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Re: Capacitor Quality Factors


From: 	Barton B. Anderson[SMTP:mopar-at-mn.uswest-dot-net]
Sent: 	Wednesday, November 26, 1997 7:28 AM
To: 	Tesla List
Subject: 	Re: Capacitor Quality Factors

Tom,
Just want say a big "THANK YOU" for these tid-bits and facts on caps and
construction tips. This is excellent information and will be very useful.
Bart

Tesla List wrote:

> From:   Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
> Sent:   Wednesday, November 26, 1997 1:23 PM
> To:     tesla-at-pupman-dot-com
> Cc:     pacster-at-ibm-dot-net
> Subject:        Capacitor Quality Factors
>
> Tesla List members,
> Every once in a while I respond to something on the Tesla-2 List
> that I think would also be useful for the members of Chip's
> Tesla list. So here is a post dealing with capacitors.
>
> My apologies to those who already got a copy because they are members
> of both lists.
>
> Fr. Tom McGahee
>
> ----------
> > From: Tesla-2 <tesla-2-at-emachine-dot-com>
> > To: tom_mcgahee-at-sigmais-dot-com
> > Subject: How does one make high Q
> > Date: Wednesday, November 26, 1997 5:29 AM
> >
> > From: pacster-at-ibm-dot-net
> > Date: Wed, 26 Nov 1997 10:29:37 GMT
> > Subject: How does one make high Q caps?
> >
> > Hello !
> >
> > Finally!, my exams are over for the year, so now I can get coiling again (I
> > definitely prefer coiling to exams). I was reading some posts on capacitors
> > and it was said that for a 15KV cap. the polyethelene should be around
> > 90mils thick. Coming from a 'S.I. units' country I'm a little confused, is
> > that thickness 24.5/1000 x 90 = 2.28 mm? If so then its no wonder that my
> > caps cook after only one minute of coil operating, I was using 0.45 mm of
> > poly between plates, oops!
> >
> > What is the recommended thickness for 7.5, 9, 10, 12 and 15 KV poly
> capacitors?
> >
> > I am hoping to build high Q caps from now on and after a fair bit of reading
> > apparently these can improve the capacitor Q:
> >
> > 1. Clean all sides of the poly with meth. spirits.
> Keeps the crud out.
> > 2. Round all foil edges.
> This cuts down on corona.
> > 3. Keep the plates as tight as possible .
> Increases capacitance and decreases chance of bubbles.
> > 4. Add lay sheets of foil between the layers of poly.
> This enhances charge distribution across dielectric layers.
> > 5. Add sheets of oil soaked Craft paper next to each live foil plate.
> This cuts down on air bubbles.
> I add the kraft paper between all surfaces. The idea is to reduce
> air bubbles, and these can form anywhere. It may be overkill,
> but I like the idea of oil being wicked up between ALL surfaces.
> > 6. Provide the biggest terminal ends for each plate.
> Maximizes current transfer to the outside world.
> By the way, I personally do not like the use of "tabs" on the
> capacitor plates. I prefer to use the entire edge of the plate
> as my connection. This requires that the plates extend out
> opposite ends and uses a bit more aluminum foil (whoopie-doo!),
> but it maximizes current path and makes the cap plates easier
> to build. (Making tabs is a pain in the you-know-what).
> > 7. Put the caps in series.
> Above 10KV you really can't make a single cap that will stand up
> to the corona. Over time the dielectric fails, especially at the
> ends. Making many caps and putting them in series makes it
> so each cap has a low enough voltage that corona is not a major
> problem.
> > 8. Submerge the cap in light mineral oil.
> The light mineral oil penetrates more readily between layers than the
> heavy mineral oils. The oil helps in the exclusion of air, which
> in the presence of HV RF results in spot heating and nasty corona.
> > 9. Use equal numbers of + and - plates.
> Otherwise one set of plates is being wasted :(
> > 10. When using caps in series they should be the same individual part
> > capacitances.
> Very important! The capacitors in series will share voltage in
> inverse proportion to their relative capacitance. In simpler terms,
> the smallest capacitor will get the biggest voltage. Zap!
> > 11. Vacuum pressurising the caps.
> Not necessary with most flat cap designs. In fact, a dangerous thing
> to attempt if your case has flat sides! The rolled cap designs in
> cylindrical PVC pipe take the vacuum well, however.
> > 12. Keep all dust and dirt out of the cap.
> Goes right back to #1. Don't let crud get inside EVER! It becomes
> a site for corona generation and mucks up the oil.
> >
> > What is the most useful in improving the Q and what has little affect on the
> > Q, is there anything else that improves the Q?
>
> OK, my own personal thoughts on factors affecting the Quality of the capacitor.
> In groups, with like items together.
> Relative scale of 1-5, where higher is better:
>
> First the things that directly affect capacitance :)
>
> (5) Size of plates: larger is better
> (5) Number of plates: more is better
> (5) Distance between plates: shorter distance is better
> (5) Dielectric material: higher dielectric constant is better
> (5) Dissipation factor of dielectric: lower is better
> (5) Breakdown voltage of dielectric: higher is better
>
> Things that make it work better and live longer
>
> (5) Use multiple thin dielectric sheets between plates
> (2) Use inactive floating plates between multiple dielectric sheets
>
> (3) Low inductance: flat caps are better than rolled caps here
> (4) Low resistance internal connections: larger surface connections are better
> (3) Low resistance external connectors: larger is better
>
> (5) Exclusion of air by using oil (reduction of corona)
> (5) Exclusion of air by using vacuum or compression
> (4) Exclusion of air by allowing it to escape (vertical flat plates best)
> (3) Exclusion of air by using Kraft paper around active plates
> (2) Exclusion of air by using Kraft paper around inactive floating plates
>
> (5) Reduction of corona by using multiple series-connected capacitors
> (5) Reduction of corona by allowing adequate extra poly around end overlap
> (5) Reduction of corona by allowing adequate extra poly around top/bottom
> (2) Reduction of corona by using rounded edges on plates
> (3) Reduction of corona by using adequate container insulation
> (3) Reduction of corona by excluding small foreign matter
> (2) Reduction of corona by excluding large foreign matter
> (1) Obsessive-compulsive cleanliness of all surfaces and materials
>
> Other factors
>
> (3) Size (smaller is better)
> (3) Weight (lighter is better)
> (2) Portability (the more portable the better. No messy oil spills)
>
> (1) Aesthetics (the way *You* think it looks)
> (0) Aesthetics (the way *Other* coilers think it looks)
> (-5) Aesthetics (the way *NON* coilers think it looks)
>
>
> > Normally I roll my caps, but
> > I think that if I should be using a thickness 2.3mm of poly this could be a
> > bit of a task. But perhaps I could make ten 10nF caps and parallel them up
> > to 100nF and then series four sets of them down to 25nF?
> >
> > I would also like to take a look at my coil on the scope when it is running.
> > To do this can I just connect a piece of wire to the 10x probe and dangle
> > the wire in the air about 10 to 15 feet away from my coil
>
> Yes.
>
> > (do I need to
> > connect the scope earth to anything ?),
>
> Yes. Earth. Mr. Ground. The Ultimate Reference point.
>
> > and then fire up my coil, with
> > hopefully my new high Q caps (so that I can watch it for more than a minute
> > at a time). Is this O.K. or is there a safer and better method?
> >
>
> Safest method: Don't build a coil. Watch TV instead.
> Better method: Fire it up and enjoy the sparks. Tweak for maximum Fun Factor.
>
> > Thanks all for your time everyone!
> >
> > Mark
> >
> Mark,
> I hope the above comments are helpful.
> Fr. Tom McGahee
> >
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> >
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> >