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Re: How do you measure coupling
Original poster: "Mike Tucknott" <michael.tucknott-at-virgin-dot-net>
Thanks for the reply, I should be getting the scope and sig gen some
time over the weekend, I`ll show Brian your post and hopefully we
can start to get some measurements of the coil..
Thanks Mike Tucknott
> Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
> Tesla list wrote:
> > Original poster: "Mike Tucknott" <michael.tucknott-at-virgin-dot-net>
> > How do I measure the coupling between the primary and the secondary.
> > I can get hold of a sig gen and a scope, I`v no idea how to operate
> > two bits of kit, I need the basics and some theory..
> A method that takes everything into account, and also tunes the
> Assuming a conventional Tesla coil (magnifier below):
> Assemble everything, primary coils and primary capacitor,
> secondary coil in place with top terminal and ground, spark gap,
> but do not connect the power supply (that would be across the
> spak gap).
> Connect the signal generator across the gap, and connect a small
> resistance across the gap too (as 1 Ohm). This is to lower the
> output impedance of the signal generator to the order of the
> effective resistance of the gap.
> Set the signal generator to a square wave with a frequency around
> 500 Hz, with the highest amplitude.
> Connect the oscilloscope across the primary coil, with the trigger
> input connected to the output of the signal generator (or to a
> trigger output if it has one).
> At each transition of the square wave, you will see a waveform as
> This is the same waveform that would appear at each discharge of
> the primary capacitor, scaled down.
> Adjust the primary tap position until you see full notches, as in
> the picture. The system is then correctly tuned.
> You can take streamer loading into account if you connect a wire
> with the expected shape to the terminal, or simply approach your
> hand to the terminal.
> Count the number of cycles b in each of the beats, between the
> notches (about 8.5 in the picture). Let a=b-1. The coupling
> coefficient is then k=(b^2-a^2)/(b^2+a^2), and the coil is
> operating in mode a:b, meaning that it resonates at two
> frequencies with the ratio b/a.
> If the output of the signal generator is too small, making the
> measurement too noisy, you can try to make a "tuner" circuit:
> For a magnifier, follow essentially the same procedure. But a true
> magnifier has another point to tune, that is the capacitance
> across the secondary coil. If the system is properly tuned,
> the voltage across the secondary coil when the voltage at the
> primary coil is zero is zero too. See the waveforms at:
> This voltage can be measured with a 10x probe in the oscilloscope.
> Increase or decrease the capacitance across L2 until you see
> coincident notches at the primary and secondary voltages. This is
> not difficult if the magnifier is designed considering this from
> the start. If you will use my formulas, choose a mode of the
> type a:b:c, where b=a+1 and c>b+1. This results in smaller
> capacitance across C2, that is easier to obtain with distributed
> capacitances. The program mrn6, at:
> can evaluate the formulas for you and plot the expected waveforms.
> Or, if you just want k, simulate the structure. Programs as Acmi,
> Fantc, or my Inca (same site above) can calculate k from the
> geometry better than you can measure it.
> Antonio Carlos M. de Queiroz