RE: New cap info
Awsome info! Did Beay recommend any particular part numbers?
Are these capacitors the same as used in induction heaters?
Do they make them in metal cans?
|To: Benson Barry; "Tesla-list-subscribers-at-poodle.pupman-dot-com"-at-PMDF-at-PAXMB1
|Subject: New cap info
|Date: Thursday, December 05, 1996 3:04AM
|<<File Attachment: 00000000.TXT>>
|From DavidF4797-at-aol-dot-com Thu Dec 5 00:15:59 1996
|Date: Wed, 4 Dec 1996 14:03:22 -0500
|Subject: New cap info
|Warning: Huge Post!
|I just blew up my cap - which had been working just fine for about 8 hours
|cumulative TC use by switching to a rotary gap with my neon. I had heard
|that a rotary gap with a neon put a heavy load on the neon and would likely
|burn it out, but since I had repotted my neon in oil I figured that it
|handle it and it did. It was the cap that couldn't :(
|I then spent 5 or 6 hours (!!) with one of the capacitor engineers (Beau
|Meskan) from a company here in Chicago called Plastic Caps which
|in High Voltage discharge caps for military and industrial use. He was
|helpful, took me on a full tour of the plant and was willing to share
|everything he knew about HV cap design and construction. He was also
|to put my cap (when I finish rebuilding it) in one of their vacume tanks
|"pump it down" over night along with one of their regular runs. They are
|also willing to sell their HV AC caps to tesla coilers at cost! After the
|great and informative experience I had with them I don't hesitate to
|recommend them to anyone looking for a commercial cap for their TC.
| (shameless plug) Their phone number is (773) 489-2233.
|Plastic Cap has been experimenting with various materials and designs for
|last forty years and has settled on some general rules of thumb which apply
|to high frequency AC caps these are as follows:
|1) Polyethylene (LDPE) film is no longer used because it is no longer
|available in "capacitor grade" film as of the 1970's.
|2) The 1200v/mil breakdown voltage that is reported in the literature is
|*only* for "capacitor grade" polypropelene or polyehtelene film. And
|capacitor grade polyethylene grade film is no longer made, the 1200v/mil
|figure is no longer correct. The polyproplelne (PP) and polyehtelelne (PE)
|materials that are currently available in sheet form from a plastics
|distributor have breakdown voltages only in the 450 to 700v/mil range
|of impurities. The breakdown voltage for the PE sheeting that one can buy
|the hardware store ("uyility" plastic) has an even lower breakdown voltage
|about 250v/mil because of even greater levels of impurities.
|3) One of the reason that the industry switched from PE to PP was that the
|breakdown voltage variation in PP was much less than for PE. For example,
|with 4 x 8 foot sheets of commercial grade plastic available from a
|distributor, the breakdown voltage of PE (same manufacturer) is roughly
|450v/mil on the low end to about 700v/mil on the high end which is a 250v
|variation The he breakdown voltage variation, however, for PP is roughly
|550v/mil to 600v/mil or only a 50v variation. In other words, the PP is
|more consistent in its breakdown voltage characteristics that PE.
|Also, the melting point of PP is much higher than for PE so that if "hot
|spots" do occur in the cap, PP is more resistant to melt down than PE.
|4) When building capacitors in mineral oil (without a vacume pump to remove
|the air bubbles) the plates should be no more than 3 inches long in any
|direction (for flat cap designs) or more than 3 inches wide for rolled
| This is because dielectric more than three inches wide will end up with
|spots which will heat and eventually cause cap failure. This is because the
|capillary action of mineral oil cannot easily penetrate more than 2 inches
|any direction between plastic sheets. (I noticed this effect in my flat
|the first build (which has 8.5x11in plates)).
|5) If you are determined to use a plate/dielectric size wider that 3 in,
|must then use paper to wick the oil farther between the layers (I
|I came to myself when building mine). This works well up to a 6 inch
| Alternatively, If you use use *silicone* oil instead of mineral oil, you
|may then dispense with the paper. This will also work well to about a 6
|6) If you use silicone oil (very expensive -at- $50.00/gallon) and paper, you
|may then be able to get away with a plate size as large as 8in if you allow
|sufficient time for the wicking action of the paper to completely remove
|7) While using paper for wicking has the advantage of elimination dry spots
|and therefore eliminates melt through at those spots, it also,
|has the disadvantage of raising the dissipation factor (heating effect at
|high frequencies) of the total plastic paper sandwich. The bottom line
|is that continuous more gentle heating over the entire surface of the
|dielectric (which is what happens when one uses paper) is preferable to the
|intense "hot spots" that occur in dry pockets.
|8) The best paper to use for wicking is Kraft paper which is available for
|capacitor use in various widths. It is called "capacitor grade" Kraft
|and is *very* thin. Plastic Cap uses 3 tenths of a mil thick Kraft paper
|which they feel has the best trade off between wicking action and the
|somewhat increased dissipation factor that results when it is paired with
|or PE. If you can't get "capacitor grade" Kraft paper, use the *thinist*
|possible regular paper you can find.
|9) The Kraft paper should be interposed between each layer of PP or PE but
|this is impractical then at the very least it should be used between the
|*shiny* side of the plate (aluminum foil or flashing) and the next layer of
|PP or PE. The shiny side of anything, apparently, impedes capillary action
|while "dull" surfaces (slightly rough) encourage it.
|10) The cap, whether rolled or flat, should be built in several sections
|connected in *series*. This lowers the voltage that occurs across each
|section. The voltage at which corona begins to become a problem in cap
|construction is approximately 1200v. This means that each "section" should
|not be forced to see more that 1200v. A 15kv power supply, for instance,
| would require at least 12 or 13 sections (smaller caps) wired in series.
| This combination series/parallel design makes calculating the number of
|plates for proper capacitance a bit harder but is well worth it since
|(and the heat and loss produced thereby) will be eliminated resulting in
|longer life and better performance for the cap.
|12) The aluminum foil available in the supermarket has many impurities such
|as sodium (the worst) that causes "hot spots" on the aluminum. Once again
|"capacitor grade" aluminum is the best but in lieu of that, thicker
|dielectric and the use of polypro Propelene (which is more temperature
|than polypro Ehtelyene) is acceptable.
|13) When using non "capacitor grade" materials expect heeting of the
|dielectric (PE/PP), plate (aluminum) and wick (Kraft paper). Leave plenty
|room for expansion of the oil in such a system or explosion is likely.
|Whew!!! that was a long one.
|Hope it is usefull