High-Voltage Transformers

A high-voltage transformer steps the mains supply up to several thousand volts or more, providing the high potentials needed to strike arcs, light discharge tubes or drive other high-voltage apparatus. Like any transformer, it works by electromagnetic induction between two windings on a shared magnetic core: an alternating current in the primary winding creates a changing magnetic flux that induces a voltage in the secondary. The ratio of secondary to primary turns sets how much the voltage is increased, so a large turns ratio produces a large step-up.

The best-known example is the neon-sign transformer, designed to ignite and run gas-discharge tubes. It supplies a high open-circuit voltage to strike the discharge, then settles to a lower running voltage as current flows. These transformers are built to limit their own output current, a property called high leakage reactance, so the current stays within safe limits for the tubes even when the output is effectively short-circuited by the arc. This self-limiting behaviour makes them suitable for experiments and displays such as Jacob's ladders, although safety standards have reduced the use of older designs in fixed signage.

Other high-voltage transformers serve different needs. Oil-burner and furnace ignition transformers produce a high-voltage spark to light fuel, microwave-oven transformers supply high voltage to a magnetron, and instrument and test transformers generate controlled high voltages for laboratory work. All of them present serious shock hazards because their outputs can be lethal, and many can deliver enough energy to cause severe injury or start fires. For this reason high-voltage transformers are enclosed, insulated and handled with strict precautions, and capacitors associated with them are discharged before any contact.

Frequently asked questions

How does a transformer raise voltage?

An alternating current in the primary creates a changing magnetic flux that induces voltage in the secondary. A secondary with more turns than the primary produces a higher voltage.

Why are neon-sign transformers current-limited?

They are built with high leakage reactance so they restrict their own output current, protecting the discharge tubes and the transformer even when the output is short-circuited by the arc.

Why are high-voltage transformers dangerous?

Their outputs can reach lethal voltages and deliver enough energy to cause severe shock, burns or fire, so they must be insulated, enclosed and handled with great care.