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DESIGN QUESTIONS

 * Original msg to: Esondrmn-at-aol-dot-com

 Es> From: Esondrmn-at-aol-dot-com
 Es> Subject: design questions

 Es> Richard,
 Es> I hope you don't mind, but <I have> several more questions.

 Es> Spark gap - I had planned to make this from brass screws or rods
 Es> turned down to points on the ends.  I saw a comment from someone 
 Es> where you mentioned holes in the face ends?  Instead of points?  
 Es> What diameter? 

Never use points: parallel flat plates, parallel sections of hard 
copper water pipe, large diameter steel ball bearings placed close 
together (conductive spheres) even the large flat heads on some brass 
bolts... but never points. Point gaps break down at too low a voltage 
and are all but impossible to quench. The get hot, they wear out fast, 
they have high resistance... they have everything you DON'T want in a 
spark gap. 

 Es> On the primary, with 3/8 copper tubing - is 7/8 centers ok?  
 Es> This leaves 1/2 inch air gap between windings.

Nahh, I would go with 1/4 or 3/8 inch spacing between the turns on
the primary conductor. I use 1/2 inch spacing between turns on my 
large primary with 20,000 volts through the coil.

 Es> I need a discussion on EMI & RFI filtering.  I know I need a good
 Es> isolated ground.  What size wire to the ground rod?  #12 AWG ok? 

Try some 1/2 inch braided tinned copper ground strap. Make sure
the ground itself is equal to the load that the strap can carry.
 
 Es> I plan to connect one side of the primary to the lower end of 
 Es> the secondary.  

This circuit is not good for beginners. It is very difficult to tune,
and it allows potentially lethal 60 cycle currents to contaminate the 
clean RF of the secondary discharge.

 Es> I think I still need an RF choke in series with the other output 
 Es> from the transformer.  

You have got me lost here. The high voltage outputs on transformers
are isolated from the transformer case with an insulator known as
a "bushing". For every bushing connected to a coil circuit you 
should place a ferrite core RF choke in series, between the bushing
and the coil. One bushing connected, one RF choke; two bushings,
two chokes...

 Es> 15,000v probably 60ma transformer.  

If you use this transformer with the circuit you mention above you
will only be able to use one side (50%) of the transformer output. 
You have to ground something. You cannot ground the base of second-
ary to the primary and not ground one side of the primary. If you
ground one side of the primary you cannot use both high-voltage
bushings on a neon sign xfmr. Are you aware that all neons have
a grounded center tap between the high voltage bushings? You need 
to look at a another circuit. 

 Es> Do I need a small (300 to 500pf) capacitor from the output to 
 Es> ground also?  

A small capacitor from the HV bushing to ground is a good idea.
 
 Es> Do I need RF chokes in series with the 120V AC line to the 
 Es> transformer?  What design parameters?  I had planned to put 
 Es> .01mf 1000v ceramic capacitors from each AC line input to 
 Es> ground - OK? 

Somewhere between the breaker box and the step up transformer you 
need a filter. I think that for the time and money I would shop 
around for some surplus heavy-duty line filters. You simply 
place them in the line somewhere between the breaker box and the
step up transformer. Place them in the line backwards. These line
filters are marked on the case: one side says "LINE", the other side
says "LOAD". Put the load side towards the breaker box. Put the line
side towards the step up transformer. These line filters are designed 
to filter RFI out of a piece of equipment, we are trying to keep the
RFI out of the breaker box.

 Es> I understand your plans on the torroid.  How do you mount it 
 Es> to the secondary?  On a plastic spacer?  

Yes, or a porcelain insulator, either simply placed or glued to
the top plate of the secondary coil.

 Es> How tall? 

This is determined by experiment, looking for the best performance.
The best distance is different on every coil and depends on the 
coil, the size of the toroid, the coupling coefficent, and the power 
level of operation.

 Es> How do you make the electrical connection to the secondary?

The top of the coil is left with an extension of wire. This wire is
wound through the air, just like an extension of the secondary winding,
but with much greater spacing. Usually two to four turns the same dia-
meter as the coil, until it touches the botoom plate of the toroid. The 
wire is then brought to the center of the toroid and a connection is
made with a small clamp, nut, etc..

 Es> I have seen the schematic drawn with the spark gap in two 
 Es> different places. I plan to connect the transformer directly 
 Es> to the primary and to ground on one side. The other side con-
 Es> nects to the primary coil through the capacitor with the spark 
 Es> gap on the side of the capacitor that is connected to the trans-
 Es> former.  i.e. the spark gap is across the output terminals of the 
 Es> transformer (after the RF choke if we use one). Is this what you 
 Es> reccommed?  Thanks,  Ed Sonderman

What you should do is this: put a ferrite choke on the end of each 
bushing on the neon. Take a wire from each choke and connect it to
one side of the main system spark gap. Now your gap is across the 
line, your line is choked, and everything so far is wired with a 
low current feed line. Take a high current tank circuit connector
from one hot terminal of the spark gap to the coil. Take the other 
end of the coil and connect it (with a high current connector) to 
one terminal of your pulse discharge capacitor. Take the second 
terminal of your capacitor back to the spark gap with another 
high current connector. This wires your tank circuit.

The base wire from the secondary coil is connected to the dedicated
"system ground" via the 1/2 inch braided ground strap. The tank 
circuit is floating ungrounded, which is just fine. The tank circuit
will oscillate quite well without a ground connection. For safety's 
sake we use a safety gap on the back end of the tank circuit. The
safety gap has a grounded center post which is connected to system 
ground via the same 1/2 inch strap that grounds the secondary base 
wire. The safety gap is placed across the two high voltage bushings 
of the step up transformer, behind the ferrite core chokes. This
way in the event of a circuit imbalance, or a kickback towards the
step up transformer, the tank circuit can find a low impedance
ground path where then energy can be dumped without damaging anything.

Richard Quick


... If all else fails... Throw another megavolt across it!
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