[Prev][Next][Index][Thread]

Re: flashing vs. whatever


From: 	Esondrmn-at-aol-dot-com[SMTP:Esondrmn-at-aol-dot-com]
Sent: 	Wednesday, November 26, 1997 7:22 AM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: flashing vs. whatever

In a message dated 97-11-26 04:06:59 EST, you write:

<<  Subject:        flashing vs. whatever
 > 
 > To all, a question for rolled polyethylene caps.
 > I'm getting ready to begin plans for building these caps. My transformer
 > matched cap value is .256uF. I'm planning on building three .083uF caps
 > to achieve the (near) value. The plate area required is 2592 sq. inches
 > (18" x 144"). I also plan on using four 4 mil sheets each side of plate
 > for total of 8 between plates. In transformer oil the caps should prove
 > durable assuming I get the air out.
 > 
 > It seems those of you who have built this type of cap have used roof
 > flashing. I'm not real familiar with roof flashing, but isn't it a bit
 > thick for the purpose? At least the pulse caps I've seen have an
 > extremely thin aluminum sheet. What advantage wood roof flashing give
 > over say aluminum wrap bought at a grocery store?
 > 
 > Just curious,
 > Bart Anderson
  >>
Bart,

I agree with Bert about the voltage rating you are likely to achieve with
this design.  It certainly is better to use multiple thin layers than one or
two thick layers of poly.  I think the finished cap will only be good for
maybe 7 to 8 kv in Tesla coil service.  Two caps in series should be good for
12 kv and maybe up to 15 kv.  I have built three rolled poly caps and they
always come in under the calculated value.  I think it is because we never
really get the spacing to be the same as the dielectric thickness, it is
always a little more.  I have found if you use 2.0 for K, the calculated and
measured values come out the same.  Using 2.0 for K, I calculate .072 uf for
your design.

I haven't tried aluminum foil, always used aluminum roof flashing.  The
heavy, industrial grade of foil, would probably work ok and make the caps
much easier to roll - and cheaper to build.  I don't remember anyone on this
list talking about using foil for these caps, only in the flat plate caps.
 The big difference being in flat plate caps the maximum size of each plate
is only maybe 12" or 18".  Your cap will have plates 144" long.  With thin
foil plates, this may not make the best high current pulse duty capacitor.
 The best construction method may to allow the foil to hang out on each side
of the roll and connect it all together (called end foil construction?) like
they use in commercial caps.  Has anyone on this list built rolled caps this
way?  If you build these, let us know how it comes out.

Ed Sonderman