**Original
23 October 2000
Revised 27 January 2001**

**PRIMARY
CIRCUIT**

__GENERAL:__**
The primary Circuit in this discussion includes T1, L1, L2, S.G,
Cp and T2. T1 supplies the input AC power, L1 and L2 provide RF
isolation between T1 and the main oscillating components, Cp and
Lp, the primary of T2. The primary of T2 will be referred to as
the Primary Circuit inductor Lp. The Spark Gap S.G. should be
considered an open circuit when C is charging, and as a short
circuit when Cp and Lp are exchanging energy (oscillating).**

**PRIMARY
CIRCUIT CAPACITOR**

**The
Primary Circuit capacitor is the heart of the Tesla Coil. The
amount of electrical energy it contains is determined by its
capacitance and magnitude of voltage charge.**

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**For
example, a 0.037uF capacitor charged to 13kV will contain 3.1
Joules of energy****.**

**PRIMARY
CIRCUIT INDUCTOR**

**In
the Tesla Coil arrangement the Primary Circuit inductor Lp
receives electrical energy from the primary circuit capacitor in
the form of discharging current when S.G. is shorted. This
discharging current goes from zero to maximum at a sinusoidal
rate. At the peak value all energy is transferred and is in the
form of a magnetic field. This energy is expressed as a function
of inductance and current.**

**Peak
current in Lp can be calculated since the energy it received from
the capacitor (3.1 Joules) and its inductance (50uH) are known.
My pocket scientific calculator tells me it is 352 Amperes.
During energy transfer some is lost due to circuit resistances,
capacitor dielectric losses, radiation, etc. so the actual peak
current will be less than that value. **

**OSCILLATION**

**The
transfer of energy from the capacitor Cp to the inductor Lp just
described is the beginning of oscillation. The first 90 degrees
of a complete cycle occurred at a sinusoidal rate. At this point
the voltage in the capacitor is zero. With no capacitor voltage
left the circuit current is at a maximum. The current continues
to flow back into Cp charging it up in the opposite direction at
a sinusoidal rate. The self induced voltage in Lp due to its
collapsing magnetic field keeps the current flowing back into Cp.
This is the energy transferring back to the capacitor. The
current correspondingly decreases at a sinusoidal rate until it
again reaches zero. When it does, Cp will have charged up to a
peak voltage of the opposite polarity and 180 degrees of
oscillation will have been completed.**

**The
above process continues for the next 180 degrees at opposite
voltage polarities and current directions to complete one cycle
of oscillation. If no energy is dissipated in the form of heat
due to circuit resistances and radiation, the oscillation would
continue indefinitely. However, There are always resistance and
radiation losses in the circuit, so the energy will decrease
exponentially which is normally regarded as damped oscillations.
(This is regarded as "Ring-down" in some circles) **

**The
circuit oscillating frequency is determined as follows:**

**For
example, the resonant frequency of a 0.037uF capacitor and a 50uH
inductor is 117kHz Such oscillations would continue indefinitely
if it were not for resistances in the circuit which dissipate the
energy in the form of heat. Energy will also be lost in the form
of electromagnetic radiation which is not desireable in terms of
intereference (EMI). In other words, a damped oscillation takes
place.**** **

**The
values mentioned so far are for Tesla Coil operation. Note that
the capacitor must withstand voltages in excess of 10kV and be
capable of handling charging and discharging peak currents in
excess of 300 Amperes through its terminals. Furthermore, the
dielectric must have very low loss at the RF frequencies involved
The inductor must withstand the same high currents. As such,
these components are not normally available from electronic and
radio parts distributors. That means they must be designed and
constructed to Tesla Coil specifications.**

__LOSSES
AND Q__

**Losses
in the primary circuit are mostly due to its resistive components.
The circuit Q is a measure**** ****of these resistive
components. Q = XL/R where XL is the inductive reactance of Lp
and R is the effective resistance in series with Lp. Q's in
excess of 50 should be strived for. Well constructed Primary
circuits will have Q's of between 50 and 100. Copper tubing coils
and low density polyethylene capacitor dielectric material give
good results. Low density polyethylene sheet material has
excellent RF characteristics.**

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