Spark-Gap Tesla Coil (SGTC)

Introduction

The Tesla coil is one of Nikola Tesla's best-known inventions. It is a resonant transformer designed to generate very high-frequency, high-voltage alternating current.

Operating Principle

A supply of several kilovolts charges a capacitor in the primary LC circuit. When the spark gap fires, the capacitor is connected briefly across the primary coil and discharges rapidly. Energy oscillates between the primary inductance and capacitance, inducing a much higher voltage in the secondary resonator. This process repeats thousands of times per second and produces damped oscillations.

The approximate resonant frequency is f0 = 1 / (2π√(LC)). The primary and secondary circuits must be tuned to nearly the same resonant frequency. A secondary coil commonly has hundreds or thousands of turns, while the primary usually has approximately 5–20 turns. A toroid or other smooth conductive top load lowers the secondary resonant frequency, stores charge and helps control breakout.

Types of Tesla Coil

SGTC means Spark-Gap Tesla Coil and is the traditional, simplest form. SSTC means Solid-State Tesla Coil and replaces the spark gap with IGBT or MOSFET switching. VTTC means Vacuum-Tube Tesla Coil. Other variants include OLTC and interrupted SSTC systems. Solid-state designs are quieter and more compact, but their electronics are more complex and require careful gate drive, filtering and protection.

First SGTC Replica

For my first coil, I wound the secondary by hand on a small plastic tube and coated it several times with acrylic varnish. The secondary was 131 mm high, approximately 28 mm in diameter and had about 700 turns. The primary had five turns. The resonant capacitor was 4.7 nF / 6.3 kV and the spark gap consisted of two screws in a small enclosure.

A 10 kV supply produced direct streamers of roughly 2 cm and faint corona around the toroid. A later test with another black-and-white TV transformer produced stronger streamers of up to approximately 5 cm and a blue corona extending around a 10 cm top load.

Second SGTC Replica

The second coil used an ATX computer supply for the low-voltage electronics, a transistor driver and a flyback transformer. Twenty 1N4007 diodes were connected in series as a high-voltage rectifier. The spark gap used two aluminium discs.

The secondary was 24 cm high, 4 cm in diameter, wound with 0.25 mm wire and contained approximately 912 turns. The primary was 3.5 cm high, 7.5 cm in diameter and had eight turns. An NE555 driver with an IRFZ44N operated from approximately 9 to 30 kHz. The flyback transformer produced around 16 kV and the resonant capacitor remained 4.7 nF. Because the primary and secondary circuits were not yet perfectly tuned, the streamers reached about 8 cm.

Further experiments were performed on 5 June 2015 and are documented in the photographs.

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