New cap. design
After a bit of tweeking, lowering torroid, lowering the coupling and
a couple of retunes (thanks for the advice malcolm) I finally managed to
increase my spark length to about 18 inch. The spark was still a spindly
blue/purple colour though, not the big hot sparks seen in richard quicks
video. I seem to remember in a past post of richards that this is a normal
charactaristic of glass/ salt water caps. I've now dismantled the system for
including a better shaped primary made from copper pipe and a better capacitor.
It is the capacitor that i'm looking for advice on.
The basis of this capacitor is plastic A4 Document wallets. I found a cheap
scource of supply for only a couple of pounds per 100. These are made from
polypropylene film 0.003 of an inch thick. To fit inside a readily available
plastic container the pockets were trimed in to 8 1/4" x 5 7/8" , open on
three sides and closed along one 5 7/8" side. An Aluminium foil plate 5" x 9
3/4" was cut and slipped inside.A second film and plate was placed over the
top, but with the exposed end of the plate on the opposite side. The
measured capacitance was right on 0.003 MFD. With a total film thickness of
Next, the basic cap was connected across a 10Kv neon and variac. The variac
is calibrated 0-100 percent, so the output voltage can be easily estimated.
The voltage was slowly increased to determine the breakdown voltage. The
first thing noticed was at around 500V the plates pulled in hard against
each other by electrostatic attraction and created an amazing amount of
noise. In darkness quite an amount of faint corona was seen around the edges
of the plate and coincided with the noise. Obviously the plates were acting
as an electrostatic loud speaker with the corona discharges proving the
varyation in voltage needed to excite the plates. The voltage was further
increased to 1Kv and although the corona and noise increased the cap. Held
together. The voltage was increased again to 2KV and the same increase in
corona and noise was noted. At 3KV occasional sparks were starting to jump
around the 7/16" boarder, far longer paths than would normally jump in a
plain air gap but probably assisted by the ionised area around the edges of
the plates. At 4KV the sparks were getting very fierce and loud (more so
than a spark gap across my beer bottle cap at 10KV). Finally at around 5KV,
with a bang that made me jump two feet backwards, the film punctured and the
current blew a hole about 1/4" Dia. staight through both layers of film and
foil, not at the edges where the corona and sparks should have been heating,
weakening the film, but fairly close in to the centre of the plates.
Now my problem is this, i was origionally basing my cap. On the assumption
of arround 1KV working voltage per pair of plates on 20 x 0.2 mfd caps in
series. Each cap. Comprising 67 plates. giving me a final cap. Value of
0.01mfd -at- 20KV working. But on the results of my experiments it appears that
the film is capable of withstanding much higher voltages than was first
assumed. Note the plates were dry and not submerged in oil(supply scourced
but not obtained yet)
and i can only speculate on what the breakdown voltage would be if the
plates were submerged. If i can assume a safe working voltage of 2KV I could
build two 0.01 caps out of the same materials and place both in the same
container saving space and oil.
My questions are:-
can anybody advise on what would be a safe working voltage to assume with
this thickness of film.
What would be the best arrangement as far as dielectric loss is concerned,
20 X 0.2 mfd -at- 1KV per cap.
10 X 0.1 mfd -at- 2KV per cap. (half the area but -at- twice the voltage)
Is the differance in loss between the two caps. Too small to worry about and go
for the smaller cap anyway.
If the breakdown voltage question cannot be answered then i shall do
another destructive test, submerged in oil (the cap, not me (:-) ) and if
so what should i assume to be the working voltage i.e. half the breakdown
My normal working voltage is arround 10Kv, possibly 13Kv in the future.
Please feel free to add any comments about any other aspects of this cap as
it is still very much in the design stage and thanks in advance for any
answers to my questions.
Ian Hopley ----> i_hopley-at-wintermute.co.uk