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Help needed for primary...

>Date: Sat, 10 Oct 1998 11:23:13 -0600
>To: tesla-at-pupman-dot-com
>Subject: Re: Help needed for primary...
>From: Tesla List <tesla-at-pupman-dot-com>
>
>Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>=20
>
>
>Making a smaller primary will have virtually no effect on overall coil
>efficiency. Substantiall reducing the turn-turn spacing will
>significantly increase the odds of a primary turn-turn flashover, will
>require more primary turns to achieve the same primary tune-point, and
>will increase the coupling coefficient somewhat. With the smaller
>diameter primary, you may need more than 16 turns to achieve the same
>tuning point, all other things being the same. HV RF will jump a longer
>distance in air than low frequency AC or DC, particularly if you've got
>any surface imperfections that promote corona - it would be quite
>difficult to prevent turn-turn flashovers at a 1mm turn-turn spacing.
>
>
>You should be able to reduce the turn-turn spacing to 1/4" if you're
>running below 5 KW, and smaller spacings at lower power levels, and
>there's only marginal space advantage in trying to reducing this below
>1/8". You will need to think about how you can easily change the "tap"
>on the primary without reducing clearances or shorting to adjacent
>turns.=20

  Ok. So I guess I'll just stick to my original design... Are you sure 
there is enough voltage difference from turn to turn to cause sparks to 
form between the turns? Since the primary is basically a short circuit, 
I would expect it to have the same voltage all around...


>Wood is a very poor insulator for HV and RF. Because of this, most
>people actually make their coilforms out of some type of plastic or
>dried and well-sealed cardboard tube (Sonotube). Typical plastic
>coilform materials used include PVC, ABS, HDPE, Polycarbonate, 
phenolic,
>or acrylic (PMMA). While it IS possible to make a coilform from wood if
>the material is thoroughly dried and well sealed throughout with
>multiple coats of polyurethane, this should be done only as a last
>resort. There are MUCH better materials available now - Tesla would 
have
>immedialtely switched over to PVC and LDPE had they been available!  
:^)


 But is the primary voltage large enough to pose a problem? Or would my 
dry, varnished, Plexiglas covered coil form be good enough? I need this 
coil to be very efficient!



>Throw away any sparklength versus voltage tables you may find - they
>simply don't apply to Tesla Coils - there's no simple relationship!
>Sparklength is a complex and not fully understood function of voltage,
>frequency, toroid size and voltage standoff characteristics, breakrate,
>etc.=20
>
>However, you can estimate the maximum voltage your coil can output if
>you know a few things about your system.=20
>
>   Vout (max) =3D Vgap*SQRT(Cp/Cs)
>
>where Vgap is the gap breakdown voltage, Cp =3D primary capacitance, Cs 
=3D
>combined secondary and toroid capacitance. Alternatively, if you know
>the primary and secondary inductance you can also estimate Vout as
>
>   Vout (max) =3D Vgap*SQRT(Ls/Lp)
>
>As a refernce point, coils generating 5-7 foot streamers typically
>operate at voltages of only 300-400 kV. Only very large, high power
>systems can be expected to break 1 MV. A reasonably efficient system
>should be capable of achieving 1 meter long sparks with 500-700 watts.
>Your mileage may vary... :^)

 Some time ago (not very long) I saw this spark length/power input table 
based on actual Tesla Coil Results on this list. Could someone re-post 
it for me???

>A coil capable of achieving 1 MV can deliver substantial amounts of
>current! The topload "looks" exactly like a capacitor, and like any
>capacitor, can store energy. A topload that can support a buildup to 1
>MV will be quite large, and will have significant capacitance. The
>current that's available will depend upon how rapidly you discharge 
this
>capacitance - a streamer may discharge it over a period of several
>hundred microseconds, while an arc to ground may do so 100 times more
>rapidly. Maximum steamer currents may be in the 1-10A range, while a
>power arc can easily be 10-20X greater (but last for a much shorter
>time). Because these discharges only exist for a relatively short time,
>the average system power is much lower - but still 10's of KW for a 
1MV+
>system.


>Since the Tesla Coil output is RF, you'd need to find diodes that can
>turn off quite quickly. Most recitifiers are made for low frequency
>power and won't do the trick. TV rectifiers are very low current, and
>may still not be fast enough for your higher-frequency coil. Long
>strings of HV avalancehe diodes could do the trick if immersed in oil.
>However, this is far from a trivial problem to solve, and it won't be
>cheap. Your best (most cost-efficient) alternative for reaching 100's 
of
>kV DC would be to use a HV electrostatic generator, like a Van de Graaf
>or Wimshurst machine. Try "Homemade Lightning" by R. A. Ford (Tab 
Books)
>for construction plans.


 I know how to build a Van de Graff, but what I really need is a high 
power, continuous DC source... What kind of diodes would be ideal for 
this purpose. Anyone knows what the output would be like???


 Thanks a lot for answering all these questions!!!


 Sam Barros.

 BTW, I have a 8inches diameter, 25 inches winding height, 40inches 
toroid/strike rail clearance primary, wound with 1000 turns of 22 
(0.60mm) thick magnet wire secondary and I will build an 18 turn, 1 inch 
primary-secondary clearance primary, wound with 0.32inches diameter 
copper tubing and 0.32 inches turn to turn spacing. I also have a 20uF 
toroid. My secondary has over 1mm thick insulation, with epoxy, enamel 
and polyurethane. Any idea how much power such a system could handle? 
What would the voltage/amperage output and streamer length be?

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