NT-OUT neon transformer.
Older NT-(OUT) series are simply iron-cored transformers, usually embedded in asphalt for protection and insulation. 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 supply|switch mode power supplies]] to power neon signs. These generate the same voltage and current ranges as iron cored transformers, but in a much smaller, lighter, and more efficient designs at high frequency (not the common 50-60 Hz). They are gradually replacing iron cored transformers in neon signs. Older types are still available.
