Which device uses a small DC current to control a larger AC current?

• A. Rectifier
• B. Saturable reactor
• C. Rheostat
• D. Relay

Answer: (B)

A saturable reactor changes how much current can flow in an AC circuit by using a small DC bias to alter the magnetic core’s properties. The DC current magnetizes the core and pushes it toward saturation, which reduces the core’s permeability and lowers the inductive reactance the AC current sees. Because inductive reactance determines how much AC current passes for a given voltage, shifting the DC bias lets you regulate a larger AC current with a relatively tiny control current. In other words, a small DC current sets the level of a larger AC current by changing the impedance in the AC path.

This is different from a relay, which uses a small DC current to operate a switch that controls a separate, larger AC circuit in an on/off manner rather than continuously varying the AC current. It also isn’t a rectifier, which converts AC to DC, or a rheostat, which is simply a variable resistor in the same current path without magnetic saturation control.