Cap Location

From: 	terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
Sent: 	Sunday, January 11, 1998 10:16 PM
To: 	Tesla List
Subject: 	Re: Cap Location

        Put the gap across the transformer, and the high voltages induced by
the primary coil may hurt the transformer (especially if it miss fires).
Put the cap across the transformer, and the high frequency high voltage
signal is place directly across the neon burning the secondary winding.  An
interesting dilemma.  
        My thoughts are:
                If the cap is across the transformer, the high-frequency /
high-voltage signal will kill the neon for the following reason.  The neon's
output terminals are basically very large value inductors.  The
high-frequency voltage will be distributed very unevenly across the outer
windings of the neon's secondary coils.  At 60Hz the voltage is evenly
distributed throughout the winding but at say 200KHz the voltage will be
mostly blocked by the inductance of the neon's secondary winding resulting
in most of the voltage drop on the last few turns of the secondary winding.
Then pop!...neon's got burnt windings.
                With the gap across the transformer, if the gap opens under
a high current condition (rather unlikely but....), the voltage induced by
the TC's primary inductance my go very high and also blow the insulation.  
        This is why we play with inductors, resistors, MOV networks, etc. to
try and block these nasty signals from the neon.
        With the gap across the transformer, a spark gap should be able to
save the neon from the spikes and you should be OK.  With the cap across the
transformer, you will need inductors, resistors, and other messy to design
parts.  Can't make any mistakes with these or the neon gets fried.  
        You must also be cautious of the output of the neon and the primary
capacitor resonating and producing higher voltages than you want.  My 15KV
60mA neon and 0.01725uF primary cap can reach 15KV at only 40 volts AC in
due to this.  If I were to put 120VAC into the neon it would hit about 45KV
and the cap or the neon would blow.  
        To make a long story short, go with the gap across the transformer
and use a good spark gap across the transformer to catch any induced spikes
during the break.

        In a related note, my real-time fiber-optic voltage and current
probe is now working properly after many months of dealing with bandwidth,
noise, finding parts, etc.  It can easily measure the situations we have
here.  I have been trying it in a number of situations and I will soon have
it hooked to a full power primary circuit which will easily show the
differences between the two situations described above.  I will report the
results as available.  I will try the circuits without the secondary in
place for now since I have calculators, laptop, digital scope, etc. laying
all over the place.  Not ready for any 4 foot arcs right now :-) I may need
to build a new spark gap first.  According to the probe, my present gap
really sucks!  Works for a few pulses then just acts like a continous short.
You can easily hear the transition from proper arcing to poor shorting once
you can see it on the scope as it is happening.  Very interesting stuff!!


>From: 	John H. Couture[SMTP:couturejh-at-worldnet.att-dot-net]
>Sent: 	Saturday, January 10, 1998 11:50 PM
>To: 	Tesla List
>Subject: 	Re: Cap Location
>At 10:13 PM 1/10/98 +0000, you wrote:
>>From: 	Greg Leyh[SMTP:lod-at-pacbell-dot-net]
>>Sent: 	Saturday, January 10, 1998 5:46 AM
>>To: 	Tesla List
>>Subject: 	Re: Cap Location
>>> Hmmm,
>>>  Mr. Cox says put the gap across the transformer, not the cap.
>>>  Mr. Couture says put the cap across the transformer, not the gap.
>>> I respect the opinion of both of these gentlemen, and my own experiences
>>> have been inconclusive either way. I can think of arguments for either
>>> method being superior.
>>> Anyone else have an opinion?  Discussion?
>>The operation of the primary circuit is of course the 
>>same for either scheme.  However, the spark gap placement 
>>has an _enormous_ impact on the charging circuit. 
>>It is preferable whenever possible to put the _spark gap_ 
>>across the transformer, so that when the primary circuit 
>>fires, the only voltage that gets kicked back to the xfmr 
>>is the IR drop of the gap, rather than the full voltage 
>>swing present on the primary capacitor.  This results in
>>at least a 10 to 1 reduction in the RF voltage thrown back
>>to the xfmr.
>  Greg, All -
>  Tesla in the CSNotes said he thought the gap across the transformer was
>better than the cap across the transformer as you and Mr. Cox suggest. He
>based this on the fact that the current from the cap did not have to go thru
>the gap to get to the TC primary winding. However, he did not make
>comparison tests to determine which method was preferable. Also, he did not
>address the problem of the gap OPENING.
>  The TC primary circuit contains an inductance. When a switch (gap) opens
>on an inductive circuit with current, there can be a large voltage transient
>created. This voltage is   V = L di/dt   and can be high enough to break
>down the secondary winding insulation in the power transformer. I believe
>this is what breaks down neon transformer insulation.
>  When the gap is across the transformer and it opens the TC primary
>circuit, there can be large voltage transients formed in the power
>transformer secondary winding. This voltage can damage neons and under
>special conditions even damage pole transformers. When the cap is across the
>transformer with a safety gap the safety gap will provide voltage limiting
>for the transients comimg from the operating spark gap.
>  The problem is we do not know how to design a spark gap to keep these
>transients under control. A few years ago when I saw the "rotgit" program by
>Jamison I thought it could be used to solve this problem. It is an excellent
>simulation of the spark gap but is based on theory only and I was not able
>to find a method to relate it to real world spark gap design.
>  It is obvious that more research is needed in this region of TC operation.
>As I mention in one of my books the design parameters that need to be
>coordinated in a computer program are the spark gap and primary capacitor
>charging  characteristics, and several other parameters. If a rotary spark
>gap is used the RPM, number of electrodes, breaks/second, etc, would also
>have to be considered. In fact this program would be more complicated than a
>program to design the Tesla coil.
>  John Couture