What’s a Trigger Transformer?

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A trigger transformer is a low-load ignition transformer that starts the pilot currents that produce the main starting currents for gas-filled cold cathode devices. It is usually manufactured such that the turn count in the secondary winding is several times that of the primary winding, although it is also possible that the primary winding is part of the much larger secondary winding. This large winding turns ratio allows the trigger transformer to provide trigger voltages not otherwise available from the main supply.

Power transformers are based on the principle of electromagnetic induction, in which a time-varying current in the primary winding produces a time-varying magnetic field. This magnetic field produces the time-varying current in the secondary winding. Trigger transformers are capable of generating high voltage bursts that can ionize most gases used in cold cathode devices.

Capacitors work with trigger transformers to produce the desired ringing effect or resonance that sustains enough trigger energy. This is very similar to a mechanical pendulum in which, initially pushed to one side from rest, the pendulum will periodically swing from one extreme position to another until all kinetic energy has dissipated. The energy stored in the capacitance is like the potential energy on an extreme position of the weight, while the energy stored in the magnetic field inside the magnetic core of the trigger transformer is like the kinetic energy of the weight as it passes its position rest.

An electrical ballast, which limits the electric current in a circuit, is similar to a trigger transformer. In electric lighting, for example, the cold cathode tubes are not directly connected to the main power supply. The electric ballast is connected in series with the tube so the current is limited to very safe levels.

High Voltage Self Limiting Transformer (HVSLT) is used for neon tubes in signs. In addition to being a step-up transformer, the HVSLT also limits current as a reactor does. The HVSLT does not require an additional trigger transformer as the high voltage generated by the HVSLT is sufficient to initiate current flow through the load even during cold cranking.

A popular use for the trigger transformer is flash for photography and lighting effects. A flashtube, such as a neon bulb, is set to standby when the main terminals are energized with approximately 100 volts. This level is not sufficient to ionize the gas in the tube and cause a flash. A third terminal, called the trigger terminal, on the tube is closest to the main common terminal. To trigger a flash, a burst output from the trigger transformer ionizes the gas between the tube trigger and the common terminal, resulting in a triggered flash.

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