One of these windings, the primary, is connected to an alternating voltage source. An alternating flux will be produced whose amplitude will depend on the primary voltage and the number of turns in the winding. The mutual flux will link the other winding, the secondary, across which it will induce a voltage whose value will depend on the number of secondary turns. When the primary and secondary turns ratio is properly proportioned almost any desired voltage ratio, or ratio of transformation, can be obtained. This transformer is also widely used in low power electronic and control circuits where it performs such functions as matching the source impedance and its load for maximum power transfer, isolating one circuit from another, or isolating dc current while maintaining ac continuity between two circuits.
Loads can be equipment or appliances.
That then provides the right level of ac line voltage to the main transformer of the equipment by connecting one of the primary windings through the triggered triac. The operational range in this circuit, in the lower ac line voltage condition, is from 100 Vrms to 150 Vrms. This means that the triac driving the winding of the main transformer for 110 V would keep itself triggered whenever the input voltage across the control transformer is within that 100 to 150 Vrms range. The operational range in the higher ac line voltage condition is from 180 Vrms to 250 Vrms, with the corresponding triac-trigger range. If the circuit is working outside of these limits it will probably be unreliable.