What device uses a small DC current to control the flow of a large AC current?

The device that uses a small DC current to control the flow of a large AC current is a saturable reactor. A saturable reactor is a type of inductor whose inductance can be changed by applying a DC control current. When the DC control current is applied, it causes the magnetic core of the reactor to saturate, which reduces the inductance and allows more AC current to flow through the device. This principle is used for various applications, such as in regulating power in electrical circuits and controlling the power factor in AC systems.

In contrast, a transformer operates on the principle of electromagnetic induction to transfer electrical energy between two or more circuits and does not use a DC current for control. An inductor itself stores energy in a magnetic field and cannot control large AC currents based on a small DC input. Similarly, a capacitor stores electrical energy in an electric field and does not function in the same manner as a saturable reactor for controlling current flow.