What Is a Trigger Transformer?

A trigger transformer is a low-duty firing transformer that initiates the pilot currents that produce the major start-up currents for gas-filled cold cathode devices. It is usually manufactured such that the turn count in the secondary winding is several times higher than in the primary winding, although it is also possible that the primary winding is part of the much bigger secondary winding. This large turns ratio of windings allows the trigger transformer to deliver trigger voltages otherwise not available from the main supply.

Electrical transformers rely on the electromagnetic induction principle, where 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 able to generate bursts of high voltages 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 where, initially pushed to one side from rest, the pendulum bob will periodically move from one extreme position to the other until all the kinetic energy has dissipated. The energy stored in the capacitance is like the potential energy on one extreme position of the bob, while the energy stored in the magnetic field inside the magnetic core of the trigger transformer is like the kinetic energy of the bob as it passes its resting position.

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

The high-voltage self-limiting transformer (HVSLT) is used for neon tubes in signboards. Besides being a step-up transformer, the HVSLT also limits current like a ballast does. The HVSLT does not require an additional trigger transformer because the high voltage generated by the HVSLT is sufficient to initiate current flow through the load even during cold start.

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