Capacitor Design Question

 * Original msg to: Tahoetom-at-best-dot-com

Quoting tahoetom-at-best-dot-com (tom renko):

> Richard; My friend and I have built a Tesla coil with plans 
> from B&L Scientific. It is a air core transformer, designed to
> produce 600,000 volts. I'm using a 15,000v neon xfmr -at- 60 ma. I
> bought four caps from a local aviation surplus store, which 
> were rated at the proper voltage per the plans. They promptly 
> exploded. 


I purchased my first plans from B&L in the early 1970's. The coil
project ended up a disaster and B&L was solely responsible for my
failure to produce GOOD sparks for a full ten years. Shortly
after I lost my plans from them (a good thing) they moved and
left no address (another good thing). I still have things to say
to them and I want their address.

> The following dialoge is from my friend who is trying to help 
> me save this project:

> " I have read your posted Capacitor info dated 28 Aug 1995. I 
> was most interested in the part about the plate capacitors that
> Bill Richards of TCBOR designed. I really like the idea of the
> vertical plate capacitor because: it does not require a vacuum
> pump, and that it is essentially modular in design, and can be
> quickly inspected and/or repaired(unlike the rolled cap design)

> Assuming that I stick with the vertical plate design, there are
> two ways to assemble the plate/dielectric assembly. One of the
> designs uses 90 mil Polyethylene dielectric with the plates 
> wired in parallel and the other design uses 2 layers of 3 mil 
> "zip lock bags" with the many plates stacked in series to get 
> the required voltage rating. As of now, I favor the full 
> thickness 90 mil design, because of the simplicity of 
> construction and the "tune ability" of the design.

> One of the ways I can tune the Tesla tank circuit is by making
> the capacitor larger than called for in the plans (our plans 
> call for (four) .05uf caps in a series). I could then put the 
> cap into the Tesla,  operate it and observe the sparks. I could
> then off-set the plates, to lower the plate area overlap to 
> lower capacitance and again operate the Tesla coil. The sparks
> should lenghten as I approach resonance. As I keep doing this,
> and pass resonance, the sparks will start to decrease, and I'll
> know I went too far. The problem with this approach is that 
> I'll have to over-build my cap to allow room for plate 
> movement. This extra room would require extra (expensive) 
> mineral oil to fill. The other way I thought of to vary
> capacitance is to build several smaller caps, and add or 
> subtract caps as needed. 


This was a serious problem with B&L plans and theory: they lead
you to believe the best way to tune is to change the value of the
tank circuit capacitor against a fixed secondary frequency and
primary coil (of about 5.5 turns). This led me to design
capacitors exactly as you are doing, and this is NOT the correct
way to design, tune, or operate. B&L and I need to talk.

The correct way to tune is to build a large primary coil,
allowing lots of turns flexibility (this keeps your capacitor
value small and allows lots of frequency flexibility) and to use
various sized discharge terminals on your secondary coil which
allows you to reduce your natural 1/4 wave resonate frequency of
the secondary winding. 

I have found the B&L secondary design to be severely lacking.

> Now that you know some background, I have a couple of questions
> that I hope you could answer for me. The most important one is:
> Given a 90 mil Polyethylene dielectric plate type capacitor
> submerged in mineral oil; WHAT IS THE PLATE SURFACE AREA I 
> WOULD NEED TO GET .01 uf OF CAPACITANCE? I have tried 
> calculating the area using textbook  formulas, but I have to 
> make too many assumptions to get a reliable answer. Even better
> than the calculated value would be an actual caps specs.

We need to talk on the phone.

> We can be reached by fax at (510) 577-5026   Tom Renko
> Work phone----------- (510) 577-5398   Tom Renko 
> 7:00am-3:00pm pst E-Mail--------------- tahoetom-at-best-dot-com

> P.S. Richard, send me your fax, and I'll send you a copy of the 
> plans from which I built the coil.

Richard Quick

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12