What type of transformer is used in the filament circuit?

In a filament circuit, a step-down transformer is utilized to reduce the voltage from a higher level to a lower level that is suitable for the filament. This type of transformer is designed to provide the necessary low voltage required for the operation of incandescent bulbs or other similar devices that use filaments. A step-down transformer typically features more turns of wire on the primary coil compared to the secondary coil, allowing it to decrease the voltage while increasing the current, which is essential for heating the filament to produce light.

Using a step-down transformer ensures that the filament receives the appropriate voltage, which is critical for its safe and efficient operation. Employing a different type of transformer, such as a step-up transformer, would increase the voltage instead of decreasing it, potentially damaging the filament. An isolating transformer is primarily used for safety and electrical isolation, rather than for adjusting voltage levels. Lastly, a three-phase transformer is used for three-phase systems, and is not applicable in scenarios that involve single-phase filament circuits.