TESLA COILS:

   In order to charge the capacitor bank enough to fire the gap each cycle, you will need a source of high voltage. Since a minimum of 4kV is required to fire the spark gap reliably, electrical outlets (120V-240V) are ruled out. Since it is unlikely you own your own high voltage generator or dynamo, step-up transformers are your best option.

   Step-up transformers operate similar to a Tesla Coil (in fact, a Tesla Coil is a type of transformer): a low AC voltage is applied to a primary coil, which creates a alternating magnetic field that induces a higher voltage in a secondary coil via electromagnetic induction. For Tesla Coil use, usually 4kV to 20kV are used, however it is possible to use voltages even higher (100kV  x-ray transformers have been used). Unlike a Tesla Coil, though, most transformers do not require a resonant capacitor or drive circuit (flyback transformers and ignition coils are among the exceptions). Most transformers have four terminals: two primary coil inputs ("positive" and "negative") and two secondary coil outputs. Some transformers have only three terminals, in which case the missing terminal is the other secondary output, and is generally connected internally to the iron core or metallic case. Below is a list of the most common transformers used to power Tesla Coils, along with their specifications and other important details:

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Neon sign transformer (NST):

• 3kV-15kV

• 10mA-60mA (although rare, 120mA models also exist)

• More expensive ($50-$200)

• Found in two forms: solid state (plastic, lightweight, small, 

      practically useless in a Tesla Coil) and 60Hz/50Hz (heavy,

      metallic, usable in a coil, and are the type this site refers to).

• Internally current limited by magnetic shunts within the transformer core, which, with some effort, can be removed to allow more current (for more on this, check out this YouTube video).

• Single units are commonly used in smaller coils, while a bank of paralleled (phased) NST's can drive even large coils (for a demo on NST phasing, click here).

• Lower output currents allow for easier gap quenching, meaning a static gap can be used with fewer issues.

• Although an excellent option, NST's are more prone to secondary failure

Microwave oven transformer (MOT):

• 1.8kV-2.3kV

• Over 1A when unballasted

• Most models available to buy are salvaged/used and are quite affordable ($20)

• Most units have only three terminals (the missing fourth terminal is the other secondary output, and is internally wired to the metal body).

• Non-current limited (although ballasting isn't always needed). A popular ballasting option is to connect one or more MOT's (with the secondary shorted) primary(s) in series with the primary of the main MOT. This limits to power to 1-2kW (8-17A at 120V), but generally causes a lot of heating in the ballast MOT and the Tesla coil output may be severely decreased.

• Since 2kV isn't usually enough to fire a spark gap, 2-4 MOT's are usually connected with secondaries in series and primaries in parallel to yield 4-8kV (some coilers have even connected eight or more MOT's successfully). To make a dual MOT bank, electrically connect the metal bodies of two transformers, ground the bodies using the ground terminal on a wall socket (the rounded, third prong which some appliances don't have) or the earth directly, and use the two wire terminals as outputs. If no voltage (or excessively low voltage) results, switch the connections on one of the transformer's primary winding. This essentially gives you a 4kV center-grounded transformer with two 2kV outputs that are 180 degrees out of phase (i.e. when one terminal hits +2kV, the other hits -2kV, giving you 4kV between them. This prevents the individual units from facing the full 4kV, which could potentially damage them). With any bank over two MOT's, it is smart to submerge the transformers in oil or wax to help prevent voltage breakdown, interwinding arcing, etc. (even a dual MOT bank can benefit from oil submersion). 

• The high currents produce by MOT's make static gap quenching more difficult, so rotary gaps are preferred in MOT coils.

• Most commonly used in medium to large coils.

Oil/gas burner ignition transformer (OBIT):

• 10kV-14kV (although harder to find, models as low as 6kV are available)

• 20mA-40mA

• Relatively cheap (usually $30 or more)

• Internally current limited

• Single transformers may be used in small coils. However, OBIT's can be paralleled in the same manner as NST's to get more current. Paralleled OBIT banks, like NST banks, can power almost any coil.

• Relatively low currents allow single units to work with a static gap.

Flyback (television) transformers (FBT):

• 10kV-40kV

• Approximately 5mA-30mA

• Units have several terminal pins. To power a flyback transformer, a drive circuit (ZVS drivers are the most popular for Tesla Coil use) is need, which can easily be bought online for under $40. Although most flyback transformers have built-in primary coils, a DIY primary coil is usually wrapped around the exposed ferrite core (shown in the image at right). The secondary coil has one end coming out the transformer top (the long, thick, red wire in picture at right) while the other end is one of the pins (you may test for continuity with a multimeter or find which pin pulls an arc from the other secondary wire to determine which pin is correct).

• Flyback transformers operate at relatively high frequencies (usually over 20kHz). At frequencies this high, the primary capacitor will simply act like a short, and not function correctly in a resonant circuit. To overcome this, the current must be rectified (converted to DC, usually by placing a diode or diode array in series with the output). Many flyback transformers have a built-in rectifier diode, however, so this is generally not an issue.

• Generally used in small to medium coils. While more difficult, it is theoretically possible to parallel several flyback transformers to get enough power to drive a larger coil.

• The low currents and high frequencies associated with flyback transformers allow them to be easily run with a static gap. As power and voltage increase, though, a rotary gap might become the better option.

Distribution transformers (pole pigs):

• 3kV-35kV

• 200mA to over 3A (without a ballast)

• Non-current limited. Requires a ballast (otherwise it will trip almost any size breaker).

• Very expensive ($500+) and hard to find

• Usually used as step-down transformers, stepping down the high voltage from powerlines to be used in homes. For Tesla Coil use, simply input the mains voltage into the secondary (usually found on the unit's side as two to three metal terminals), and high voltage comes out the primary (the large bushings found at the top of the unit).

• Most often used in large coils, but can be used with any coil size (especially if ballasted).

• The high output currents mean a rotary gap is nearly essential (heavily quenched static gaps may still be used).

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   Other transformer models can/have been used in Tesla Coils (such as bombarding, x-ray, plate, or potential transformers), but, due to their rare or hard to implement nature (x-ray transformers, which can run up to 150kV, are a good example), they will be omitted from discussion.