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Re: Q, Coupling and things
From: Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: Wednesday, November 05, 1997 1:15 AM
To: Tesla List
Subject: Re: Q, Coupling and things
Hello Viv,
Congrats on your first firing. Sorry the info is not as
complete as perhaps it might have been. The article was based on 3 - 4
months of pretty heavy research and experiment and embodied eveything
I knew at that stage - the most important thing being able to make
one that worked. I have since refined my understanding considerably
but the crude basics are there.
> From: Vivian[SMTP:V.C.Watts-at-btinternet-dot-com]
> Reply To: Vivian
> Sent: Tuesday, November 04, 1997 2:13 PM
> To: Tesla List
> Subject: Q, Coupling and things
>
> Hi all,
> I joined the list back in May and just watched what was being said.
> I read and read again Malcolm Watts article of 1995 in Wireless World. In
> September I left the list as I must admit I couldn't cope with all the email
> and build my coil. Well a few days ago I fired it up and it worked. I'm
> sorry to say I got big white sparks about 18" first time. No breakout of
> the secondary, nothing going bang. I have yet to open up the gap to see if
> I can get them longer but rain has stopped play as my earth spike is
> outside. So why am I writing? Well you've got to tell someone haven't you.
>
> Spec,
>
> Neon Transformer 240v 50Hz, 10,000V 50ma
> Primary Capacitor 15nF flat plate, polythene sheet (37Kv breakdown)
> Primary coil 8" start, 10 turns taped at 7.5, 1/4" copper pipe 1/4"
> spacing.
> Primary Inverse conical 30 degrees
> Secondary coil 4.25" diameter, 20" long 24 A.W.G (0.54mm)
> Secondary turns 870, Inductance 16uH
> Toroid two stainless steal dishes, creates 16" by 10" toroid
> Resonant frequency 270kHz
>
> I have access to a Scope, signal generator and Meters so I was able to tune
> it spot on before firing it up. An interesting observation was the
> secondary top load. With the first dish in place you get a certain
> capacitance. With the second dish on top inverted the capacitance increases
> significantly showing how isotropic the toroid and seemingly unaffected by
> how close to the floor it is.
>
> So now a few questions for the experienced.
> I have read that the Q of the secondary should ideally match that of the
> primary. In Malcolm's article It tells you to measure the frequencies at
> the 70% of Max points of the resonant freq.. But not what to do with them!
> Guessing is it Fres/(Fupper-Flower)?
To first order, yes. Sorry about that. I never had a reader's query
on that one before :(
I can't find my old electrical books.
> My values are
> Secondary 70% points 272 and 278Khz
> Primary 70% points 271.3 and 273.5, centre 272.4
>
> When the secondary is in place, the coil has a double humped response of
> 253Khz and 298 kHz. Can anyone explain how this is? Is it the two coils
> now having different resonant frequencies?
If you look on the scope while the coil is running (preferably single
shot - one bang), you see a double-sideband suppressed carrier
envelope. It is a result of the primary cap emptying while the
secondary is filling up over several cycles. That is the *time
domain* picture. In the frequency domain you can decompose that
waveform into the two frequencies you see using the sig gen. However,
the frequency domain picture says nothing about the relative phase of
the two frequencies.
It is a consequence of overcoupling the coils. You *have* to do
this to get sparks. If you adjust it so that the system is critically
coupled *before* you get an output spark, you will find that the
transfer proceeds too slowly to get energy into the spark (secondary
fill time becomes infinite).
> I hear about coupling. Is this akin to matching the source impedance to a
> load? If so take the two extremes. No coupling = no power transfer.
> Overcoupling is equivalent to what?
Completing a transfer in a finite time determined by the coupling
constant. The closer k approaches 1, the quicker the secondary fill/
primary empty time. At k=1 (impossible to realize) fill time becomes
zero.
Nice goin' and thanks for the feedback,
Malcolm
<snip>