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> >I can get 630WVDC 0.10uF polypropylene film capacitors for $0.43 each.
> >They are designed for current pulse, high frequency, low inductance
> >applications although not quite what we have in mind ;-) They are
> >Panasonic ECQP6104JU types. They are about 1 x 1 x 0.5 inch in size.
> >
> >What I had in mind is this. If I string 8 parallel rows of 40 caps in
> >series (320 caps) I would get one large cap rated at 25kVDC (18kVAC) at a
> >value of 20nF. It would cost $138 plus a container, oil, and a bunch of
10M
> >ohm balance resistors. Perhaps it would be ok left in free air??
> >It wouldn't have the 3X rating we like but these are commercial caps with
> >the usual 5 seconds at 250% over voltage rating. Each string may see
> >around 30 amps peak during a firing pulse. It would be interesting in that
> >each cap or string could be easily replaced if something blew.
Hi All,
I have now built up a 10nF 15kV version of this. I have done some testing
that I wanted to pass along.
I destructively tested two caps and found that they failed at 3100 VDC and
3900 VDC. Very impressive considering that the polypropylene film is only
0.5 mil thick (I took one apart). The polypropylene is rather stiff and is
clear as glass. Obviously of extreme quality. The 630 VDC rating is
conservative. Note the the film's dielectric breakdown is 6000V / mil and
they are designing for 1260 V / mil!!
I have 10M ohm 1/2 watt 5% YAGEO carbon film resistors across each cap.
They break down across their outer surface giving about a 0.2 inch arc. I
was hoping that they might breakdown sooner to protect the cap but it looks
like they will easily withstand very high voltage. They can get to 2200
VDC before they reach their power rating. Their rated working voltage is
350V and their overload ratting is 700V. Also conservatively rated.
The cost is working out as follows:
Caps (160) $70
Resistors (160) $2
Epoxy glass perf board $20
Binding posts (8) $10
So it looks like about a hundred bucks per 10nF. Mine is pretty fancy with
the board and posts which are not really needed. A 20nF could run $145 if
you left out the fancy stuff.
I think it will run in free air just fine (no oil). The balance resistors
seem to work well (time constant = 1 sec) and I am placing it on thick
Styrofoam which has much the same properties as air (you don't want the
caps arcing to anything or being messed with by surrounding electrostatic
fields). Since the caps are 5% the final capacitance values were almost
perfect. Each varies around 2% but they average very close to the rated
value. Panasonic obviously knows what they are doing! The balance
resistors bleed the charge down after a moment which is very nice. It
would be hard to short out each cap to drain them :-)
I hooked it up to a primary circuit (no secondary in place) with a current
probe and measured the ringdown time. This is an indication of how much
internal resistance the cap has and if there is any significant difference.
It appears that the new cap performs equal to or slightly better than my
nice ceramic caps. The ringdown times were the same. The gap is really
where most of the primary resistance is as some other testing I have done
has shown. Everything stayed at room temperature and I could not detect
any heating of the caps at all.
I then put the primary in place (the tuning was poor) and fired up the
coil at low power. I tried both type of caps under the exact same
conditions. The performance and primary current waveforms were exactly
identical. The output (just at breakout for this test) were exactly the
same. There was no heating of either caps (the ceramics have always run
very cool, they are high Q transmitter doorknobs).
Now I put 10nF of ceramic and 10nF of my new cap in the circuit so my coil
could tune properly and let it rip! I ran at 150 amps peak of primary
current with about 20 inch streamers for a minute (the room is to small to
really turn it up). The caps stayed ice cold (the rotary gap gave them a
very gentle breeze).
One thing I really like is I can easily select strings of 2.5nF each to
adjust the primary cap value. This is very nice in the work I do. The
ceramics were hard to add or remove and I never had quite enough of them.
I will have to do more testing (perhaps even Chip's "Halloween" test :-))
and get some hard run time on this before I really recommend it. Tesla
caps have a way of needing a few revisions before they are really ready for
prime time. However, so far, everything is going super well!
I took a bunch of digital pictures and posted them at:
http://www.peakpeak-dot-com/~terryf/tesla/misc/
They are manyc001.jpg to manyc011.jpg
Terry