**Original
date****.
****21
October 2000
Last revision 24 January 2001**

**MEASUREMENTS**

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**The
following measurements are described on this page:**

**1.
Secondary Coil Inductance**

**2.
Self resonant frequency of Secondary Coil**

**3.
Resonant frequency of Secondary Circuit with Terminal attached**

**4.
Resonant frequency of Primary Circuit**

**5.
Frequencies for determining coefficient of coupling (k)**

**6.
Ring-up and Ring-down Oscilloscope observations**

**Note:(
the coils undergoing the following measurements should be a metre
or more away from obstructions)**

**1.
MEASURING SECONDARY COIL INDUCTANCE**

**The 0.1uF capacitor and the coil
form a series LC circuit. Vary the signal generator frequency
until a maximum signal is observed on the oscilloscope and note
the frequency. Calculate L using the formula given above with the
measured frequency and the 0.1uF capacitance. The resonant
frequency of the above circuit measured at 2250Hz.**

**2.
MEASURING SELF RESONANCE**

**If
the secondary coil is mounted with the primary coil, the primary
coil must not be connected to its capacitor to form a primary
resonant circuit. The open spark gap will suffice for this
condition.**

**Vary
the Signal Generator frequency until the signal observed on the
oscilloscope is at a maximum. The coil described in the Secondary
Coil Construction page was used in this measurement. It was self
resonant at 155kHz. Calculate the coil self ****capacitance using the measured frequency and
the calculated Coil inductance (50mH) the formula at the right.
The self resonant frequency of the 50mH coil measured at 155kHz.
The calculated self capacitance worked out to 21pF**

**3.
MEASURING SECONDARY CIRCUIT RESONANT WITH A TERMINAL**

**The
same set-up was used to measure Secondary Circuit resonant
frequencies with different Terminals. A flexible 110mm diametre
aluminium pipe forming a toroid provided a resonant frequency of
117kHz. Tuning was achieved by stretching and compressing the
flexible pipe. Using the formula for C above with the measured
frequency and 50mH total Secondary Circuit capacitance calculated
to be 37pF. Subtracting from this value the self capacitance (21pF)
gives a Terminal capacitance of 16pF.**

**This
is a special case of one particular coil which matched a
particular primary circuit frequency. The same technique can be
used for many different size coils and frequencies.**

**4.
MEASURING PRIMARY CIRCUIT RESONANT FREQUENCY**

**The
Primary circuit must be isolated from the secondary circuit for
this measurement. Simply remove the secondary coil. Connect a
short jumper wire across the spark gap to place the primary coil
in parallel with the primary capacitor. The set-up for doing this
and making the measurements is shown at the right.**

**Adjust
Signal Generator for maximum amplitude on the Oscilloscope and
read the Primary Circuit resonant Frequency on the Counter. Q of
the circuit can be determined by measuring the frequencies which
give an output of 0.7 x maximum level obtained at resonance on
the oscilloscope. (-3dB points) Dividing the difference between
these two frequencies into the resonant frequency yields the Q.
The actual Q will be slightly higher because of the 10k resistor
in the measuring circuit. A Q of about 50 should be expected.**

**5.
FREQUENCY MEASUREMENTS FOR DETERMINING COUPLING COEFFICIENT(k)**

**The
diagram on the right shows the set up for locating the two
frequencies associated with a tuned RF transformers. The primary
and secondary coils are in their normal configuration concentric
to one another, but the spark gap is jumpered across. . For these
measurements to be valid, the primary and secondary circuits must
be independently resonant to the same frequency as measured in 3.
and 4. above. (The Oscilloscope probe is attached to a metre of
wire and dangled about 1 metre from the secondary coil. )**

**Locate
two frequencies which give a peak signal on the oscilloscope. One
of these frequencies will be above the original resonant
frequency and the other will be below it. These three frequencies
can be used to calculate the coefficient of ****coupling
(k) using the formulas at the right. If the values of k are
significantly different (about 10% or more) using the two lower
expressions, either the primary or secondary circuits were not
tuned to the same frequency**** ****f****r, ****or
there was an error in measurements. **

**Source
for these formulas: ****RADIO ENGINEERING HANDBOOK HENNEY**

**..................................................****3rd Edition 1941
page 149**

**6.
RING UP AND RING DOWN MEASUREMENTS**

**The
ring up and ring down measurements involve: **

**1.
replacing the high voltage transformer (Neon sign transformer
secondary ****....****winding) with a 680 ohm charging
resistor and a 12 Volt dc power supply.**

**2.
Substituting the spark gap with the contacts of a vibrating reed
relay **

**3.
Observing waveforms in the Primary and Secondary circuits with a
dual trace ****....****oscilloscope**

**The
circuit I used is shown below:**

**The
audio oscillator was set to 250Hz which resulted in K1, a reed
type relay to vibrate at the same rate. With K1 contacts open,
the Primary Circuit capacitor charged up through a 680 ohm
resistor to 12 Volts dc. When the contacts closed,**** this capacitor dumped its charge into the
Primary Circuit coil and commensed to oscillate. At the same time
a negative sync pulse triggered the Oscilloscope. e The Primary
and Secondary Circuit oscillations were fed to A and B
oscilloscope inputs. In this circuit the trigger occurred about
50 milliseconds ahead of K1 contact closure. The built in
oscilloscope trigger delay allowed for wave form centering and
easy observation. **

**The
circuit above is not an optimized design. It was based on what I
had in my junk box (static assets) at the time. I did experience
relay contact bounce, but I was still able to see energy moving
back and forth between the Tesla Coil Primary and Secondary
circuits. A more sophisticated solid state design using MOSFETs
with milli-ohm source to drain resistances when switched on would
be much better. The effects of spark gap quenching could be
simulated using appropriate MOSFET input waveforms. **

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