# Re: Am I Right ? (fwd)

```

---------- Forwarded message ----------
Date: Wed, 15 Jul 1998 10:09:36 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Am I Right ? (fwd)

Hi Fred,
Over the last few years I have been investigating and
experimenting, I have evolved my own general way of looking at TCs
which I find extremely useful in designing a system to meet particular
requirements.

> Date: Tue, 14 Jul 1998 02:36:43 -0400
> From: quantum3 <quantum3-at-geocities-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Am I Right ?
>
> I think I understand now what is going on inside the Tesla coil, but
> correct me if I am wrong.
> We need to achieve maximum current transfer through the primary coil
> (Tesla coil). To achieve this we  need to achieve resonance between the
> primary and secondary coils. Maximum current transfer occurs only at
> resonance where the impedance is at the lowest level. With nearly zero
> resistance  to current flow, maximum field can be achieved and induced
> directly into the secondary winding.

Since overcoupling in the RF sense needs to occur for a coil to work,
one can take as granted that energy from the primary cap (minus
some losses) ends up totally in the secondary and terminal self
capacitance. From this point of view, di/dt doesn't really enter the
picture at all. One can nominate an output voltage, look at one's
available power supply, then choose a value of secondary self
capacitance to produce a particular output voltage. Exactly what
proportion of secondary capacitance ends up in the terminal as
opposed to the coil is up to the builder. We can at least calculate
the capacitance of the secondary coil and nominate a value for the
terminal to satisfy our requirements. Primary L/C ratios only matter
from a loss point of view. From an output voltage point of view, one
can say "I have 5 Joules to play with, I want to get to x00,000 Volts,
what Csec do I calculate to get there?".  This leaves a lot of
latitude for choosing both primary and secondary values while all the
time being able to live within a design constraint. Note also that in
nominating an output voltage, one effectively nominates a minimum
total height for the secondary system as well. I say "total" because
in the magnifier, a proportion of the output voltage is generated by
the secondary with the rest being left to the free resonator. Using
this approach, one doesn't even need to choose an operating frequency
beforehand. For example, knowing the value of my primary cap, I could
decide on system frequency by nominating a surge impedance for the
primary (e.g. 50 Ohms). Both primary inductance, frequency and
seconday inductance follow naturally from this.
Further to this, one can nominate a primary energy (Joules)
tradeoff for BPS to abtain a specific power throughput (Ep x BPS).

> The secondary winding having many more turns then the primary, converts
> all available current to voltage (ohms law). So resonance regardless of
> coil dimensions must be achieved regardless of all other factors, though
> they are important. The spark cap and the capacitor provide resonance
> for the primary windings. If I look at this from a different angle
> however (for example; audio circuits), most of the circuits contain a
> capacitor also on the secondary winding to adjust the resonance on that
> side of the circuit too. However in many designs of Tesla coils I do not
> see this (cap on the secondary). Why is this ?

The spark gap is just a high current switch. The secondary
capacitance is there alright. Part of it is inherent in the resonator
and the rest can be obtained with a suitably sized and shaped top
terminal.

Malcolm

```