Spark Gap Tesla Coil

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Basics of the Spark Gap Tesla Coil

A Tesla coil that uses a resonant circuit including a capacitor, coil and of course a spark gap. These coils are most common for beginner Telsa coil constructors, or "coilers". They are a favorite among beginners because of the low cost and relatively easy accessibility of parts.

Power Sources

Neon sign transformer (NST)

See Neon sign transformer
They are commenly used, current limited transformers, with a large range of voltages 3-15 kV@ 30-120 mA (under 10 kV in Europe). For a table top unit use at least 4 kV@30 mA, for easy breakdown in the spark gap. For higher powers use at least 6-7.5 kV @ 30 mA. Do not forget to protect your NST (they are very sensitive at RF and peak voltages). Use a RC filter and a safety gap. You can use more identical NST's with the secondaries in parallel (same output voltage) to get more current output.

Microwave oven transformer (MOT)

See Microwave oven transformer
Often (relatively) low voltage. The voltage is so low, 2kV, that it is difficult to make a proper spark gap. A high current output makes the gap difficult to quench. For a small TC you can series two MOTs (with primaries in anti-parallel, and the secondaries in series (connected cores with HV at the outer terminals) for at least 4 kV, and limit the current with an inductive ballast on the low voltage side, or a capacitive ballast on the HV side. For higher power systems, use two MOTs and a doubler or a 4 MOT "Quad pack". This requires adequate insulation, usually under oil, with appropriate current limiting.

Oil burner ignition tranformer (OBIT)

See Oil burner ignition transformer
10 kV @ 20 mA. Is sometimes used in paralleled stacks. They heat up in a few minutes , so the run time is limited.

  • PT (potential transformer) 10-14 kV, up to 500 mA. These are powered "in reverse." They are lethal, take the necessary precautions. Use an external inductive ballast on primary side. Very robust transformers, safety gaps are superfluous.
  • Pole pigs (distribution transformer) Single phase in USA, 3 phase in Europe. Range in voltage from a few kV to several tens of kV's. Require passive or active external ballasting. Extremely dangerous, avoid contact at all costs.


The maximum predicted spark length for a given input power is  spark\; length = 1.77 \times \sqrt {Pin} where the spark length is in inches and Pin is in watts. (Freau formula)

Secondary Parameters

These, corellated with the used power, are needed to calculate the primary tank parameters

Lp * Cp = Ls * Cs (resonance condition)

The secondary dimensions depends by the power you want to use:

Dimensions By Power
Input Power Secondary Diameter
<500 VA <4"
500-3000 VA 4-6"
>3000 VA >6"


P = Ec \times BPS

{Ec = C\times(Up)^2 \over 2}  = energy\; in\; the \;primary\; cap\; when \;the \;spark\; gap(s) \;breakdown

Beats Per Second = number of discharges cycles per second in the spark gap eg: for a static gap, if mains frequency is 60 Hz,  BPS = {120 \over sec}

Secondary Coil

  •  {diameter \over wound \;length} =  {1\over3}\;to \;{1\over5}. Longer secondaries will give slightly higher outputs (longer sparks). Short, thick secondaries will handle higher input power.

Dos and Don'ts

  • Do use PVC, acrylic pipe, and other types of plastic as coil forms.
  • Do use thin walls when possible as the RF losses are greater in thicker material.
  • Do sand the coil form before winding, so the surface will be smooth. You can also varnish it at this stage.
  • Do wind round 800-1500 turns
  • Do varnish the coil after winding, if you do not it may unravel. Varnish also provides insulation.
  • Do not use black PVC as it contains carbon.
  • Do not use very thin wire (<0.2 mm), it will increase the ohmic resistance and decrease the voltage gain (Q) of the system. Thicker wire will prevent the racing sparks as well.
  • Do not use metalic screws which penetrate inside the PVC pipe. Only nylon or other plastic screws are permited

A common problem is racing sparks. These occur when a coil has one or more of the following:

  • Overly high coupling
  • Overpowered system
  • Poor quenching in the spark gap
  • Out of tune, too large toroid
  • Overly large primary cap

Primary Coil

Dos and Don'ts

  • Do use thick solid copper wire or 1/6" to 1/4" soft copper tubing, thicker for larger coils.
  • Do use short, solid, thick connections between the primary tank capacitor
  • Do not use metal in the primary supports
  • Do use many primary turns, this will allow for easier tuning
  • Do use a tap with a large surface to reduce resistive losses
  • Do not use a cylindrical primary, they are prone to breakdown between primary and secondary, if the insulation is not adequate
  • Do use a flat (pancake style) primary
  • Do raise the secondary, if racing sparks occur, continue raising until they diminish.
  • Do use conical primaries, they are a compromise between higher coupling and primary to secondary insulation
  • Do leave at least 1/4" between primary turns and at least 1 inch between the inner turn and the secondary. If the bottom of the secondary arcs to the inner turn, raise the secondary or remove the inner turn of the primary.

Tank Capacitor

Dos and Don'ts

  • Do use high quality commercially made polypropylene film capacitors when possible. Home made capacitors can work as well, however with greater losses.
  • Do use a MMC, or specially desiged high voltage pulse capacitors (for larger coils ~5kVA+).
  • Do not use: mylar, window glass with a <3 mm width, ceramic, paper-oil capacitors, photo flash capacitors.
  • {voltage \;rating\over \;string\; of\; capacitors} \; = Urms * 2 * \sqrt{2} V
  • Do use paralleled strings of capacitors to increase current capability.
  • Do measure the temperature of the capacitor(s) after each run to prevent overheating.
  • Do add more capacitors if they get hot after a short run.
  • Do use short high current connections between capacitors.
  • Do use bleeder resistors on each capacitor, around 10M 1/2 watt, to eliminate the residual charge after run.
  • Do shield the capacitors in the case that they were to fail violently.
  • Do use a larger than resonant capacitor value. The value of the resonant cap can be calculated with this formula: f is Fres

 C = {I\over  2* \pi * f * U}

  • Do use 1.5 times resonant cap for static gapped coils.

See also: Capacitor


Dos and Don'ts

  • Do use a toroid it provides capacitance plus better corona suppression on the top turns.
  • Do use a bigger toroid, it will give longer sparks, up to a limit.
  • Do use corrugated aluminum pipe, cover the surface with aluminum foil tape and smooth it very well.
  • Do use a break out point for a single streamer output.
  • Do use a toroid pipe diameter that is similar to the diameter of the secondary, use a length similar to the secondary length for the toroid's outer diameter.
  • Do try several different toroid sizes and shapes to achieve best preformance.
  • Do stack your toroids if they are too small.

Spark Gap

See the article on spark gaps for more detailed coverage.

  • simple gap
  • non-quenched
  • quenched
  • -blown gap
  • -sucker gap
  • multiple gap (indicated for beginner)
  • non-quenched
  • quenched
  • -blown gap
  • -sucker gap
  • asynchronus
  • -AC charging
  • -DC charging
  • synchronous
  • -AC charging
  • assymetric and asynchronous
  • -DC charging
  • COMBINED (rotary + static gap, used usually in magnifiers for fast quenching)
Better quenching in the gap = longer streamers.
But at low powers, too much quenching can sometimes decrease the performance.
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