A neon-sign transformer (NST) is a transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 made for the purpose of powering a neon sign

Neon sign

Neon signs are made using electrified, luminous tube lights that contain rarefied neon or other gases. They are the most common use for neon lighting, which was first demonstrated in a modern form in December, 1910 by Georges Claude at the Paris Motor Show. While they are used worldwide, neon signs...

. They convert line voltage from the 120-347 V range up to high voltages, usually in the range of 2 to 15 kV. Most of these transformers generate between 30-120 mA.

Types

Older NSTs are simply iron-cored transformers, usually embedded in asphalt to reduce noise. The core has a magnetic shunt which serves to current-limit the output, allowing them to run indefinitely in short-circuit conditions. They can also run indefinitely with no load. Iron cored varieties are quite heavy, for example a 15 kV, 60 mA device may weigh up to 20 kg.

Since the 1990s, manufacturers have been producing switch mode power supplies to power neon signs. These generate the same voltage and current ranges as iron cored transformers, but in much smaller, lighter designs at high frequency (not the common 50–60 Hz). They are gradually replacing iron cored transformers in neon signs.

All NST's are designed to produce a high voltage starting pulse to a tube, then limit the current through the tube when it has started.This is opposite of most line transformers, which will produce full voltage to a load even if overloaded, unless the resistance of windings is too great to allow the excess current or until a winding burns out.

Other uses

Besides the obvious purpose of powering neon signs, iron cored NST's are often used by hobbyists for:

• Tesla coil
  Tesla coil
  A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. Tesla coils produce higher current than the other source of high voltage discharges, electrostatic machines...
  
   power supplies – used in small to medium sized tesla coils as the main source of high voltage.
• Jacob's Ladder – a climbing arc device often pictured in older horror films.
• Charging Capacitors – an NST makes a useful high voltage power supply to charge high voltage capacitors. Although the output of an NST is AC
  Alternating current
  In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
  
  , it can be rectified by the proper diode
  Diode
  In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
  
   or bridge rectifier.

.

Safety

• Electrocution
  Electric shock
  Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
  
   – The shock from a neon sign transformer could be lethal. The high voltage allows a large current to flow, even with light contact against dry skin. The transformer is current-limited, but typically to a level well above the threshold for ventricular fibrillation
  Ventricular fibrillation
  Ventricular fibrillation is a condition in which there is uncoordinated contraction of the cardiac muscle of the ventricles in the heart, making them quiver rather than contract properly. Ventricular fibrillation is a medical emergency and most commonly identified arrythmia in cardiac arrest...
  
  . Disconnect power to the transformer before servicing. Use appropriate insulation around connections; typical insulation, including standard electrical tape and most insulated wire, is rated only for much lower voltages.

• UV Light – The ultraviolet light emitted from the high voltage electrical arc can be harmful to ones eyes. It is recommended that the arc be viewed through the appropriate welding
  Welding
  Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
  
   goggles or at the very least a high quality pair of sunglasses
  Sunglasses
  Sunglasses or sun glasses are a form of protective eyewear designed primarily to prevent bright sunlight and high-energy visible light from damaging or discomforting the eyes. They can sometimes also function as a visual aid, as variously termed spectacles or glasses exist, featuring lenses that...
  
  .

• Ozone
  Ozone
  Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
  
   – the production of ozone can be noticeable when there are problems with a luminous tube transformer installation. Ozone usually indicates failed secondary wiring, loose connections, high capacitance coupling
  Capacitive coupling
  In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
  
  , or a failing transformer.

• Fires – The arc length on 15 kV is in excess of 2 inches. Keep wiring inside grounded 1/2" metallic conduit (watertight if applicable). Be sure to use proper enclosures for transformers. Stray electric arcs will ignite combustible materials.