Re: How do you measure coupling

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike Tucknott" <michael.tucknott-at-virgin-dot-net> 

Hi Antonio

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
these
 >  > two bits of kit, I need the basics and some theory..
 >
 > A method that takes everything into account, and also tunes the
 > coil:
 > 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:
 > http://www.coe.ufrj.br/~acmq/tesla/tefpreg1.jpg
 > 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:
 > http://www.coe.ufrj.br/~acmq/tesla/tuner.gif
 >
 > 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:
 > http://www.coe.ufrj.br/~acmq/tesla/mag345.html
 > 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:
 > http://www.coe.ufrj.br/~acmq/programs
 > 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
 >
 >