came out of the oven (~0.1 torr pressure and 10 hours at 140º C plus
heat up and cool down), I drilled holes through the un-overlapped sections,
tapped them for 1/4"-28 thread and added some threaded rod and nuts. The
finished capacitor had a measured capacitance of 5 nF.
Next, I hooked it up to my 30,000 VDC, 3 mA power supply, a spark gap,
a multimeter with 40,000 V probe and a 23 watt, 23 megaOhm resistor (to
limit charging current) and tested it to destruction. Starting with the
spark gap set for 10,000 V, I charged the capacitor until the gap fired
and then increased the gap in 1,000 V steps. At 22,000 V the spark gap
flash and bang was nearly as violent as the spark from my late 20 nF commercial
cap at 30,000 V! (Can you say "low self inductance"? I knew you could.)
Finally, at 23,000 V, a weak pop emanated from the capacitor and it would
no longer hold a charge.
Other than trimming the corners, I took no special precautions making
this test cap. For my big cap, I also filed the corners smooth and then
polished the edges with a 220 grit sanding pad. The slowest part has been
checking each plastic sheet for defects. I am using the sort of film normally
used as a vapor barrier in housing construction and it has lots of defects,
including holes, gels (little ball shaped particles of cross-linked plastic)
black-specks (flakes of burned plastic and metal filings) thin spots and
even a few pieces of steel wire. I hold each piece up to a light and reject
any (about two thirds) with obvious problems. Some of the better rejects
will go on the top or bottom of the capacitor in stacks of 80 sheets (~1/2").
With 15 sheets between each plate, instead of the three in my test cap,
it should be able to handle 100,000 VDC with no problem. Whether or not
it stands up to Tesla coil service remains to be seen.
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