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RE: Regulating coil: here's a big one!
Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>
Let's please call these "tuning coils" - nothing is being regulated.
Now here's the good news. No power or performance is being wasted. While
the magnetic field from the tuning coil is not likely being coupled to the
secondary, it is not going to waste. Only if there is resistance, will
there be heat generated and hence wasted energy. The off-axis inductance
of the tuning coil will merely somewhat reduce the pri-sec coupling, and
the secondary can be moved closer to the primary to compensate.
The same misconception also applies to parasitic inductance (a.k.a. E.S.L)
.within capacitors. This does no harm in a Tesla Coil application, though
it is significant in other high energy pulse-discharge applications.
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Monday, May 27, 2002 9:53 AM
Subject: Regulating coil: here's a big one!
Original poster: "Scott Hanson by way of Terry Fritz <twftesla-at-qwest-dot-net>"
In an attempt to help put an end to the "variable capacitance" debacle, I
thought I'd add a bit to the "regulating coil" thread.
Saturday I picked up the largest variable inductor I've ever seen at the TRW
electronics swap meet in So Cal. Unlike the commonly seen ex-military roller
inductors, this one is "really big", and utilizes utilizes a pair of solid
copper "shoes" on a slider to make contact with the windings. The windings are
rectangular section solid copper, .185" thick X .325" wide. The coil is 6" long
X 6.5" in diameter, with 14 turns total. As measured using a Sencore Z-meter
II, the range of inductance adjustment is from .1uH to 20uH. Weight is just
under 20 lbs.
Some photos of this inductor can be seen via the following links. The files are
150 - 200 kb in size, so they may take a little time to load.
Technically, using a regulating coil "wastes" some of the energy in the primary
circuit as its magnetic field is not coupled into the secondary. However, this
loss might be offset by the ability to fine-tune the primary circuit's resonant
frequency as the coil is actually operating.
The photos show several views of this massive inductor. An interesting feature
is shown in the photo labeled "corona burn". Clearly visible burned into one of
the insulating end plates is a flame-like defect originating at a sharp corner
of the square slider-shaft. The distance from this corner to the brush plate
(opposite terminal of the coil) is slightly over 2". It appears that this coil
was operated for some extended period of time with considerable corona
originating from this point.
As soon as I can rig up a gear motor to allow this to be adjusted from a
distance, I'll add it to my 6" X 32", 120ma NST-powered coil and see if dynamic
tuning is worth the effort.