Surefire - V5.0

Randy Peterson
16671 T.R. 51
Mt. Cory, Ohio 45868
419-358-1222

Theory

In the simplest terms, the basic operation of the Tesla coil is as follows. The power transformer increases the 120 volt, 60 cycle current to twelve thousand volts. This voltage is needed to charge capacitor C1. It does so 120 times a second, once each half cycle. Towards the end of the first quarter cycle, C1 has charged up to a voltage equal to the breakdown voltage threshold of the spark gap SG1. The air between the spark gap electrodes ionizes and the spark gap fires creating a conductive path between the capacitor and the primary winding. The energy that took 4 Ms to be stored in the capacitor is now dumped into the primary in only 3 Us, creating an expanding, and counsequently, a collapsing magnetic flux whose rate of fluctuation is consistant with the resonant frequency of the capacitor and primary coil combination. The changing magnetic field's lines of flux cut the windings of the secondary producing the radio frequency high voltage at its top terminal.

Power Transformer

A 12,000 volt, 30 Ma neon sign transformer is used to charge capacitor C1. This type of transformer includes magnetic shunts making it self current-limiting and therefore does not require an external current limiting reactance as would a conventional power transformer. It has its center tap or mid-point connected internally to case ground. This permits simpler construction since the secondary terminals need only be insulated from ground by one-half the end-to-end voltage. The transformer case is always to be used as AC line voltage ground and RF ground for the Tesla secondary winding. Since both ends of the power transformer secondary are used (as opposed to one end and ground), two sets of RF suppression networks and safety gaps are required to protect the transformer.

Capacitor

In this project, the highest quality capacitor available was used. The capacitor C1 is a high Q mica transmitting unit rated .006 Mfd. at 15,000 V.A.C. This type of capacitor is especially suited to Tesla coil operation. The plate material is all copper and it uses mica as the dielectric which means less plate area is requaired for a given capacitance. This translates to a low internal inductance and a high discharge current. The whole unit is potted for superior insulating quality. This type of capacitor far exceeds the performance of the usual foil/glass variety and weighs much less. It is, however, a much more costly and hard-to-find item.

Primary

The primary coil consists of 15 turns of #8 insulated stranded wire closewound on a 12 1/2" O.D. PVC form. The large gauge copper wire meets the requirements of the high discharge current of capacitor C1. It is tapped at the 14th and 13th turns for tuning purposes. The taps and ends of the coil connect to terminals on an insulator bar set a safe distance from the primary winding. The terminal separation permits solid connection to the ends and taps without risk of arc-over.

Secondary

The secondary is 14 3/4" of closewound #27 enameled magnet wire on a 6 1/4" O.D. PVC form and is self-resonant to approximately 240 kHz including the top aluminum cap. Prior to winding, four strips of 1/2" double-stick Scotch tape are placed lengthwise on the form separated by 90 degrees. These are to secure the coil to the form. The 'cold' end of the coil terminates to an 8-32 X 3/4" machine screw to which is attached a length of high voltage wire. This wire passes through a hole in the upper level and is to be connected to the transformer ground terminal only. This is to be done in such a way that the wire is routed away from all other potentials. The 'hot' end of the coil terminates to the top cap 1/4"-20 center bolt. To this bolt may be attached various shapes and sizes of discharge electrodes. Do not overtighten connections to this bolt. Finger tight is all that is required. Coefficient of coupling of the primary and secondary is adjustable by means of a split inside sleeve that supports the bottom of the secondary and secures it in place to the upper level. Six sets of mounting holes separated by 1" each are located on the sleeve so that the secondary may be moved in or out of the primary to affect the proper coupling.