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Re: Tesla Coil Q Factors
to Bert
John's second equation is the foundation for all of Tesla's magnifiers:
The output potential of an RF inductor is equal to the applied potential x
its dynamic operating Q factor. The third equation is perhaps the most
interesting of all. It allows all coilers to easily find their dynamic
operating Q factor of any system (classic or magnifier) by simply using a
storage scope or shooting some camera pictures of a regular scope's display
and then finding the decrement factor, ie, log amplitude 1/ amplitude 2.
Compute this value for a number of sample shots for the correct log
decrement value and divide into pi to obtain the actual dynamic operating Q
factor. The value is usually accurate to within 1% for most systems. Once
this Q factor is determined the peak currents can be easily calculated and
other interesting data drops out as well including max peak system
potential, system characteristic impedance, etc. The derivation of this
equation is via simple substitution with the energy conservation equation,
ie, 1/2cv^2. Morecroft and Termain both discuss this Q decrement factor
equation but John is the first among modern theorists to discuss its
application.
I've also noticed on this List that ocassionally some "builders" like to
"pick on" John as a theorist --- much like experimental physicists
jealously like to "pick on" theorists in their field. Many years ago I
provided John with the hard data on dozens of our commercial coils -- data
that he has carefully and thoughtfully distilled to produce his manuals.
His manuals and books are correct from both a practical and theorectical
standpoint and will withstand scrutiny from any builders standards. John's
books and manuals provide excellent guidelines for all coil builders and
are especially invaluable to the new coil builders on the Tesla List. It is
unfair and most of all blatantly incorrect to assume John is an ivory tower
builder. His data is correct and not all derived for theorectical
constructions. Enough said on this subject (ps Bert -- I wasn't implicating
you in any manner). Your insights are always a welcome treasure to the
List.
DR.RESONANCE-at-next-wave-dot-net
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Tesla Coil Q Factors
> Date: Tuesday, August 25, 1998 5:26 PM
>
> Tesla List wrote:
> >
> > ----------
> > From: John H. Couture [SMTP:couturejh-at-worldnet.att-dot-net]
> > Sent: Monday, August 24, 1998 11:57 AM
> > To: Tesla List
> > Subject: Tesla Coil Q Factors
> >
> > To All -
> >
> <SNIP>
>
> Q = 6.283 x F x L Q = Vs/Vp Q = 3.1416/ Log Dec
>
> <SNIP>
> > With real coils the secondary voltage Vs increases when the input
wattage
> > is increased. This means that in the equation Q = Vs/Vp the Q Factor
would
> > increase with an increase in wattage. However, with real coils just the
> > opposite happens. The Q Factor decreases as the wattage increases. I
show a
> > graph in one of my books of this relationship where the Q Factor
decreases
> > with an increase in the wattage. To my knowledge there is nothing in
the
> > present literature that shows why there is this apparent discrepancy
with
> > the equations.
> >
> <SNIP>
>
> John,
>
> The first equation is not quite right - it should be:
> Q = (6.283 * F * L)/Reff
> where Reff is the combination of AC resistance in the circuit due to
> resistance, skin effect, winding proximity effects. As you indicate,
> determining the Q's of the primary and secondary circuits in a sparking
> coil is tougher, but can be estimated by looking at waveforms and
> back-calculating.
>
> However, output voltage is not directly relatable to input wattage,
> although you will see a degree of correlation: larger systems tend to
> use larger tank caps and higher primary voltages, and need more power to
> achieve similar breakrates. Output voltage is more directly a function
> of bangsize and system losses, since disruptive coils do not build-up
> secondary energy from bang to bang. Sparklength is another matter...
>
> I'm not familiar with Q = Vs/Vp - how was this equation derived?
>
> -- Bert --
>