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RE: TESLA DESIGN

 * Originally By: Froula-at-cig.mot-dot-com
 * Originally To: Richard Quick
 * Originally Re: RE: TESLA DESIGN
 * Original Area: UUCPE-Mail
 * Forwarded by : Blue Wave v2.12

Date: Fri, 20 Jan 1995 14:06:37 -0600
From: Don Froula <froula-at-cig.mot-dot-com>
To: richard.quick-at-slug.st-louis.mo.us
Subject: RE: TESLA Design


>  DF> I have a few specific questions.  I'm planning on building a 
>  DF> 6" coil, 24" winding length wound with 22 ga. enamelled magnet 
>  DF> wire. I was planning on using 60' of 3/8" copper tubing for the 
>  DF> primary.  I am tentatively planning on a single 15,000 V, 60ma. 
>  DF> neon transformer to power it, using a balanced circuit like in 
>  DF> your big coil.  Each leg of the circuit will use two .012 uF, 
>  DF> 25,000 V. oil filled commercial capacitors in parallel.  This 
>  DF> should provide .012 uf of tank capacitance at a DC voltage 
>  DF> rating of 50,000 volts. - Is the rating on the commercial caps 
>  DF> conservative enough for the 15KV transformer?  I could step down 
>  DF> to a 10 or 12 KV unit. 
> 
> Is the commercial cap DC or pulse AC rated? That is the question.
> Wired as you stated, they should hold up even if they are DC rated,
> but even my big pulse rated caps came without any warrantee :-)
> The next question is... Even if they are DC rated and they hold up,
> will they give the best performance? My experimentation shows that 
> a well designed homemade capacitor will blow away most "surplus"
> caps that have the voltage rating. Most surplus caps, especially
> with a DC rating, are not designed to pulse. 

The caps are DC rated at 25,000 Volts.  I decided to do a little phone
investigation this morning to try to find out more about these units.  I spent
some time on the phone with Sprague in North Adams, Mass.  They sent me to a
number of smaller firms to whom they had sold their high voltage cap business.
I was finally referred to one engineer who had been with the company for 30
years or more.  He was retired, but still working as a consultant.  I
outline the application I was planning on using the caps for.  We had a very
enjoyable conversation, and was very helpful. He offered this information:

                - The caps were made about 1965, so are at least 30 years old.

                - They are filled with a very high quality mineral oil (the
                        meaning of the "Vitamin Q" designation).  The oil 
                        includes stabilents and moisture scavenger materials.
                        Absolutely NO PCBs are in this stuff.  That's a 
                        real relief, given the glass envelope.  The oil is
                        still in use today.

                - The units use an impregnated craft paper dielectric and are
                        made up internally of two or more series connected
                        sections.

                - Internal inductance is low, on the order of .5 microH.

                - He cautioned against wiring the units in parallel. A failure
                        in one will cause the energy in its mates to dump
                        into the failure point.

                - He felt that the units would hold up in the given application
                        but cautioned to use a shield for protection.  He has
                        seen dielectric failures near the glass surface shatter
                        the envelope.

                - The units were designed for use as RF bypass caps on high-
                        power transmitters.

        Given this information, I think that I will relegate these to use as
        bypass caps in the protective circuits, and go with a homemade rolled
        polyethylene cap or a surplus commercial unit.  I'll check a few of the
        Chicago-based sources that you mentioned.

>  DF> - Is there any real performance advantage to winding the
>  DF> primary in a saucer rather than a pancake configuration? 
> 
> Primary design is dependant upon the aspect ratio of the secondary.
> Smaller diameter secondary coils with aspect ratios over 3.5 work 
> best with saucer primaries. As the aspect ratio of the secondary
> coil comes down to 3, then the primary needs to flatten out to the 
> pankcake type coil to keep from overdriving the high inductance
> secondary. In general, a six inch diameter secondary needs a saucer 
> shaped primary, an eight inch diameter secondary (and larger) works 
> best with a flat pancake type coil. This assumes you are using a 
> good high Q capacitor.

I'll go with a 30 degree saucer primary.

>  DF> - I was planning on eliminating the bypass caps on the filter
>  DF> circuit, using only a protective gap and chokes for tank 
>  DF> circuit isolation (along with a line filter on the neon 
>  DF> primary).  Is this adequate protection for the transformer 
>  DF> in a circuit like this?
> 
> Not with Tesla's balanced circuit. This particular circuit is the
> worst when it comes to RF kickback, and is hardest on the neon.
> You will have to have bypass capacitors if you use the balanced
> circuit. If you put the spark gap in series with the primary, you
> can get away, maybe, without bypass caps. 

See above...

>  DF> - You mentioned excessive kickback unless the capacitive
>  DF> values of the two tank legs were carefully balanced. 
>  DF> How important is this in a coil of this power level?
> 
> How important are your neons at this power level? 

Enough said!!

>  DF> - I was planning on using your "air blast" single quenched gap
>  DF> design from the GIF.  Will this supply reasonable performance 
>  DF> for this coil?  How about using a high-power vacuum cleaner 
>  DF> blower fo quenching?
> 
> Compressed air gaps give wicked, wicked, performance. Unbeatable
> with neons, and they work with heavier xfmrs as well. But they
> have drawbacks: noise, compressor requirements, and dielectric
> stress on the capacitors. Wicked performance in the gap puts great 
> demands on the capacitor. The cylinder series static gap, assembled
> with seven parallel electrodes spaced at about .028 -.030 inches
> will work well, and does not have the drawbacks of the compressed
> air quench gap. The cylinder series static gap in the video (the
> one that did not quench well) was designed as a single component 
> in a sophisticated gap SYSTEM; the system had about a dozen static 
> gaps, plus a rotary, and worked extremely well. A single cylinder
> static gap designed to work alone is quite serviceable in these
> powers levels and give good, solid, reliable, cheap, performance.
> Vacuum gaps are also excellent performers, but they are a bit
> harder to engineer. 

I'll go with the series design in the GIF.  Is this OK with a 15KV neon,
        or do I need two units?  Can I increase the gap on a single unit to
        work at 15KV with a single series gap unit?
> 
>  DK> - How important is the variac for tuning a coil of this size? 
>  DK> I have a 7500 V, 30 ma unit I could use for lower power
>  DK> tuning.  Can I get by without?
> 
> Trust me. I ALWAYS use a variac, a good ground, and a line filter.

The Variac is back on the must-have list.  Newark's price is too dear, but
I'll canvas the engineers around here...someone probably has a suitable unit
in the basement.

By the way, the address for Jefferson Electric in Downers Grove that was given
in your list of resources has changed.  They are now located in Elk Grove
Village, Illinois - phone is (708) 806-6500.

Thanks again!  I want to work out the realities of this project in my head
so I know how to budget my time, money, and other resources.  Four children
demand tight budgeting in all of these areas!

Many thanks,

Don, WD9DMP
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