Which component is used to shift a low-current control signal to switch a higher-current circuit while providing isolation?

• A. Transformer
• B. Relay
• C. Resistor
• D. Fuse

Answer: (B)

The idea being tested is using a device that lets a small, low-current control signal drive a larger-load circuit while keeping the two sides electrically apart. A relay does exactly this: an electromagnet coil on the control side draws only a small current to energize, while its switch contacts on the load side handle the higher current. The coil and the contact circuit are isolated from each other, so the control signal is kept separate from the high-current circuit, providing galvanic isolation. This makes it possible to run a control system (like a microcontroller or PLC) safely without exposing it to the higher voltages or currents of the load.

Transformers can shift voltage and provide isolation, but they don’t inherently switch a load on or off by themselves; you’d still need a separate switch on the load side. Resistors don’t provide any switching or isolation, just limit or drop voltage. Fuses protect by interrupting current in fault conditions, not by enabling normal switching with isolation.