The patented atg airports (Tann) Transformer Switching Unit (TSU) is a fully encapsulated solid state electronic switch, capable of shunting the secondary winding of a series isolating transformer. The unit is used in airfield ground lighting circuits for the SmartControl of aircraft on taxiways, and as required within CAP 168.
These units comprise two optically isolated circuits, the signal input circuit, and the power switching circuit. An input signal from a nominal 50V AC or DC source, when applied, passes through a suitable circuit filter to the light emitting diode of the opto isolator.
For normal type TSUs the power switching circuit receives this signal from the opto isolator and passes it via switching and filter circuits to the gate of a TRIAC, which in turn shunts the secondary winding of the Series Isolating Transformer, and extinguishes the relevant lamp.
For inverse type TSUs the signal is used to disable the gate of the TRIAC, hence removing the shunt on the secondary of the series isolating transformer and thus allowing the relevant lamp to illuminate.