Principles of Tesla Coil Construction

Part II - The Power Source

Induction Coil

The most common form of induction coil used in amateur made Tesla coils was the Ford Model “T” spark coil. This was a 6-volt d.c. functioning transformer which depended upon a simple vibrating device for its operation. The spark coil is still available from antique automotive dealers and if one is lucky, a spark coil is sometimes seen at flea markets. The Model “T” spark coil is suitable for small Tesla coils.

Automotive Ignition Coil

A modern-day automotive ignition coil is even better suited for small Tesla coils but requires a rapid make and break device for its operation. The required effect can be achieved by mechanical means or from a solid state circuit. Older models which ran on 6-volts can be obtained at small expense from automotive grave yards. The newer models run on 12 volts. Batteries or battery chargers are good sources for powering direct current induction coils. They can also be run on standard a.c. circuits through a step down transformer and a rectifying system. The make and break device mentioned above must still be used.

Oil Furnace Ignition Transformer

The oil furnace ignition transformer is, perhaps, the most desirable source for powering small to medium sized Tesla coils. For one, it is readily available.* For another, used transformers can be purchased at a very small cost. Thirdly, they include a current limiting arrangement which makes it just about impossible to burn them out. However, on Tesla coil circuits, an rf choke must be used in series with the output terminals to prevent a breakdown of the insulation by high voltage-high frequency currents. This applies to any power source being used on high frequency circuits. Oil furnace ignition transformers produce 10,000 volts at .025 amperes and function on common house circuit currents.

Neon Sign Transformer

Most of what has been stated about the oil furnace ignition transformer can be applied to the neon sign transformer. The latter, however, provides for a wider variety of outputs and can be used on small to large Tesla coils. The largest transformer usually available from neon sign dealers produce 15,000 volts at .060 amperes. Very large Tesla coils can be powered by two transformers hooked in parallel.

Communications Power Transformer

The communications power transformer used to energize transmitter circuits is another good source for Tesla coil circuits. They come in a wide range of voltages and currents. Unlike oil furnace and neon sign transformers, they do not contain a current limiting arrangement. Unless a reactance coil is used in series with the incoming line, power arcs are apt to occur at the spark gap. On the other hand, some can be used to power very large Tesla coils by hooking their secondaries in series.

RADAR Transformer

While not readily available to the average experimenter, the radar transformer is quite suitable for powering very large Tesla coils. What has been stated about communications transformers can be applied to the radar transformer.

Distribution Power Transformer

This is the pole transformer one can see along power lines. Unless an experimenter has been into large Tesla coils for some time, it is best to avoid their use. They are heavy, expensive, and carry lethal currents! One error in handling a live unit is certain death! They cannot be used on regular house circuits as they draw between 100-200 amperes!

* During an interview, a oil company employee informed me that they discard (throw away) 200 transformers a year (that are still operable)!