
Fres Finder
Fres Finder
When tuning resonant circuits it's often best to just hit the
circuit
with a variable frequency and look for a response, as this saves a lot
of
math and gives a more accurate result. If you want to do it via math
you will need exact component values, but since component values
deviate
from their ratings and homemade components have no values
printed on them this is difficult. To add to the difficulty many
circuits also have considerable parasitic L or C components, greatly
increasing error. Having an extra 7HC14 kicking around I made simple
square wave oscillator, with a frequency range of 50kHz to
1.6MHz, a good wide sweeping range for TCs in my opinion.
The circuit can be
used to tune for both parallel or series resonance. When tuning Tesla
Coil secondaries one looks for the series resonant frequency. This resonant frequency can be found
by connecting the base of the secondary in series with a 10
 50k resistor which goes to the fres finder. The secondary base is
connected to the scope as well. Both the scope and the fres finder are
then grounded to the TC/ earth ground, as shown in the schematic below.
Then by sweeping through the
frequency range and looking for the frequency which gives the lowest
amplitude one can find resonance. Amplitude will decrease as the
frequency increases because of the scope lead's capacitance and fres
finder series resistance, so don't be fooled.
The real resonance will easily stick out from the surrounding
frequencies. When the amplitude increases
near fres you have found a
parallel resonance. Remember that series resonant circuits have their
lowest impedance at resonance, while parallel resonant circuits have
maximum impedance at resonance. With the given circuit layout the
signal amplitude will naturally decrease as it is shorted more and more
to ground. The TC secondary couples to ground through it's topload
capacitance, leaving it connected to ground in a sense. The circuit can
be viewed as a voltage divider, with the TC secondary as the "lower"
resistor. The voltage at the scope probe will decrease as the secondary
impedance decreases.
I haven't needed
to tune for parallel resonance yet, but I would do so by connecting the parallel resonant
circuit in parallel with the fres finder, with a ~1k
resistor in series with the fres finder output and PR circuit. The PR circuit would then be scoped, and a waveform like
this should be present at resonance.
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