Re: Flyback Winding

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

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Mark,

On 2 May 2003, at 10:55, Tesla list wrote:

 > Original poster: "Mark Dunn by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<mdunn-at-wmwmeyer-dot-com>
 >
 >
 > Gentlemen:
 >
 > I am new to the "List", but I've been building HV units for some time to
 > power Plasma Globes and Jacobs Ladders with old Flybacks and Ignition Coils.
 > I haven't yet graduated to Tesla Coils.
 >
 > Currently, I'm trying to wind my own flyback secondary(I do this instead of
 > sleeping) to improve performance and have a number of questions specifically
 > related to that.  Old flyback secondaries were short axially and big in
 > diameter.  In many cases there is plenty of room on the core above and below
 > the secondary to use.  Why not make them tall and big in diameter?  Is the
 > danger that they will arc to the ferrite core?   If  I carefully wind the
 > first course right hand and run the second course back the other way over
 > the top(left hand), then the last winding in the second course is directly
 > over the first winding in the first course(If I wind 10 courses then this
 > same issue is present in every other course on either side)  Are these apt
 > to arc because of the differential voltage between these windings?  Someone
 > told me a rule of thumb that the flyback will generate about 2-4
 > volts/winding in resonance with a 12 Volt primary.  Thus if I put 100 turns
 > in each course, then the subject windings will be 400-800 volts apart.  Any
 > comments?  Does the 4 volt/winding rule make any sense or is it strictly
 > dependent on the gain associate with the system's resonance?
 >
 > I am using 30 ga enameled magnet wire.  Is another film coating better?  I
 > seal each course in epoxy and wrap with epoxy impregnated brown paper to
 > give a guaranteed gap between courses.  Any suggestions on the thickness for
 > that gap or a superior construction method?  It seems to me that the
 > dielectric constants aren't that far apart for these insulating materials.
 > Since I am embedding in epoxy anyway, a 1 mil coating on the wire is not
 > important.
 >
 > So far I've been lucky enough not to blow any of my prized old disc shaped
 > secondaries.  Therefore, I've never disassembled one.  I've been told they
 > typically have 1200 turns, but they don't seem to be big enough to support
 > that number.  Anybody have different information?  I've sawed in half the
 > potted newer style flybacks(circa 1970) and find nowhere near 1200 turns in
 > their secondaries.
 >
 > Thanks for any help
 >
 > Mark D.

I have built flyback secondaries to produce up to 20kV using about
3/4 the width of the core. The winding should be graded so that the
first layer covers 3/4, the next a couple of turns less, the third a
couple of turns less and so on. Interlayer insulation is 0.125mm
thick PTFE sheet covering the full width of the winding window. Each
layer of windings should be varnished and allowed to dry before
applying the PTFE and the next layer of windings. You can make end
cheeks for the finished secondary using some of the PTFE sheet with a
hole cut in the centre to accomodate the core leg.
      Winding the primary as a single layer beneath the secondary gets
best coupling to the secondary and allows the height of the secondary
to reach a maximum (about 1/4" from the opposite core leg. apply
plenty of tape or some other insulation on top of the finished
winding to suppress corona.

Malcolm

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