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Re: Cheap 20nF 20kV pulse caps

Terry,
Nice job on your film caps. Your results are surprising to me. I wouldn't
have thought
film caps including the ones your using would have resulted in problems
such as
popping a cap or two maybe more frequently than one would like. Maybe too
soon to tell
yet, but your results so far are encouraging. Have you made an educated
guess at what
the limitations of film cap banks will have?
Bart

Tesla List wrote:

> Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
>
> Hi All,
>
> >From my original post for background:
>
> > >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