SGTC – Design

The Spark-Gap Tesla Coil

This was my first coil, and required a fair bit of research before building it. It follows the same principles as Tesla’s original coils and turned out to be very impressive when running.

The spark gap acts as a very high power, high voltage switch which triggers itself as the voltage across it rises to it’s breakdown point, it does this around 50 times a second. Each time it fires, it discharges the stored energy in the tank capacitor into the primary coil. It is this rapid pulsing which sets up the high voltage gain in the secondary coil. This is a very simplified description as the full operation is quite complex. To stand a fair chance of it working, I used new components wherever possible and did as much testing as possible before connecting everything together.

Following lengthy research on the web and with the kind help of the members of Tesla Coil Builders of the UK, I managed to conjure up a reasonably easy to build design using readily available parts.

+++Primary Coil+++

12 turns 8mm copper tube
Outer Diameter – 606mm
Inductance – 0 to 64uH

+++Secondary Coil+++

Core material – 110mm grey pvc pipe
Wire – 0.5mm enammelled copper
Winding height – 550mm
Overall height – 650mm
Turns – 1000
Inductance – 19.865mH
Capacitance – 8.394pF
Resonant frequency – 239kHz
Insulation – 3 heavy coats of polyurethane varnish

+++Power Supply+++

10kV 50mA Neon sign transformer
Standard line filter at transformer supply, earthed to RF ground
Power factor correction – 25uF 440v
275v MOV accross supply
Custom R/C filter in HT lines to main spark gap
Safety spark gap across transformer output, centre electrode to RF ground

+++MMC Tank Capacitor+++

Value – 16.7nF at 21kv
Made up of 5 strings of 14 * 0.047uF 1500v
polypropylene capacitors

+++Control Box+++

Standard line filter at input, earthed to supply ground
275v MOV across supply outputs
Variac power control
Input ‘Live’ and ‘Ready to fire’ warning lamps
Output voltage and current meters
Key operated safety switch
Two simultaneous press-to-fire buttons and a long extension lead !!

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