I had thoroughly enjoyed building and using my mini-coil. It was time to further plumb the small end of the coil spectrum. 

I had been collecting bug zapper transformers (BZT's) whenever I found one cheaply at a flea market, so I had a couple to choose from.  Some of them had a ceramic disk capacitor externally wired across the BZT's secondary, and others had the capacitor embedded within the potting enclosing the secondary coil.  The purpose of the cap is to provide a charge reservoir to put some snap into the zaps.  While some Tesla Coils also have their tank capacitors wired across the transformer, a ceramic disk capacitor is worthless at the frequencies of interest (much too lossy), so it had to be removed.

Above is what the BZT looked like after gouging out the unwanted capacitor.  A BZT is similar to an NST in that it's current limited.  The short circuit current of this unit measured 8.7 mA.  I attempted to measure the secondary voltage by feeding 120VAC into the secondary and measuring the primary voltage.  That method resulted in an estimated Vsec of 4.8kV..A recent thread on the TCML now makes me question the accuracy of that.

For a tank capacitor, I just used what I had.  I constructed an MMC out of five 0.047uF/1600V polypropylene caps and 10M bleeder resistors - .0094uF@8kV.

Spark gap is a pair of 5/8" copper pipes, 20 mil separation.  With such low power, forced air flow through the gap isn't necessary.

Secondary coil is 0.84" diameter PVC pipe wound with 3.4" of #38AWG magnet wire, taken from a small relay.  3.37mH

Primary coil is 5 turns of #18 solid vinyl insulated hookup wire, helical wound on a PVC pipe cap.

Top load is a pseudo-toroid (a toroid with the hole filled in) turned down from a 3/4" x 3.4" pine disk, covered with aluminum tape.

Secondary base (RF ground) just connects to the mains ground.  No filters or protection circuits.

Operating frequency, from memory, was about 1.5 MHz.

Best spark to a grounded target: 2.25"

Future plans: Replace #18 solid primary with a Litz wire primary


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