Cap Location

From: 	Antonio Carlos M. de Queiroz[SMTP:acmq-at-compuland-dot-com.br]
Sent: 	Monday, January 12, 1998 12:03 AM
To: 	Tesla List
Subject: 	Re: Cap Location

John H. Couture wrote:

>   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.

Strange. In any configuration the three elements capacitor, gap, and primary
inductor are in series. The difference is where the power transformer is.
>   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.

I think that this can really happen in a rotary spark gap. In a fixed gap
the gap would just fire again as soon as the current in the primary circuit
charges the parasitic capacitances. The voltage will never exceed the normal
firing voltage of the gap. Anyway, this is a good reason for the filter
connecting the power transformer to the primary circuit.
This also makes me think about the following: Several people report seeing
oscillations in the MHz region while measuring waveforms in Tesla coils
with oscilloscopes. Can these oscillations can be caused by a parasitic LC
tank formed by the primary inductor and parasitic capacitances that appear
when the gap opens?
>   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 capacitor alone is a good protection against overvoltage. It can't charge
to more voltage than it had at the start of the cycle.
>   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.

The spark gap model used in that program is not documented, but appears to
be very simple, just discharging completely the capacitor as soon as it fires.
The primary inductor is not considered. The firing voltage is proportional to
the distance between the gap electrodes. That model is useless for the
complete circuit.
>   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.

I agree. The first thing needed is a good model for a spark gap. This is
very probably already known, but I really don't remember seing a sound model 
I imagine a model with firing voltage and minimum current depending on the
temperature of the gap, with the temperature depending on a lossy integral of
the power dissipated in the gap. This is simple to model with the elements in 
most circuit simulators.

Antonio Carlos M. de Queiroz