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Re: Bang the rocks together harder lumpophiles

Hi Bob,

	I have been looking over your test idea and had a few questions.

I can set up the aluminum foil on a concrete floor easily enough but I
assume I should slit it so as not to have the eddy current / shorted turn
effects.  I figure the ground point can be in the center.  I want to keep
the coil vertical to represent a real Tesla coil case.

I have a <1 ohm output generator but it is 1 volt at 20mA so driving a coil
directly over currents it.  It is active output so there is no real output
impedance as a normal generator would have (a marvel of science).  Your
isolator should take care of this perfectly!  I can put a 50 ohm resistor
on its output and then a 1 ohm resistor to ground.  That will only give a
10mV signal to the coil but that should be enough.  The coil would have to
pump 1 amp through the resistor to mess with the generator which would be 1
watt which is not possible, so all should be well.  Does that sound ok?

One big problem is measuring the output voltage.  With the self capacitance
of the coil in the 9.5pF range.  Dropping a 16.6pF 10:1 Tektronix probe on
the open load end would cause terrible loading and distortion.  I think the
one at work is 2.5pF at 100M ohm (tek P6109) but that my still mess with
the measurements.  I also worry that the wire to the probe will pass right
through the self capacitance fields at the top of the coil but they may not
be a big deal.  Any thoughts on how to measure the top voltage without
disturbing the thing would be appreciated.

Your square wave test where one observes the returning square wave edges
should me much fun!

Cheers,

	Terry



At 01:36 PM 04/16/2000 -0400, you wrote:
snip...>
>The best I can do is as follows.  Use the isolator described below with say
>470ohm in place of the 4.7ohm. Starting at 50 ohms and increasing in say 30%
>steps until you get close (or use a 1k pot), connect various resistors
>across the output of the coil to ground until the output voltage is equal to
>the input voltage. At that point I think you have no reflections from at
>least one of the ends.and hopefully its the terminated end. Replace the
>divider at the input with a divider that has an output impedance equal to
>the
>value you found above. check the input and output voltages are the same if
>not you may have to adjust the divider again.  You only need to be within
>say 10% if you only want to see the delay and not make accurate
>measurements.
>
>Now your ready if you are using the resonant frequency of the coil you will
>be able to observer the 90deg phase shift.  QED and conversion. (assuming
>you don't invent a spurious reasons to account for this and the coax)
>
>You can now perform various experiments that you must have missed at collage
>like open circuit resonance, short
>circuit resonance, effects of C or  L and  determine the dispersion i.e.
>velocity against frequency. You can also switch back to a square wave, drive
>the coil from a high impedance source, say ten time the characteristic
>impedance, and leave the other end open circuit you will be able to observe
>at the driven end the progressive step up of the square wave edges  to the
>source voltage as each reflection returns from the open circuit.  Similarly
>if the end is shorted you will be able to observe the decaying transients
>in voltage this time caused by reflections that are inverted at the short
>circuit. Just like the primary transients when the gap closes.
>You could also determine the characteristic impedance at different
>frequencies.
>
>Note that the square wave must be at least ten time the resonant frequency
>or there will not be time to observe the effects before the next edge
>occurs. Note also that the edge may be smeared out because of the dispersion
>i.e. the harmonics will travel at different velocities.
>
>Some potentially important points:
>
>1) You don't appear to have a ground plain this could produce all sorts of
>spurious responses that could be very confusing. Ideally you should do the
>experiment over a large metal plate or aluminium foil.  Preferably although
>not essential with the coil laying parallel to it. You could use the open
>circuit primary. The generator ground, scope probe grounds  and any
>terminating resistors or short circuits should be connected to it preferably
>at a common point.
>
>2) You should have an isolator on the output of the generator at the point
>it is connected to the coil. Particularly if your using coax to connect to
>the generator and the generator has a 5 ohm output.  You should also
>terminate at the generator if its output impedance does not match the cable.
>Again use a 50ohm termination assuming 50ohm coax.  This may be essential if
>you doing square wave experiments as the reflections in the cable will
>degrade the edges of the signal and can upset the generator. Its interesting
>to speculate which  point  in your set up is equivalent to the point in the
>coax experiment where you measured the 90deg phase shift.
>
>3) Use the scope probes on X10 and a direct connection you only need 0.1
>volt in or less depending on the ambient electrical noise.
>
>The isolator can be made in one of two ways depending on how you want to
>drive the coil from a high impedance source or low.  In the low impedance
>case which you need to determine Tesla resonance (the coil is supposed to be
>shorted at one end) and assuming 50ohm coax, use a pie net work consisting
>of a 50ohm(or 47) resistor across the generator and a 470ohm and 4.7 ohm in
>series across that. (for the coax experiment you my need to use 1ohm to get
>better results but it will make tuning sharp).The coil is connected to  the
>junction of the 470 and 4.7 resistor.  For a  high impedance drive use a
>50ohm (or 47) across the generator and a 10kohm resistor from it to the
>coil.
>
>Note that for 50 ohm coax  4.7ohms is  only 10% of the cable impedance so
>it does not represent a good short.  A better value would be 1ohm with a
>series 100ohm.  This should give you a good standing wave but the tuning
>will be more critical
>
>I did say it was tricky but it should teach you a lot. Suddenly all those
>spurious effects can be easily be explained.  The funny ringing on the spark
>is produced by waves bouncing round the primary.  A new layer of complexity
>and wonder will be open to you.  Then all you will have to do is convince
>the other non believers.  Well I have done my bit.
>.
>Regards Bob
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