# Spark Gap Tesla Coil

## 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.

## Mathematics

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"

### Power

$P = Ec \times BPS$

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

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.

### 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.

• STATIC GAP
• simple gap
• non-quenched
• quenched
• -blown gap
• -sucker gap
• multiple gap (indicated for beginner)
• non-quenched
• quenched
• -blown gap
• -sucker gap
• ROTARY SPARK 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.