Mini Tesla Coil

Why build a miniature Tesla coil? My sister Helen is into miniatures (doll houses that is) and I thought this would amuse her (it did). Of course, now she needs to build a miniature mad scientist laboratory to go with it.

The base is made from two 4" square pieces of 3/8" thick UHMW polyethylene separated by three 3" lengths of 1/2" OD X 3/8" ID polycarbonate tube and one 2 3/4" length of 1/2" copper tubing. The polycarbonate tubes are countersunk 1/8" into the PE squares and the two squares are fastened with three 1/4"-20 nylon threaded rods and a 1/4"-20 stainless steel rod. The threaded rods are capped with delrin swivel feet on the bottom and nylon nuts on top. The primary capacitors and spark gap are mounted directly to the lower PE square. The spark gap is made from five 1/4" X 3/4" X 1" aluminum blocks, each drilled and tapped with two 1/2" deep 4-40 holes on the bottom and a 6-32 hole on top which connects to a 1/8" hole which carries a tungsten electrode. The two end blocks also have a 5/32" hole to accept banana plugs from the power supply. The left hand block (high voltage) sits on a copper sheet which connects to the primary capacitors. The right hand block (ground) connects to the copper tubing. The upper PE square has a central 1" X 1/8" countersink to accept the secondary and a series of 8-32 tapped holes which accept nylon screws to hold down the primary. Another piece of copper sheet (not visible) runs under the upper PE square and connects the base of the secondary to the primary and copper tube. The secondary is wound on a 5" length of 1" OD X 7/8" ID polycarbonate tube and terminated with copper sheet tabs on top and bottom which are epoxied to the tube. The strike rail is made from 1/4 copper tubing and sweat fittings. The toroid is an aluminum foil tape coated "roller bumper", held in place with 1/4"-20 nylon rod, nylon nut (under the PE square) and a nylon wing nut.

The power supply is mounted in an 11" X 11" X 5" polycarbonate enclosure. An oil burner ignition transformer, rated 20 mA at 6,000 volts, is protected by an iron core inductor with seven 5" long layers of 20 gauge wire wrapped on a 1" OD X 7/8" ID polycarbonate tube and a safety gap made from two 1" steel ball knobs.

Specifications of my Mini Tesla Coil
Diameter of secondary coil (1")	25.4 mm
Winding height of secondary coil (4")	101.6 mm
Wire Diameter of secondary coil (30 gauge)	0.2794 mm
Spacing between windings	0.00 mm
Secondary turns	364
Secondary wire length (95')	29 m
Secondary inductance	0.74 mH
Approximate resonant frequency	4264 kHz
Secondary quarter wavelength resonant frequency	2585 kHz
Secondary self capacitance	1.88pF
Toroid capacitance required to form quarter wave length coil	3.23 pF
Primary capacitance	0.0035 uF
Primary resonant frequency	2585 kHz
Primary conductor diameter (1/8")	3.18 mm
Primary turn to turn spacing (5/16")	7.938 mm
Spacing between the inside turn of the primary and the secondary	3.18 mm
Transformer secondary voltage	6,000 V
Transformer secondary current	20 mA
Transformer frequency	60 Hz
Primary needs to be tapped between turns	3 and 4

And here is a picture of the mini Tesla coil in operation. The four static gaps are each set at 0.012" (with a feeler gauge). The output gap is ~2.5".

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