Neon-sign transformer

An iron cored neon-sign transformer, with a 9-volt battery for scale.

A neon-sign transformer (NST) is a transformer made for the purpose of powering a neon sign. They convert mains voltage in the range 120-347 V up to high voltages, in the range of 2 to 15 kV. These transformers supply between 18-30 mA; 60 mA on special order. The high-voltage electricity produced is used to excite neon or other gases are used in luminous gas discharge tubes.

Types

Older NSTs are simply iron-cored transformers, usually embedded in asphalt to reduce noise. The core either has a magnetic shunt, or a gap in the iron core, both of which serve 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 (44 lb). Some newer manufactured iron cored NSTs include a large capacitor in parallel with the output for power-factor correction (PFC). This serves to correct the shift in the phase of voltage and current caused by the large inductance of the transformer.

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 NSTs 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 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, it can be rectified by the proper diode or bridge rectifier.
• Lichtenberg figures can be created in wood with high voltage transformers, using water and salts to make the wood more conductive.
• Because of the lower current, neon sign transformers are safer than microwave oven transformers which output 2100-2500 V at 500 mA.

Safety

• Electrocution – The shock from a neon sign transformer can 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.
• UV Light – The ultraviolet light emitted from the high voltage electrical arc can be harmful to the eyes. It is recommended that the arc be viewed through the appropriate welding goggles or at the very least a high-quality pair of sunglasses.
• Ozone – 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 capacitive coupling, or a failing transformer.
• Fires – The arc length at 15 kV is in excess of 5 millimetres (0.20 in).

Operational problems

Gas-light transformers have two unpleasant features: at low load overheating of windings is possible, which is one of the main reasons for their failure, in turn, overloading the transformer leads to the development of high-frequency oscillations in the electrical circuit, which deteriorate the quality of line voltage. Interference that can affect the operation of capacitors (resulting in breakdowns) and protection devices (resulting in false tripping of circuit breakers).

Errors in the source data have three sources. First, inconsistency in line voltage standards. Most European, American and Asian transformer models are designed for 230 V mains voltage. Other parameters may also differ, e.g. mains voltage frequency (60 Hz instead of 50 Hz) and tolerance values. Therefore, the characteristics of gas-light transformers required for the calculation of light lines and external circuit elements are only valid for the networks of the country of manufacture. Second, despite the international character of technical knowledge, the forms of its representation in technical documentation may differ markedly due to problems with professional translation. Thus, any technical data needs to be comprehended. Third, we are actually always dealing with a "transformer - light circuit (load)" system. In case of deviations from the recommended load, the current and power ratings of the transformers will differ from the nominal ratings, which are provided to the customer as a reference.

Reliable operation of gas-lighting systems depends significantly on the operation of the power supply systems. The stability of the main elements of these systems is determined not only by their quality, but also by the correct choice of electrical parameters that ensure the electrical stability of the circuit "transformers - neon lamps", the stability and quality of line voltage and ensuring normal heat exchange of transformer surfaces with the environment.