Power Stealing Transformer Core Design

[elgersmad]

At that point, you could use a single E core, if you wind the primary on the first leg and the secondary on the third.  Then all you would do is have a gap placed in the center of the E core of about 1mm to 2mm if it's a large core.  Smaller cores would have smaller gaps.  Using a good machine file should allow to do that.  The gap will cause the magnetic lines to prefer travelling all the way past the gap to the secondary.  Only when Faraday's Law and Lenz's law are in effect will there be magnetic lines there to cross the gap.  The more power you use on the secondary, the more power the Thief Coil will produce.  THey actually make the E cores that way for flybacks and resonant cores.  But, you need absolutely need three separate windings, one on each leg.

At most, you might need a Zener diode that can handle all of the current from the generator coil.  You want to wait to tap it, so in order to do that, following a bridge rectifier, you'd place a zener diode in series with the load before you got to ripple filter capacitor.  If the secondary and the Thief Coil have the same number of turns, the zener diode breakdown voltage should be nearly equal to the output voltage of the secondary or slightly higher.  Basically, there will be twice the number of magnetic lines to jump the gap.