Problem with common leads in dual channel scopings?

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

Original poster: Harvey Norris <harvich-at-yahoo-dot-com> 


Recently I was doing some scopings on the alternator
source frequency air core transformer, that uses only
resonant components in LC values that will be resonant
to the frequency of the AC converted alternator. These
"continuous wave" tesla coils can actually be operated
in what might be considered a very low frequency of
application at a frequency of operation that does not
seem very applicable for the possibilities of simply
applying the laws of resonance for each component.
Turns ratio voltages between primary and  secondary
are exceeded  by the predictions made in the
ferromagnetic model.  Also it may be true that the
tightest coupling between the air core inductors may
not necessarily be the best mode of operation.
Grounding may enhance the operation of the secondary.
So to counter any off topic charges here: I am simply
indicating that the simple source frequency air core
transformer has many common traits with the ordinary
tesla coil of high frequency. In this particular
transformer, a sort of magnifier effect seems present
by connecting the endings of LsCs to another tank
circuit thus making the circuit a triple resonance. In
the present embodiment, the primaries are constructed
in two opposing series resonances using two high
induction coils as secondaries connected in parallel
with reverse windings respectively.

It seems that it is possible to scope out the action
of the primary voltage inputs so that one of the
probe leads  can be connected to the actual souce of
stator voltage to the circuit, and the second probe
set can be connected to a voltage rise of the primary
circuit, but because here two primaries are being used
we can try just looking at one coil at a time. I also
had an amp meter on that coil being scoped, to see the
ratio of amperage split between sides of the primary.
The idea of simultaneously scoping the voltage on the
outside of the circuit vs the inside resonant voltage
rise, is that the ratios shown with a dual channel
scoping of this show how "in phase" each voltage is,
and to be perfectly resonant the phasings will be
identical. The ratios of voltage rise also give an
indicator of the "acting Q" of the primary system,
which is dramatically reduced when coupled to the high
induction coils in space as the secondary.

Using a HP-1740A scope that I have now incapacitated
by improper practices, I noticed that connecting up
the voltage rise portion of the circuit, in
conjunction to the other probes outside voltage
connections: they will always have one lead from each
probe common to the sensoring circuit.  In fact this
means each probe connection should be in identical
polarity connections. When I was doing this with an
appreciable power input, arcing over from the
secondaries LsCs outside windings went through the
insulation of a probe lead being used for these
primary measurements.  The scope still seemed okay
however. Later I observed that one of the probe leads
was smelling funny, and it was too hot to touch. Some
kind of short was taking place, thought to be brought
upon by making dual channel scopings: with a common
probe lead. The amperage for that coil being scoped
would go to zero after a point when the volume of
primary amperage was increased.

This brings me to a question of scope operation: can
anyone simply say why grounding is necessary? These
scope leads were only monitoring voltages from an
alternator stator being under 12 volts or so, so the
voltage ranging precautions of the scope were
available, BUT it might be true that since it is
monitoring a circuit of higher amperage delivery,
might any special precautions be necessary?  Here it
seems like some kind of short occured, so after the
scope was trashed I tried my TEK 2213 on the same
circuit. In this case the short starting acting every
time, and such a dual channel scoping by that method
was impossible.  In fact what strangely occurs with
these primaries, what happens is that when a certain
volume of amperage rise occurs by increasing the
alternator input via DC variac to field, all of a
sudden the primary being monitiored simply stops
resonating. Could it be possible that the internal
impedance of the scope might act differently when
observing a 480 hz signal? Doesnt sound too likely,
more like a problem involving the probe connections I
thought...

In any case a solution to these monitorings was found,
by making the dual channel leads connected to the
voltage rise of the circuit, WITHOUT any of those
probe leads having a common connection point. In the
past I think I was able to past this obstacle just by
using two different scopes for each probe lead, and of
course centering each sweep line,  then visually
comparing each scope sweep; which would be necessary
to circumvent this short problem when observing only a
single phase of series resonance. But because here we
have 180 phased dual series resonances, instead we
have the opportunity to measure just the voltage rise
between the inductors in series resonance, and then
those probe leads are not having a common joining pt
from those on the outside of the circuit.

When doing this the shortcomings of the method were
quickly shown. At the lowest input voltages, a
ordinary digital voltage meter records 8 volts across
the primaries voltage rises, given the minimal 2 volts
made in operation of spin. One channel of the scope
can show that 2 volt signal from the alternator. But
however once the second channel midpoint voltage of
scoping is added, the measured voltages from both
meter and scope drop down to ~6 volts. When the
primaries are tuned for more effective resonance, the
drop upon scope monitoring drops the action 50 %. so
for this case example anyways, it is definite to
conclude that the action of scope monitoring the
primaries resonance, acts to significantly drop the
possible amount of resonance obtainable given a fixed
stator voltage input. This scope has a 30 pf internal
capacity. I have seen that at higher input
frequencies, the internal impedance of a meter can
influence what is recorded.  At some 20,000 hz  from
solid state neon transformer; a convential needle AC
voltage meter will record a different result for each
voltage scale, and these values vary widely. Can
anyone make a comment on the problems of the internal
impedance of the measuring instrument affecting what
is being monitored? I have also noticed that when
observing  inductive high frequency signals near the 5
volt range, that when we make the change to 10x on the
probe this can also dramatically change the frequency
that is observed by the scope, so the reasoning
becomes that less internal capacity made by the 10x
probe change, and it then hinders the observed
vibration less, since the recorded frequency in those
cases is increased. The changes in one sample of
spirals tested for resonant frequency were readings of
550,000 hz to over 800,000 hz, so surely this is an
issue to be contended with, if the measuring
instrument can deliver so different an answer
depending on which range we choose, then the scoping
of course looses quite a bit of relevance????
Different scopes of different internal capacities also
interprete these hf signals differently.

HDN