Quoting Mark Conway
MC> I have just bought the 6 inch plastic pipe for it - it is grey
MC> stormwater pipe,not schedule 40 but it is a bit thicker than the
MC> usual pvc downpipe used in houses. Do you think that this will be
Well since generally PVC is the worst plastic you can use anyway, you
really can't go wrong... (Grin) Just sand the form smooth. Make sure
the plastic is very, very, dry, (gently baked even) and seal the bare
plastic coil form (before winding) with several coats of a clear two-
part epoxy paint, or clear polyurethane boat or "Spar" varnish.
MC> Looking at your coil in the video it seems to be about 24
MC> inches high but I'm not sure of that or what kind of wire
MC> you used to wind it or how many turns you wound. Also what
MC> is the total length of the 6 inch plastic pipe (including
MC> the unwound length) i.e how long do you leave the unwound
MC> section of the pipe? Looking at your video it seems that
MC> you only leave about an inch or so unwound on either end.
The coil form of that particular coil is PVC drain pipe. The
outside diameter is 6.25 inches and the total coil form is a
little over 33 inches long. The actual winding is 30 inches long.
The winding is close wound with #21 magnet wire. This particular
wire has a double coat of Formvar enamel insulation, and winds
with 33 TPI (turns per inch). There are about 990 turns on the
coil, the resonate frequency as measured with a scope and frequency
counter is around 220 kilohertz. The aspect ratio of 4.8 to 1 is a
little high for a 6 inch coil, but I had a lot of the surplus #21
wire that I wanted to use up. Because the aspect ratio is a little
high, this coil requires a primary that couples a bit tighter.
MC> I cant locate any aluminium flashing over here but I can get
MC> copper flashing ( 12 inches wide and 0.08mm thick (what you would
MC> call about 4 mils thick)- a bit thinner than the 10mil aluminium
MC> flashing you use over there). Do you think that it will be ok?
Yes, this substitution will work just fine.
MC> Also ( sorry about all these questions!) in a posting you put on
MC> fidonet about making polythene capacitors written by Bert Pool he
MC> mentions that it is much better to use multiple polythene sheets to
MC> prevent breakdown in case of a bad patch on the polythene. I notice
MC> that in one of your postings you say that multiple sheets affect th
MC> efficiency of the capacitor. Do you think that I should just use a
MC> single or multiple sheets if I want to get spark like in your video
That depends on the grade of plastic and how much you intend to abuse
the capacitors. The plastic I get is still rolled on the cardboard tube
from the manufacturer. It has not been scratched, cut, or dented, or
impregnated with sharp particles of dirt. I inspect all of my materials
for defects before the capacitor is assembled, and I make sure that the
caps are rolled up free from dirt. After the capacitor is completed, I
do not run them at full voltage for at least six months, instead I build
two capacitors at a time, and run them in series with no more than 20 kV
across the pairs.
Bert Pool on the other hand feels that it is the design, not the
construction, that makes a capacitor durable. From talking with him
I know that he does not take the same care in choosing his materials
or in the cleanliness of his construction. He takes his completed
caps and fires them with 15 kV across the dielectric shortly after
he completes them... and he has complained when the capacitors
failed after a few seconds.
We have both had our share of capacitor failures, and we have both
solved them, albeit by different methods. The capacitors we have
written about are the same basic design.
MC> a supply in this country. BTW how big do you make the breather
MC> hole in your capacitor lid- I'm a bit worried that the breather
MC> hole may let in water vapour which the transformer oil will suck
MC> up - Auckland tends to be very humid...
Then go ahead and leave the breather hole out of the construction.
The hole I use is tiny, and the only purpose of having it is to
make the tank easier to uncap for topping up oil, and replacement
or repair if the capacitor blows.