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.
Read more about this topic: Neon-sign Transformer
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